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ISSN 1023-1935, Russian Journal of Electrochemistry, 2009, Vol. 45, No. 5, pp. 497–511. © Pleiades Publishing, Ltd. 2009. Original Russian Text © M.E. Kompan, 2009, published in Elektrokhimiya, 2009, Vol. 45, No. 5, pp. 531–546.

Luminescent-Technique Study on the Structure of Cation Subsystem in the Ionic Conductor Na5RESi4O12 (RE is a Rare-earth Cation Tb3+, Ho3+, Er3+, Gd3+, Eu3+)1 M. E. Kompanz Ioffe Physicotechnical Institute Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia Received July 14, 2008

Abstract—Results of experimental studies of luminescence in a group of materials with common formula Na5RESi4O12 (RE is a rare-earth cation Tb3+, Ho3+, Er3+, Gd3+, Eu3+ entering the stoichiometric formula of the substance) are summarized. The luminescence spectra give information on the mobile-cation sublattice structure and dynamics. It follows from experimental results that the current opinion on the disordering of mobile sublattice is not quite correct. At relatively low temperatures (T < 100 K), a small number of typical local configurations of cations can be elucidated, which in aggregate can describe the Na+ cation sublattice to rather high degree of accuracy. At higher temperatures, the number of spectral lines and their positions change, which formally corresponds to changes in the symmetry of sites of the lattice, hence, is the sign of a second order phase transition. This phenomenon is given an explanation based on a suggestion on the local dynamic averaging of the mobile cation electrical fields. Key words: superionic conductors, optical spectroscopy, self-assembling of dynamic structures, cation sublattice DOI: 10.1134/S1023193509050012

INTRODUCTION Spectroscopic studies of superionic materials started much before the specificity of the materials has been realized. The materials from this group were studied as solid-state objects, because they are dense and in many cases have crystalline structure. A detailed overview of optical properties of silver and copper halides [1] exemplifies this approach. This review is free from references or record concerning specific manifestation of superionic conduction of silver iodide, a model material, despite it catches one’s eye. An interest to superionic materials aroused among researchers in spectroscopy in 1980s, as a part of general interest to materials of all kinds of disordering. Numerous theoretical works were focused at this aspect when interpreting phenomena in superionic conductors (see, e.g., [2, 3]). At lower temperatures ionic conduction is suppressed, and this analogy works, indeed; in some optical studies there were found phenomena similar to those observed in semiconductor solid solutions [4–6]. As mentioned above, these studies were carried out at low temperatures; and the spatial scale of the inner characteristics and structures in the materials under study (e.g., excitons) exceeded the ion hopping 1

Published by report at IX Conference “Fundamental Problems of Solid State Ionics”, Chernogolovka, 2008. z Corresponding author: [email protected] (M.E. Kompan).

length by 1–2 orders of magnitude. This approach is the reason why the works of this orientation yielded no more than analogies between two classes of materials; it have hardly promoted better understanding of the nature of superionic conduction. The phenomenon of superionic state and superionic conduction is very interesting; it is not fully appreciated. Among the superionic materials, the most striking is silver iodide [7]. This material passes to the superionic state through a first order phase transition at a temperature of 147°ë (its melting point is 555°ë); the conductivity jumps up by three orders of magnitude and reaches a few S/cm. The ion mobility in the superionic AgI is so high that the conductivity even drops down by 7% following the material melting. The transformations similar to those occurring in silver iodide at 147°ë are adequately called “the sublattice melting”. Other materials show somewhat lower carrier mobility (for example, RbAg4I5 [8]) and not always reach the highconduction state via the phase transition (e.g., β-alumina [9]). The central point is the very (nontrivial) possibility of the observing of high ionic conductivity in solids, which is comparable to the conductivity of liquid electrolytes. The explanation of the phenomenon of superionic conduction implies most of all the explanation of low activation energy of ionic transport (for example, 0.05 eV for AgI [7], 0.15 eV for RbAg4I5 [8]). Why are

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ions almost free in some substances, while require nearly 1 eV for their hopping in other substances? What determines the basic difference between two salts of univalent metals and haloids—an insulator NaCl and superionic conductor AgI? (Despite both materials have similar (cubic) crystal lattices; moreover, the mobile ion in the latter material is nearly one order of magnitude heavier than the sodium cation in the former.) The mere criterion suggested, the specific degree of the bond ionicity (~0.7 [10]), explains nothing. In the phenomenological approach, the low activation energy of ionic transport is considered as a source entity; the ionic interactions in the crystal lattice are specified in such a way that the potential relief of an ion diffusion path was minimally modulated. This approach was realized, in particular, in work [11]; however, no correlation in the mobile ion sublattice was taken into account. And yet, generally speaking, this interaction cannot be neglected because mobile ions are point objects, their charge density is not diffuse, unlike that of lighter particles (electrons). The charges of ions in the mobile and immobile crystal lattices are close to each other; the distances between mobile ions differ but insignificantly from those between the mobile and immobile ions. The rather incorrect approaching the problem is likely to come out from the adopted manner of the X-ray-diffraction-analysis data presentation. A typical formula “energy equivalent positions” proceeds from the statement on the equality of average occupancies, which are determined from the X-ray-diffraction-analysis data. However, the X-ray-diffraction-analysis data are produces in a long experiment, hence, they are averaged over time and, additionally, over crystal bulk. An instant equality of the occupancies of “partially occupied positions” is impossible because of the charge discreteness of charge carriers; it does not follow from any experimental data. The equality of the average occupancies allows the existing of conditions of correlation in the occupancy of particular positions, for example: when the position of the type A is occupied, the position of the type B is free,—and more complicated. Noteworthy is that the superionic conductors are not unique in this regard. Any object classified as a two-level system [12] must conform to these rules. Although the taking into consideration of the correlation between mobile ions generally is recognized as necessary, the works directly dedicated to this problem are scarce. For example, the concept of the ionic transport correlation were used [13] in the explaining of the data on the β-AgI high-frequency conduction. However, in work [13] and alike, only the obvious type of correlation in the mobile ions’ subsystem was considered, in particular, the correlation of the direct and reverse ion jumps. The correctness of the taking into consideration (or neglecting) of the contribution from different sublattices to the activation energy was discussed in work

[14] by example of α-AgI. It was concluded for certain that the activation energy of ionic conduction can only be correctly calculated when the ion correlation, up to the four-particle one, has been taken into consideration. Actually, the case in point is that the coordinated crowdings or exhaustings of the mobile cation local density could change the activation energy of an ion jump over certain region. However, the analytical approach [14] was mainly of methodological character. The origin of the described approach to superionic conductors lies in the absence of methods that allowed revealing self-forming stable dynamic configurations in the mobile sublattice [15]. By definition, macroscopic methods (e.g., the conductivity or heat capacity measurements) cannot provide this information. While the microscopic information is mainly taken from the X-ray-diffraction-analysis data that yield information on spatially periodic structures, yet are insensitive to local structures. In the summarized series of works we used in the study of ionic conductor Na5RESi4O12 the method of laser spectroscopy with selective excitation, which was initially developed for the analyzing of some homogeneous subgroups of a disordered set (in terms of spectroscopy, uniformly broadened components of a nonuniformly broadened band). This approach was applied to molecular spectra in condensed media for the first time [16]. Additionally, in this series of works we used the site-selected spectroscopy combined with timeresolved detection, which allowed performing direct kinetic experiments involving changes of cation configuration in the sublattice in time. IONIC CONDUCTOR Na5RESi4O12: ITS STRUCTURE AND PROPERTIES The experimental studies were carried out with single crystals of the ionic conductor Na5RESi4O12. Here RE denotes a rare-earth cation Tb3+, Ho3+, Er3+, Gd3+, Eu3+ entering the stoichiometric formula of the substance. Europium was introduced to the samples as an additive (20%) to optically inactive trivalent cation Y3+; gadolinium, at the same ratio, as a co-activator to Ho3+. The single crystals were grown by hydrothermal method (Dimitrova, O.V., Moscow State University); they were hexagonal bipyramids sized ~0.3–0.5 mm. Nowadays, the structure of materials under study (Na5RESi4é12, the double silicates of sodium and rareearth elements) is studies in detail [17]. Their crystal lattice relates to the R 3 c spatial group; the elementary cell contains 18 formula units, the cell averaged parameters are: ‡ = 2.2 nm, Ò = 1.26 nm. The rare-earth ions take regular crystallographic positions in the crystal lattice. The first coordination sphere of the rare-earth ion is composed of 6 oxygen anions that form slightly distorted octahedron, the average distance from RÖ3+ to O2– in the octahedron is 0.225–0.230 nm, depending on the rare-earth ion nature. The rare-earth–oxygen octahe-

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Na5 Na6

Na+6 Na+4

499

B

Na+6

–

O2

Na+5

B

R

Re3+

Na4

Na+4

c = 12.61 Å

Na+6

3.05

G

–

O2

3.42

Na+6 Na+5

1.0

1

0.85

Fig. 1. Fragment of crystal lattice of superionic conductor Na5RESi4O12. The oxygen-ligand octahedron in the first coordination sphere is moved apart for clarity (to demonstrate the rare-earth ion). Shown are the separate positions of Na4 cations and merged group of positions of Na5, Na6, Na6’, and Na5’ cations in two channels neighboring the rare-earth–oxygen octahedron.


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drons, in their turn, are located along the screw triad axes that are parallel to the hexagonal C axis of elementary cell. The octahedrons have no common members [18]. The substances of this family conduct electrical current via Na+ ions, the activation energy being 0.21– 0.24 eV [19]. According to the current opinion, the cationic transfer predominantly occurs along cavities in the crystal lattice, which form quasi-one-dimensional channels parallel to the elementary cell bulk diagonals. In Fig. 1 we present a fragment of crystal lattice, which includes a rare-earth–oxygen octahedron and segments of two stepwise one-dimensional channels located in its immediate proximity. The positions in the conduction channels are well diffuse; in the maps of cationic density [20] they appear as maximums passing into each other. The dense crowdings can be formally divided into four positions denoted as Na5, Na6, Na6’, and Na5’. The probability of occupation is close to 1/3 for each of these positions. The more detached positions used to be denoted as Na4, their probability of occupation is ? Under such dividing of the mobile cation sublattice into groups (the segments of diffusion paths), no mobile cation position remain beyond the limits of the groups. In Fig. 2 we demonstrate the relative arrangement of the channels and positions therein. As to the cationic positions distribution in the distance to rareearth ion, the coordination sphere is clearly distin-

2.8

Fig. 2. Structure of the conductance channels in Na5RESi4O12 (taken from [20]). The disjoint channels cross the crystal lattice in three directions. The indexes R, G, and B mark the beginning and the end of segments of the corresponding channels. These indexes are used in Table 1 and Fig. 8 for the denoting of positions in the corresponding channels.

guished: ten nearest cation positions are located at a distance of (0.35 ± 0.02 nm) to the rare-earth ion; the next nearest position, at 0.53 nm. Thus, in this crystal the rare-earth ions, firstly, are located at regular positions in the crystal lattice; secondly, the positions are situated in highly symmetrical (octahedral) environment of oxygen ligands of 1st coordination sphere. The paths of conduction diffusion of the mobile ions are located in the immediate proximity to the rare-earth–oxygen octahedrons. The positions in these diffusion paths are filled by the mobile cations probabilistically. It is the Na+ mobile cations that occupy these positions probabilistically and thus form 2nd coordination sphere of rare-earth ions. No. 5

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The above-described situation is so nontrivial and favorable for the optical probing of the mobile sublattice structure that it deserves more detailed discussion. The rare-earth ion spectra in visible region are caused by optical transitions in their unfilled inner 4f-shell. In crystals, the levels of ions are split under the action of electrical fields of the crystal lattice ions (in terms of spectroscopy, crystal fields). The particular position of the ionic sublevels and the lines in the spectra, corresponding to the transitions between the sublevels, depends on the type of ions and their local coordination environment in the crystal. The crucial factor is the crystal field strength and symmetry in the point of the ion location. In traditional objects of spectroscopy, e.g., glasses and crystals activated by RE-ions, the optically active RE-ions often supersede ions with different charge; moreover, they often differ from the superseded ions in their radii. As a result of this substitution, rareearth ions appear in defect positions whose symmetry is additionally lowered by neighboring defects of opposite sign, which provide the charge compensation. Therefore, such an ion is located in a strong low-symmetry field formed by the ions of the 1st coordination sphere. The field working on the ion cannot be changed by ions of the next coordination spheres. The spectra of the ion give information just on its 1st coordination sphere. On the other hand, spectra of crystals in which rare-earth ions take regular positions in the corresponding sublattice (e.g., the rare earth oxides and chalcogenides) are often less informative because of the smearing of levels by resonance interactions between closely spaced ions. Ions in Na5RESi4O12 are closed in individual octahedrons that have no common ligands. Therefore, the ion interaction is suppressed as if the impurity concentration were very low. However, the ion density in the material is by no means low, because the ions obey the stoichiometry and constitute the sublattice. As a result, the luminescence signal from these ions is well intense, which allows carrying out the experiments. The ions from the 1st coordination sphere of RE-ion in Na5RESi4O12 form a highly symmetrical octahedron, which results in a relatively weak splitting of the sublevels. Against this background, the influence of ions from the 2nd coordination sphere appears being significant. The 2nd coordination sphere is constituted by mobile Na+ cations arranged asymmetrically, at a distance only by a factor of 1.5 further than the oxygen ligands of the octahedron. Their asymmetrical field must lead to a significant splitting, hence, affect the recorded spectra markedly. Actually, Na5RESi4O12 is a fair object for optical investigation of the mobile sublattice state. EXPERIMENTAL AND INSTRUMENTS The method of site selected spectroscopy is destined to enhance signal from homogeneous subgroups of a

disordered set. It is suggested that broad spectral bands in the spectra of disordered objects result from the summing up of narrow (uniformly broadened) bands of individual oscillators (atoms, molecules, or ions) or their homogeneous groups. And the absorption bands that are used in the excitation of the luminescence must have the same structure. Even when the light used for the excitation of the luminescence has small spectrum width, it still can excite in resonance mode one of types of the oscillators that in aggregate compose the broad band. The luminescence spectrum observed must also be formed only by the oscillators of this very type, that is, instead of broad bands one must observe narrow ones. The conception of the method is illustrated by Fig. 3. In terms of computer engineering one may say that the site selected spectroscopy method (also called fluorescent line narrowing [21]) realizes addressed inquiry of the studied objects by using physical methods. As applied to this work, the objects under study are rare-earth ions in Na5RESi4O12 crystals. Owing to the probabilistic character of the filling of positions with mobile cations, the environment configuration around different RE-ions and their electrical fields must differ. In the absence of selective excitation, the spectra must contain broad bands resulted from additive summation of several spectra that differ from each other. By applying the method of site selected spectroscopy, one can isolate narrow bands in the spectrum, which correspond to the RE-ions being in certain particular configuration of their environment. When the mobile cation sublattice contains preferred configurations, the luminescence summary bands must be composed of small number of narrow lines or, at least, some types of the “narrow spectra” (the components of broad bands) will be much more intense than other. Thus, the rare-earth ions whose luminescence was registered in the experiments appeared being built-in probes sensitive to the state of the mobile cation sublattice. In compliance with the specificity of the method, the setup comprised a few different units that provided narrow-band excitation of luminescence, the luminescence registration, and the control of temperature of objects under study. In Fig. 4 we present the block-diagram of our setup. In the low-temperature measurements, a helium cryostat was used. Samples were mounted on a copper cooler and blown round by helium vapor. In the hightemperature measurements, samples were placed into a furnace. The luminescence excitation was performed by using a laser unit based on excimer laser. The transverse-discharge excimer laser was based on the Xe–Cl mixture; it generated pulses of UV-radiation with duration of 40 ns and full energy of 0.3 J. The laser radiation wavelength is 308 nm (the photon energy 4 eV). The laser radiation was used for a two-stage dye laser pumping. The pulse of the dye laser matched in its


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(a) 2

3

(b) (c)

1

Fig. 3. Illustration of the idea of the selective excitation method: (a) broad bands (black) and two narrower bands (gray and gray dashed), which constitute the spectrum; (b) excitation to one narrow component stimulates narrow-band luminescence of only the level set which exists in the excited ion type; (c) scheme of levels corresponding to the experiment: (1) ground level, (2) excited level, (3) luminescent level. Wavy line from a sublevel of level 2 to a sublevel of level 3 shows radiationless transition.

Cryostat or Furnace Eximer laser, 308 nm, 20 ns, 0.3 J

Tunable dye laser

Frequency doubler Samble

Synchronizer Shutter 6 µs

1 µs Computer

ADC

PEM

Spectrograph

Fig. 4. The setup block-diagram. See text for the allocation of the blocks.

duration to the pumping radiation pulse; its wavelength depended on the dye used. The spectral width of the dye laser radiation was less than 0.05 Å. In case of need the dye laser radiation frequency can be doubled by using a nonlinear crystal. Such a possibility was used in the experiments with selective excitation of Gd3+ ions, whose first excited level is located ~4 eV from the ground state. The luminescence signal was focused at the entrance slit of a MDR-3 or DFS-12 grating spectrograph equipped with a FEU-79 photomultiplier as a photodetector. The pulse signal from photomultiplier was amplified by a preamplifier and extracted by a synRUSSIAN JOURNAL OF ELECTROCHEMISTRY

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chronous detector; then it was registered in digital form. The using of variable delay in the synchronous detector control channel allowed recording timeresolved spectra, that is, registering separately spectra of the light emitted by the crystal at different instants after the exciting laser pulse had stroked the crystal. This possibility was used, in particular, in the studying of evolution of the resonance luminescence line. The resonance luminescence is the crystal luminescent response at the same wavelength that was used for the excitation of luminescence. The principal difficulty in these studies is the suppressing of the diffused exciting light that is spectrally indistinguishable from the lumiNo. 5

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nescence light (and exceeds the latter’s intensity by many orders of magnitude). In these studies we used highspeed mechanical shutter synchronized with the exciting laser pulse. The shutter closed the spectrograph entrance slit during the receiving of light from the exciting laser; it opened the slit for the registering the emission from RE-ions. The shutter front duration was 6 µs; this is sufficient for effective separation of the excitation and luminescence diffuse light because the rare-earth ion afterglow time is of the order of millisecond. Experiments at Low Temperatures. Studying of the Mobile Sublattice Structure In the low-temperature (up to 4.2 K) studies we assumed that at these temperatures the state of the mobile sublattice in the crystal is as if snapshot of its highly conducting state. This assumption is corroborated by the absence of phase transitions in the material. (The possibility of phase transition near 150°ë [22] will be discussed below, in connection with discussion of experimental results.) The absence of phase transitions followed from the data of work [19] and own preliminary experiments, in particular, (1) measurements of magnetic susceptibility of Er3+- and Ho3+-containing crystals; (2) experiments on circular dichroism of Ho3+containing crystals; and (3) luminescence experiments with Tb3+-containing crystals [23]. In all the experiments, no sharp change of any parameters with the changing of temperature was observed. Luminescence Experiments with Gd3+ Ions The very presence of the hidden structure of luminescence band was first found in the experiments on the luminescence of crystals containing Gd3+ ions, in the transitions between sublevels of the first excited state 8S and the ground state 6P7/2 7/2 [24]. (The ground state of the gadolinium ion 8S7/2 has zero orbital moment and does not split in crystal fields of any symmetry.) It was shown that at low temperatures the relatively broad Gd3+ ion luminescence band mainly is a sum of three different spectra with narrower lines. These experiments had a methodological specificity. At helium temperatures, excitation to any sublevel of the level 6P7/2 is subject to radiationless thermalization at lower sublevels; thus, the luminescence spectrum practically contains only one line that relates to the luminescence from the lowest sublevel. Therefore, the positions of upper sublevels were determined from the spectra of excitation of the luminescence narrow lines for the lowest sublevel. It was shown that some definite set of effective excitation wavelengths corresponds to each narrow component of the luminescence line. Because the Stokes shift of intraionic transitions is negligibly small, the effective excitation regions directly corresponded to the positions of sublevels in the luminescent ion. In other words, the positions of the interrelated

sublevel sets were determined from the differing excitation spectra of some components of the luminescence spectrum. Tentative conclusions were made on the structure of stable cationic configurations formed in the vicinity of luminescent Gd3+ ions (the probes). We analyzed [25] the possibility of manifestation of the so-called mobile cation continual distribution in the spectra. The concept of continual distribution was suggested [26] as an attempt of the generalizing of high defect (irregularity) concentration in the mobile sublattice and a wide variety of their types. The experiments were conducted at intermediate temperatures (100–120 K). At these temperatures the fine structure stopped being observed because the mobile ions stay in the positions not very long, so the configurations in the vicinity of the Gd3+ ions alternate rather often. It was shown that with the increasing of temperature the unstructured background in the spectra increased rather significantly. Because the spectrum under these conditions contained four lines (the maximal, without applying of magnetic field, number of sublevels of the level 6P7/2), the emergence of the significant background cannot be explained by any unresolved additional lines. It was suggested that the emergence of the strong background (another term: wings of the spectrum) is caused by appearance of great number of various configurations that differ drastically from the typical (the most probable) ones. It was shown that selective excitation in the region of long-wave wing results in the resonant luminescence. This fact was interpreted as a corroboration of the suggestion that with the increasing of temperature there emerges a great number of transient configurations, in addition to their ground set, in particular, with mobile cations in the interstices, as it follows from the concept of continual distribution. We found [27] that with the increasing of temperature the lines of ions Gd3+ change their positions. This is an important argument in favor of the interconnection between the line fine structure and the effect of electrical fields of cations in the mobile sublattice on the luminescent ion. Otherwise, were the line sets due to the presence of defects of different nature, e.g., growth faults, the increase in temperature would not change the ion arrangement in the environment of the luminescent ions (probes), the crystal fields at the ions would not change, as well as the line position. On the whole, the experiments with Gd3+ ions in the Na5RESi4O12 crystal demonstrated the principal types of effects caused by the influence of the mobile sublattice structure and mobility on the luminescence spectra. The results of the studies are summarized in work [28]. However, later experiments with ions Eu3+ in the Na5RESi4O12 single crystals gave more detailed and clear information on the mobile sublattice structure and processes therein, due to more informative spectra and well developed methods of the Eu3+ ion spectra calculations.


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T = 4.2 K 5D → 7F 0 1

1 – 578.98 ± 0.015 nm 2 – 579.50 ± 0.015 nm 3 – 580.02 ± 0.015 nm Intensity, arb. units

Luminescence Experiments with Ions Eu3+ Trivalent europium ions give several well distinguishable groups of lines in visible region, in particular, 7F a single line (singlet) corresponding to the 5D0 0 5 7 transition and groups of lines due to the D0 F1 and 5D 7F transitions, whose number depends on the 0 2 symmetry of crystal field at a Eu3+ ion. Importantly, each ion has all sublevel sets corresponding to the line sets. Obviously, an ion cannot emit light from several sublevels at the same time. When several lines are registered in a spectrum, this means that in ionic ensemble there exists some probability of excitation of different luminescent sublevels. However, in such a case the energy positions of the sublevels, determined from the spectra, relate to each ion (which is in the given configuration and emits the given set of lines). Hence, when some type of ions demonstrates many lines, their luminescence spectra allow obtaining more detailed information on the energy structure of their levels. Like the ions Gd3+, ions Eu3+ being in different environment configurations must demonstrate different positions of lines and different energy level positions. Spectra observed in the absence of selective excitation are rather broad (nonuniformly broadened); they must be a sum of several types of dissimilar spectra composed of narrower (uniformly broadened) lines. With the selective (narrow-band) excitation, we succeeded in the observing of dissimilar spectra for ions Eu3+ being situated in different environment configurations [29]. In Fig. 5 we give examples of spectra registered for different wavelengths of the luminescence-inducing radiation. Figure 6 presents plots of positions for lines 7F , as of luminescence due to the transition 5D0 1 functions of the exciting radiation wavelength. It is clearly seen that the plots have kinks and ruptures; we see regions where the positions of the luminescence lines are almost insensitive to changes of wavelength; however, there exist transition regions of relatively sharp passing from one set of lines to another. It is this fact that actually demonstrates the existence of several predominant types of configurations in the vicinity of the luminescent ions, that is, the probes. The uneven character of similar dependences is even more clearly demonstrated in the luminescence corre7F (Fig. 7). Our sponding to the transition 5D0 2 attention is focused on the fact that the ruptures in the dependences of the luminescence line position on the exciting radiation wavelength fall into dissimilar segments of the excitation spectrum. Such a character of the dependence is quite unexpected; this corroborates the above-formulated statement on the necessity of the analyzing of as large number of lines as possible. The experiments with ions Eu3+ corroborated the conclusions drawn earlier from the studies of ions Gd3+, in particular, that the mobile ion sublattice contains small number of stable types of cationic configurations. Unlike the ions Gd3+, in the ions Eu3+ we can distin-

503

3

2

1 588

592 596 Wavelength, nm

Fig. 5. Examples of Eu3+ ion spectra in Na5RESi4O12 crystal at low temperatures and different luminescence-excitation-light wavelengths (shown in the Figure).

guish four stable types of cationic configurations (we recall that this conclusion is based on the number of the segments between the ruptures of the measured dependences). We believe that the measurements with the Eu3+ ion give more reliable results because they are based on a greater massive of data. The fact that only three line sets were registered for the Gd3+ ion may be due to the masking of one more line set by a more intense one. Analysis and Interpreting of Spectral Experimental Data We analyzed possible types of cationic configurations in the vicinity of RE3+ ion by example of the Gd3+ ion in work [28] for the first time. This analysis was based on the considerations common to physics; on their basis we formulated simple criteria that were fully applicable to the analyzing of the Eu3+ spectra. No. 5

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Na5RESi4O12

Luminescence wavelength, nm

T = 4.2 K 592

590 7F 2

588

579

Excitation wavelength, nm

580

7

F1

7

F0

Fig. 6. Dependence of the wavelengths positions for narrow lines of Eu3+ ion luminescence in Na5RESi4O12 crystal during the tran7F on the excitation-light wavelength. Open squares: positions of sublevels calculated using equivalent operator sition 5D0 1 method. At the right-hand side: scheme of transitions of the levels that are active in the experiment (not to scale).

Firstly, we assumed that the most probable configuration types disturb electroneutrality of the crystal local region but weakly. It is known from the X-ray diffraction analysis that the full number of positions in the RE3+ 1st coordination sphere equals 10 when the summary occupancy of the 1st coordination sphere equals approximately 3.6. We concluded that the principal configuration types must have occupied 3 or 4 cation positions, so that the difference of the full charge of the region from the average charge has been as small as possible. Secondly, we assumed (also for the most probable configurations) that some number of cations are arranged in such a manner that the Coulomb repelling energy for the cations constituting the configuration would be minimal. During further refining of the analysis of energy of the possible configurations we simplified the criterion down to the simple rule: in stable configurations the cations do not occupy the neighboring positions. Basically, both criteria ensure the minimizing of the configuration energy: the former to the scale of elementary cell, the latter, to the scale of the RE-ion coordination sphere. Our using of these two (qualitative) criteria allowed us to avoid finding the rigorous solution of the

problem on the minimizing of the configuration energy in crystals. We leaved the energy of Coulomb interaction between the peripheral ions of neighboring configurations beyond the frames of the consideration. This question actually involves the discussion on the correlations of very large number of particles (~10 and more), hence, it is beyond the scope of this paper. Basing on the above-formulated criteria, we selected possible cation configurations in the vicinity of the luminescent ions (the probes). For convenience sake, additional simplifying condition was introduced: we assumed that the hopping of cations between the positions 6 and 6’ are well frequent, and the 4f-shell accepts sodium cation in the middle of the dense group of positions as “smeared” between the two positions. Only five independent configurations meet these conditions. The relative probabilities (wi) of realization of each of i configurations were determined from: (1) the condition of normalization: the sum of all probabilities equals unity:

∑w

i

= 1,


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505

5D 0

622 Na5RESi4O12 T = 4.2 K Luminescence wavelength, nm

618

614 7F 2 7F 1

610

579

Excitation wavelength, nm

Fig. 7. Same as in Fig. 6, yet for the case of the transition 5D0

(2) the condition that the average occupancy of a position of some type must coincide with that (nj) of a position (j), which is known from X-ray diffraction analysis:

∑w N i

ji /M ji

= n j.

(2)

In formula (2) Nji is the number of occupied positions of i-type in the configuration of type (j), Mji is the full number of positions of i-type in the configuration of type j; the summation is carried out over all types (j) of possible configurations. The two conditions [equations (1) and (2)] cannot give exact solution for the 5 unknown variables wi; however, an approximate solution still can be chosen. The following set of configurations met the conditions (1), (2) accurate to 0.05: 0.64, 0.27, and 0.08 (see Fig. 8). For this solution, the summary probability of all realizations is 0.957; the mean charge of ions in the 2nd coordination sphere is 3.5 e (the calculated value obtained from the X-ray diffraction analysis data is 3.6 e). Taking into consideration the approximations we used, this coincidence may be thought of as fair. In Fig. 8 we show the arrangements of cations in the three most probable types of configurations. To additionally prove the accuracy of thus found configurations, we calculated, by using the equivalent operator method [30], the position of sublevels 4Fj in the RUSSIAN JOURNAL OF ELECTROCHEMISTRY

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580

7F . 2

Eu3+ ion for the two most probable configuration types. The calculations were based on the suggested cation configurations. In Table 1 we give the calculated values of the sublevel energy relative to the sublevel 7F0, as well as the sublevel positions determined from the experiment. We see from the data of Table 1 that the calculated values of the sublevel positions agree well with the experimental ones. This allows contending that the found types of the mobile cation configurations actually are realized in the corresponding sublattices. We recall that the matter concerns dynamic self-organized configurations arranged at nearly energy-equivalent positions. Experiments at Elevated Temperatures. Phase Transitions in Mobile Sublattices The luminescence of ions Tb3+, Gd3+, and Eu3+ was studied at higher temperatures (up to 700°C) too. These experiments have dual object. Firstly, the results of these experiments may be an important additional corroboration of the interpreting of the low-temperature experiments. As we mentioned above, when the line sets observed at low temperatures really corresponded to different cation configurations, the spectrum appearance should change strongly with the increasing of temperature because this inevitably No. 5

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(b)

R

(c)

R

R G

G

w‡ = 0.637

wb = 0.267

G

wc = 0.053

Fig. 8. Recovered types of self-assembling cation constants in the vicinity of rare-earth–oxygen octahedrons. Dashed line: the position Na4 from third coordination sphere (its occupancy was not allowed for). The positions Na6 and Na6’ situated close to each other near the position group center were treated as a single position; in the figure these positions are half-blackened. The characters R and G point to what conductance equivalent channel the position belongs: (a) the configuration RNa5Na5’/GNa6Na4; (b) the configuration RNa6/GNa6Na4; and (c) RNa5Na5’/GNa6.

leads to changes in the cationic subsystem. And vice versa, invariability of spectra with the increasing of temperature may evidence that a rare-earth ion is located in the vicinity of a defect formed during the crystal growth. Additionally, despite intuitively we believe that stable configurations keep unchanged in some temperature interval, it is clearly seen that the configuration lifetime must decrease with the increasing of tempera-

ture, while the lifetime of cations located in interstices must increase. The phenomena that take place here are important for a better insight into the superionic conduction nature. We now discuss the data concerning the Eu3+ ion luminescence over wide temperature range [31, 32]. In Fig. 9 we show the Eu3+ ion luminescence spectra at several temperatures. We see from the spectra that the increasing of temperature resulted in the relative

Table 1. Positions of sublevels of Eu3+ ion in Na5RESi4O12 crystal (the comparison of calculated and experimentally measured values) Level

4F 1

4F 2

4F 1

4F 2

Position of sublevel, cm–1 experiment 244 363 470 823 869 926 1032 1225 261 344 4449 823 858 925 1035 1176

calculated

The sublevel set center of gravity, cm–1 R and B – indexes of channels experiment calculated

First intense multiplet 207 359 361 497 892 977 908 990 1050 1086 Second intense multiplet 224 352 352 451 889 964 903 972 1002 1053

356

Configuration R-Na5Na5'/BNa4Na6

985

342

Configuration R-Na6Na6'/BNa4Na6

964


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507

+

Eu3

Na5RESi4O12 5D → 7F 0 1

Intensity, arb. units


594

470 K

290 K

5D → 7F 0 1

590

586 200

300

500

T, K

4.2 K 584

588 592 Wavelength, nm

Fig. 10. The line position in spectra depending on temperature. Dashed is the line half-width (FWHM) from corresponding side (where the determining of the half-width from the measured data appears correct). The shaded region is the part of the plot where objective line-position separation is hardly possible. Dots show the position of the multiplet barycenter.

596

Fig. 9. Examples of ion spectra at different temperatures. Note that the intra-ion transitions in the 4f-shell dependence on the crystal temperature strongly (the line positions and their number change).

approaching of the triplet components (that means the decrease in the sublevel splitting) and further decrease in the number of lines. In Figs. 10 and 11 we summarized the data on the changes in the position and number of lines in the spectra. We see from the data that in the regions of the tran7F and 5D 7F the spectra change sitions 5D0 1 0 2 5 7F transforms to with temperature. The triplet D0 1 a doublet at a temperature of ~400 K. The line widths change rather weakly, which allows monitoring the changes in the line positions. The position of the triplet barycenter changes but insignificantly; this suggests that the spectrum entire change is caused by changes in the sublevel splitting, in other words, it results from the changes of electrical field at the luminescent ions. 7F transition is a quintet. The spectrum of 5D0 2 With the increasing of temperature above 500 K the spectrum transforms down to quartet; above 600 K, to triplet. In Table 2 we show the number of observed lines in the spectra; in Table 3, the number of sublevels to which the particular level is split in crystal fields of different symmetry. RUSSIAN JOURNAL OF ELECTROCHEMISTRY

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The comparison of the data of Tables 2 and 3 allowed realizing that the changes in the spectra caused by the increasing of temperature are a unique process occurring in the mobile cation sublattice as the temperature increased. At lower temperatures (T < 120 K) the ion jumps are suppressed; a few of basic types of configurations are trapped into the sublattice. Over the T = 120–420 K temperature interval, the ion mobility increased; in the vicinity of each RE-ion the configuraTable 2. The number of experimentally observed sublevels in Eu3+ and Gd3+ ions at different temperatures Observed number of lines in the temperature region Level T < 120 K

T= T= T > 600 K 120–420 K 420–600 K

7F 1

(Eu)

4×3

3

2

2

7F 2

(Eu)

4×5

5

4

3

3(?) × 4

4

4

4

6P 7/2

(Gd)

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KOMPAN

line number. It follows from our experiments that with the increasing of temperature the sublevel number decreases for all the levels of all ions whose spectra were observed. This result is but natural when we base on the adopted model assuming that the crystal field at an ion is caused by the presence of closely set cations. With the increasing of temperature the cations often leave their sharply defined positions and take arbitrary positions in crystal lattice, in particular, in interstitial sites, and at the same time they rapidly move from one position to another. As a result, on the average the cation charge distribution in the crystal lattice becomes more homogeneous and uniform, in particular, more symmetrical. This manifests itself in the spectra as a decreasing of both the splitting and the component number. (Probably, the manifestation of these processes was observed in work [22], where a “weak” phase transition in Na5RESi4O12 at 150°C was suggested.)


620 5D → 7F 0 2

615

610

200

400

600 T, K

Fig. 11. Same as in Fig. 10, yet during the transition 5D 7F . 0 2

tions have time to replace each other during the lifetime of the excited state [(1–3) × 10–3 s]. On the average, an ion–probe does not exist in any particular configuration. The site selective spectroscopy does not allow obtaining any result; the fine structure is indistinguishable. However, on the average, the ions are located in the same positions; the average occupancy is hardly changed. The average crystal field at the RE-ions does not change, as well as the number of lines in spectrum (Table 2, column 3). With further increase in temperature, the number of lines in the spectra changes. It follows from Table 3 that the crystal field symmetry type for each sublevel unambiguously determines the sublevel number, hence, the Table 3. Multiplicity of level splitting in Eu3+ and Gd3+ ions in crystal fields of different symmetry Type of symmetry 5D0(Eu)

7F 1

(Eu)

7F 2

(Eu) 6P7/2 (Gd)

Cubic

1

1

2

3

Hexagonal

1

2

3

4

Tetragonal

1

2

4

4

Lower

1

3

5

4

More detail analysis of the data given in Figs. 10, 11 shows some specific features that still deserve thorough theoretical interpreting. The temperature at which the changing of the number of lines in the spectra occurs differs for different line sets (multiplets): in particular, 7F . The temperait is 400 K for the transition 5D0 1 ture may differ even for different lines of a single multiplet: 500 and 600 K for the lines of the multiplet 5D 7F . 0 2 As we have pointed above, the change of the number of lines in a spectrum corresponds to a change in the symmetry type; hence, it must correspond to a second order structural transition (possibly, phase transition) [33]. However, all line sets relate to the same ion in the unique stoichiometric position. Why, then, the phase transition occurs at different temperatures for different sublevels of an ion? We have formulated [31] a concept of a “dynamic structural transition”. According to this concept, the symmetry is changed locally as a result of rapid changes in the luminescent ion environment. It is important in the case under the consideration that the low-symmetry component of crystal field at an RE-ion has been averaged during the ion emission process (more strictly, during the excited state life time). Different sublevels have different excited-state life times; the field averaging in these different life times requires different intensity of the ion motion in the coordination sphere. This explains why the change in the number of components for different levels can occur at different temperatures. In principle, the symmetry increase resulting from the dynamic averaging is an analog of well-known phenomenon, in particular, the line narrowing and the disappearing of any splitting when ion is transferred to liquid. The high symmetry of an ion environment in solution results from rapid averaging of many particular


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environment configurations, any of which can be nonsymmetrical. The suggested concept of the dynamic structural transition explains qualitatively the reason of the uncommon temperature dependence of the spectra. However, we believe that this novel phenomenon deserves a deeper theoretical insight therein.


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300 K 0.5 ms

4.5 ms

Intensity, arb. units

The Specific Features of the Luminescence during 7F . The Direct Experiments the Transition 5D0 0 with the Configuration Change Kinetics We studied [34] a rather uncommon in the spectroscopy case of the Eu3+ ion luminescence during the tran7F in Na RESi O crystal. The specific sition 5D0 0 5 4 12 7F is that both levels feature of the transition 5D0 0 (the emitting and the ground ones) are not split in any crystal field. On this very reason, only single spectral line corresponds to this transition [35]. In this case, with no splitting, rather fine effects can be observed, e.g., the dependence of the line position on the ligand average density at the emitting ion. In the spectroscopy this effect would be called “the nepheloxetic shift” [36]; in other disciplines, “the chemical shift”. It was 7F in shown [34] that the position of the line 5D0 0 Na5RESi4O12 crystals varies with temperature. With due allowance for the above given reasoning, this fact can be solely interpreted as a result of the changing of the mobile Na+-cations average electron density that was indirectly transferred by the ligands to the emitting RE-ion. The absence of extraneous lines in the vicinity of the 7F allowed carrying out an experiment singlet 5D0 0 with the detecting in real time of the configuration change in the vicinity of the RE-ion. The idea of the experiment is straightforward. The selective spectroscopy allows exciting luminescence of ion in a specified state (in this experiment the term “the specified state” means a specified value of the chemical shift for the levels of a chosen type). Because the luminescent levels are not split, the ions emit only single lines of the resonant luminescence in response to the excitation, rather than sets of lines. At low temperatures, no other processes occur. At higher temperatures, the ions also show the resonant luminescence in response to the excitation; however, in addition to the resonant line in the spectra, a line located some distance away from the resonance occurs. With the increasing of the recording delay, with respect to the exciting pulse, the resonant line disappears; only the line shifted away from the resonance remains [34]. This experiment can be interpreted in a straightforward way. At lower temperatures the ions’ mobility is suppressed, and the RE-ions emit only from the configuration in which they were excited. Such is the general low of the selective spectroscopy; however, in the case 7F we observed a single resoof the transition 5D0 0 nance line, rather than a set of lines. With the increasing

509

0.5 ms

4.5 ms

77 K

579

580 Wavelength, nm

Fig. 12. Luminescence of Eu3+ ion during the transition 5D 7F at 77 and 300 K with selective excitation and 0 0 time-resolution during the registration. See text for additional comments.

of temperature the ion motion process becomes more intense. Part of the ions appears in another (more stable) configuration during their luminescence; as a result, one more line occurs in their spectra. With the increasing of the recording delay, progressively fewer No. 5

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ions remained in their initial configuration; finally, the initial line disappears in the spectra. Thus, in this experiment the configuration change in the vicinity of a luminescent ion is detected in the real time. CONCLUSIONS In this review we summarized the results of our optical studies on the effects caused by the structure and dynamics of mobile cation sublattice in ionic conductor Na5RESi4O12. The study involved modern methods of experimental spectroscopy, in particular, the selective excitation spectroscopy and time-resolved spectroscopy. The state of the mobile cation sublattice was determined from the analysis of spectra of the rareearth ions affected by the electrical fields of mobile cation. As a result, the existence of small amount of selfforming stable configurations in the cation sublattice was determined. The particular structural types of these configurations were suggested and corroborated; their relative probabilities determined. The spectra changes with the increasing of temperature were studied. It is shown that the changes in spectra with the increasing of temperature are caused by frequent changes of configurations in the vicinity of the light-emitting ions, which leads to a dynamic averaging of the crystal field lowsymmetry components. This manifests itself as a cascade of second-order structure transitions with the change of local symmetry in the RE-ion nodes. All these effects were observed by us in the cited works for the first time; however, the ideas on the mobile sublattice structure were not. The interpretation, as they say, was in the air. And yet, the realizing of experiments for the purpose of the corroborating of these ideas became only possible when the ideology of selective spectroscopy was applied to the superionic conductors and specific experimental techniques were purposefully used. It is necessary to recognize that the described results were mainly obtained due to a happy choice of the object of the study (Na5RESi4O12 single crystals). As stated above, this object well corresponds to the purpose and methodology of this study. The very possibility of the transferring of the approach over the wider range of objects will mainly depend on the objects’ characteristics. ACKNOWLEDGMENTS Author is grateful to G.B. Venus, the colleague and coauthor in numerous articles, for the long and fruitful collaboration; O.V. Dmitrova, for the donating of single crystals for the experiments. The interest to the topic was stimulated, to a considerable degree, by E.A. Ukshe and L.O. Atovmyan; the joint discussion of the approaches and results played the key role. Author is sincerely grateful to B.P. Zakharchenya for his permanent interest and encouraging.

REFERENCES 1. Cardona, M., Phys. Rev., 1963, vol. 129, p. 69. 2. Gurevich, Yu.Ya. and Kharkats, Yu.I., Superionye provodniki, (Superionic Conductors), Moscow: Nauka, 1992. 3. Beyeler, H.V., Bruesch, P., Pietronero, L., Schneider, W.R., Strassler, S., and Zeller, H.R., Physics of Superionic Conductors, Salamon, M.B., Ed., Berlin: Springer, 1979. 4. Akopyan, I. Kh., Elektrokhimiya, 1990, vol. 26, p. 1495. 5. Studenyak, I.P., Kovach, D.Sh., Zinnikov, B.I., and Borts, A.N., Fiz. Tverd. Tela, 1987, vol. 29, p. 3442. 6. Kompan, M.E. and Venus, G.B., Zh. Eksp. Teor. Fiz., 1990, vol. 98, no. 1(7), p. 290. 7. Boyce, J.B. and Huberman, B.A., Phys. Rev., 1978, vol. 51, p. 189. 8. Owens, R.B. and Argue, G.R., Science, 1967, vol. 157, p. 308. 9. Collongues, R., Gourier, D., Kahn, A., Boilot, J.P., Colomban, Ph., and Wicker, A., J. Phys. Chem. Solids, 1984, vol. 45, p. 981. 10. Phyllips, J.C. and Andreoni, W., Phys. Rev. A, 1981, vol. 23, p. 6456. 11. Vashishta, P. and Rahman, A., Phys. Rev. Lett., 1978, vol. 40, p. 1338. 12. Mohua Mokur and Sujata Gohck, J. Phys. France, 1989, vol. 59, p. 431. 13. Shukla, A.K. and Funke, K., Europhys. Lett., 1989, vol. 10, p. 471. 14. Isaakyan, A.R. and Sokolov, A.V., Elektrokhimiya, 1988, vol. 24, p. 152. 15. Linford, R.G. and Hackwood, S., Chem. Rev., 1981, vol. 31, p. 327. 16. Personov, R.I., Al’shits, E.A., and Bykovskaya, L.A., Pis’ma Zh. Eksp. i Teor. Fiz., 1972, vol. 15, p. 609. 17. Pobedimskaya, E.A., Pushcharovskii, D.Yu., and Karpov, O.G., in Strukturnye tipy redkozemel’nykh silikatov, germanatov i fosfatov (Structure Types of Rare-Earth Silicates, Germanates and Phosphates), Moscow: Mosk. Gos. Univ., 1984. 18. Merinov, B.V., Maksimov, B.A., Kharitonov, Yu.A., and Belov, N.V., Dokl. Akad. Nauk SSSR, 1978, vol. 240, p. 81. 19. Atovmyan, L.O., Filipenko, O.S., Ponomarev, V.I., Leonova, L.S., and Ukshe, E.A., Solid State Ionics, 1984, vol. 14, p. 137. 20. Ponomarev, V.I., Filipenko, O.S., Chekhlov, A.N., and Atovmyan, L.O., Khim. Fiz., 1983, vol. 21, p. 1603. 21. Yen, W.Z. and Seltzer, P.C., Laser Spectroscopy of Solids, New York: Springer, 1981. 22. Ponomarev, V.I., Pis’ma Zh. Tekh. Fiz., 1984, vol. 10, p. 345. 23. Kompan, M.E., Venus, G.B., Dimitrova, O.V., Litvin, B.N., and Popova, T.B., Fiz. Tverd. Tela, 1986, vol. 28, p. 1944. 24. Kompan, M.E. and Venus, G.B., Fiz. Tverd. Tela, 1990, vol. 32, p. 3214. 25. Kompan, M.E., Venus, G.B., and Mikhel’son, V.T., Fiz. Tverd. Tela, 1990, vol. 32, p. 889. 26. Geizel, T., in Physics of Superionic Conductors, Salamon, M.B., Ed., Berlin: Springer, 1979.


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LUMINESCENT-TECHNIQUE STUDY ON THE STRUCTURE OF CATION SUBSYSTEM 27. Kompan, M.E., Venus, G.B., and Dimitrova, O.V., Pis’ma Zh. Eksp. Teor. Fiz., 1990, vol. 52, p. 1185. 28. Kompan, M.E. and Venus, G.B., Elektrokhimiya, 1990, vol. 26, p. 1484. 29. Kompan, M.E. and Venus, G.B., Zh. Eksp. Teor. Fiz., 1992, vol. 101, no. 4, p. 1424. 30. Gaiduk, M.I., Zolin, V.F., and Gaigerova, L.S., Spektry lyuminestsentsii ionov evropiya (Luminescence Spectra of Europium Ions), Moscow: Nauka, 1974. 31. Kompan, M.E., Venus, G.B., and Dimitrova, O.V., Pis’ma Zh. Eksp. Teor. Fiz., 1992, vol. 55, no. 1, p. 48.


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32. Kompan, M.E. and Venus, G.B., Zh. Eksp. Teor. Fiz., 1993, vol. 104, no. 5, p. 3693. 33. Landau, L.D. and Lifshitz, E.M., Statisticheskaya fizika (Statistical Physics), Moscow: Nauka, 1964. 34. Kompan, M.E. and Venus, G.B., Fiz. Tverd. Tela, 1997, vol. 39, no. 11, p. 1997. 35. Dieke, R., Spectra and Energy Levels of Rare Earth Ions in Crystals, New York: Interscience, 1968. 36. Reisfield, R. and Jorgensen, C.K., Lasers and Exited States of Rare Erths, Berlyn: Springer, 1977.

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