ISSN 10637834, Physics of the Solid State, 2009, Vol. 51, No. 9, pp. 1886–1893. © Pleiades Publishing, Ltd., 2009. Original Russian Text © Yu.K. Voron’ko, A.A. Sobol’, V.E. Shukshin, A.I. Zagumennyі, Yu.D. Zavartsev, S.A. Kutovoі, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 9, pp. 1776–1782.
LATTICE DYNAMICS AND PHASE TRANSITIONS
Raman Spectroscopic Study of Structural Disordering in YVO4, GdVO4, and CaWO4 Crystals Yu. K. Voron’ko, A. A. Sobol’, V. E. Shukshin**, A. I. Zagumennyі, Yu. D. Zavartsev, and S. A. Kutovoі A. M. Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia * email:
[email protected] ** email:
[email protected] Received December 16, 2008
Abstract—The Raman spectra of singlecrystal YVO4, GdVO4, and ZrSiO4 with a zircon structure, as well as CaWO4 and BaWO4 with a scheelite structure, are studied in detail over a wide temperature range 14–800 K. An inhomogeneous splitting of the A1g(ν2) vibrational lines in the Raman spectra of YVO4 and GdVO4 and the Ag(ν1) vibrational lines in the spectrum of CaWO4 is detected. It is shown that the profiles of these lines can be decomposed into two components, whose integrated intensities are redistributed with temperature and also depend on the matrix kind in which they are detected. The phenomenon observed is associated with the thermally activated processes of disorientation of the tetrahedral anions in the zircon and scheelite structures. PACS numbers: 78.30.j, 61.72.Dd DOI: 10.1134/S1063783409090200
1. INTRODUCTION At present, single crystals of yttrium and gadolin ium orthovanadates with a zircon structure doped by rareearth ions have been widely used as active media of diodepumped lasers [1–3]. Moreover, single crys tals of vanadates and also tungstates MWO4 (M = Ca, Sr, Ba) with a scheelite structure are promising nonlin ear media for the transformation of the radiation wavelength in SRS lasers [4–6]. The structural perfec tion of the aforementioned materials is a factor which can determine the capability of their application as materials of quantum electronics. Since single crystals of orthovanadates of rareearth metals and tungstates of alkalineearth metals are mainly synthesized at high
Scheelitetype structure Davydov splitting Local crystal field Factorgroup Site symmetry
temperatures from melt, they can contain defects due to thermally activated processes. The presence of such defects can lead to inhomogeneous splitting and broadening of spectral lines of rareearth activators and also cause a change in the Raman line shape of the materials under study, which can influence their laser characteristics. A specific feature of vanadates with a zircon structure and tungstates with a scheelite struc ture is the existence of the [VO4] and [WO4] groups as isolated tetrahedra in their crystal lattices [7, 8]. A strong covalent bond inside such groups permits one to consider them as separate structural elements, whose internal vibration spectra carry information on the structure and perfection of the crystal lattices.
Free anion
Zircontype structure Local crystal field Davydov splitting Site symmetry Factorgroup
C4h
S4
Td
D2d
D4h
(s) A(R) g + Bu
A
ν1(A1)
A1
(R) + B (s) A1g 2u
Eg(R) + E(IR) u
E
Bg(R) + A(IR) u
B
Ag(R) + B(s) u
A
Bg(R) + A(IR) u
B
ν3(F2), ν4(F2)
ν2(E)
E
Eg(R) + EuIR
B2
(R) + A IR B1g 2u
A1 B2
A1g(R) + B2u(s) (s) B2g(R) + A1u
Fig. 1. Schematic diagram of splitting of the vibrations of a free tetrahedral anion in the zircon and scheelite structures under the action of the local crystal field and Davydov splitting.
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ν2(E) Ag + Bg ν4(F2) Bg E g
ν1(A1)
B1g(II)
ν3(F2) Bg E
Eg(V)
g
Eg(IV) *
Intensity
CaWO4
ν1(A1)
ν3(F2) ν2(E) B2g
A1g
BaWO4
Eg(III)
ν4(F2)
(b) B1g(III)
Eg
ν4(F2) ν3(F2ν) (A ) 1 1
Intensity
YVO4
B1g(II)
Eg(V) A1g(II)
* Eg(IV)
*
Eg(II)
ZrSiO4 400
A1g(II)
B1g
GdVO4
0
Eg(II)
800
∆ν, cm–1
(c)
Fig. 2. Nonpolarized Raman spectra of YVO4, GdVO4, ZrSiO4, CaWO4, and BaWO4 crystals at 300 K. The posi tions of the lines obtained as a result of splitting of the vibrations of the tetrahedral anion in the crystal field are shown.
B1g(III) B1g(IV) B2g Eg(II) Eg(III)
Thus, the Raman spectra of the internal vibrations of the aforementioned tetrahedral groups are indications of structural transformations, in particular, the effects of disordering in the lattices of the materials under study. In this work, we investigated in detail the Raman spectra of YVO4, GdVO4, CaWO4, and BaWO4 single crystals synthesized from melts and also natural ZrSiO4 over a wide temperature range 14–800 K in order to detect and study the effects of structural dis ordering in these materials.
0
No. 9
200 300 ∆ν, cm–1
A1g(II)
400
(c) B1g symmetry lines (Z(XX) Z scattering geometry) in the Raman spectrum of GdVO4 crystal at 300 K. The vibrations are designated according to [16]. The asterisk shows lines forbidden in the given scattering geometry.
For our studies, the nominally pure YVO4 and GdVO4 single crystals 24 mm in diameter and 90 mm in length were grown by the Czochralski method from an iridium crucible (60 mm in diameter and 60 mm in height). Some peculiarities of growing the vanadate crystals are discussed in [9, 10]. The optical loss in the crystals measured at a wavelength of 1064 nm were less than 0.0012–0.0015 cm–1, which indicated a high optical quality of the 10 × 10 × 15mm samples pre pared for the studies. Vol. 51
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B1g(II)
Fig. 3. Eg symmetry lines (X(ZY) X scattering geometry) in the Raman spectra of (a) YVO4 and (b) GdVO4 crystals and
2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE
PHYSICS OF THE SOLID STATE
*
Eg(III)
1200
To compare the peculiarities of the Raman spectra of YVO4 and GdVO4 with the Raman spectrum of another compound with a zircon structure, the natural ZrSiO4 crystal was studied. The CaWO4 and BaWO4 single crystals were also grown by the Czochralski method and were previously studied in [4, 11]. The studies were carried out over a wide tempera ture range 14–1000 K. In the temperature range 14–
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Vibration frequencies in the Raman spectra of YVO4 and GdVO4 single crystals at 300 K Internal vibrations of the [VO4]3– anion
Crystal YVO4
GdVO4
External vibrations
A1g(I)
Eg(I)
B1g(I)
B1g(II)
Eg(IV)
A1g(II)
B2g
B1g(IV)
Eg(II)
Eg(III) B1g(III)
Eg(V)
891
839
817
490
–
260
265
260
164
156
–
891
840
817
490
444
379 378 395
260
267
260
163
157
137
884
825
809
483
–
261
–
246
156
123
–
884
825
809
483
438
380 376 389
261
252
246
156
123
110
Note: The designations of the vibration types and frequencies in the upper row are taken from [16]; the lower row presents the data of our experiments.
300 K, the Raman spectra were excited by a continu ous radiation of an ILA120 argon laser at a wave length of 488 nm and an average radiation power of 1 W. In this temperature range, a “Leybold Hereus” cryostat with a controlled variation of temperature was used. In the temperature range 300–1000 K, the sam ples were heated in a vertical tube resistance furnace with a heater from a Pt–30%Rh wire. In this case, the Raman spectra were excited by a copper vapor laser operating in a pulseperiodic regime (the pulse repeti tion frequency was 15 kHz, and the pulse duration was 10 ns). The radiation wavelength was 510.5 nm, and the average power was 2 W. The Raman spectra were measured using a SpexRamalog 1403 monochroma tor in the gated photon counting regime [12]. 3. EXPERIMENTAL RESULTS AND DISCUSSION The effects of disordering in crystals in which some fragments with a strong covalent bond can rotate with respect to their regular position in a crystal lattice were previously observed in nitrate crystals [13, 14]. The effects manifest themselves in the Raman spectra as an additional multiplicity of the bands of nondegenerate vibrations and asymmetric distortion of their form. However, the correct interpretation of such peculiari ties of lines in the Raman spectra is possible only for materials in which all the lines allowed in the first order Raman spectra are identified. Such a detailed identification for crystals of alkalineearth tungstates with a scheelite structure was previously performed in [5, 15]. At the same time, in rareearth vanadates with a zircon structure, the number of earlier observed bands in the firstorder Raman spectra is smaller than that predicted previously [16]. In this connection, a preliminary stage was the detection of “missing” bands in the Raman spectra of YVO4 and GdVO4 to exclude them as a possible cause of the appearance of the asymmetry of profiles of some lines because of the effects of disordering.
The scheelite and zircon structures containing tet rahedral anions with a strong covalent bond are similar in many respects. The structures of cation sublattices in both types of crystals are identical, and the transfor mation of the zircon lattice to scheelite lattice can occur as a rotation of the tetrahedral anions through 45° around their fourfold axis. This explains the exist ence of the zircon–scheelite phase transitions in YVO4 and ZrSiO4 crystals at elevated pressures [17, 18]. Fig ure 1 shows the character of correlation of the internal vibrations of free [VO4] and [WO4] groups with inclu sion of their local symmetries and the crystal factor groups for zircon and scheelite. Here, also there is a significant similarity. Both structure types exhibit an inversion center, which implies a separate observation of the symmetric vibrations in the Raman spectra and antisymmetric vibrations in the infrared (IR) absorp tion spectra; moreover, there are nonactive modes in the Raman and IR spectra, i.e., silent (s) modes. The internal vibrations of the free tetrahedral anion are characterized by four vibrational modes with the ν1(A1), ν3(F2), ν4(F2), and ν2(E) symmetries [15]. In the Raman spectra of both structures, the symmetric ν1(A1) vibration must be observed as a singlet and the ν2(E), ν3(F2), and ν4(F2) modes must be recorded as doublets. This is illustrated by the Raman spectra of YVO4, GdVO4, ZrSiO4, CaWO4, and BaWO4 in Fig. 2. When comparing the Raman spectra of the com pounds with the scheelite and zircon structures, we focus our attention on a significant splitting of the ν2(E) vibration of the free tetrahedral anion by the crystal field in the compounds with the zircon struc ture. The splitting is about 180 cm–1 in ZrSiO4 and about 120 cm–1 in yttrium and gadolinium vadanates. For comparison, in the scheelite structure (CaWO4 and BaWO4 crystals), the splitting of this vibration due to the crystal field is several cm–1 [5, 15]. The grouptheoretical analysis of the vibrations in 19
the zircon structure with space group D 4h and two for
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(b) 800 K
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(c) 800 K
800 K
600 K
600 K
300 K
300 K
300 K
77 K
77 K
77 K
14 K
17 K
14 K
A B
Intensity
600 K
340 380 ∆ν, cm–1
420
340 380 ∆ν, cm–1
420
400 440 ∆ν, cm–1
480
Fig. 4. Decomposition of the A1g(ν2) vibration line in the Raman spectra of (a) YVO4 and (b) GdVO4 crystals into components A and B and (c) the region of the Raman spectrum of the corresponding vibration in ZrSiO4 at various temperatures.
mula units in the unit cell gives the following vibration spectrum [16, 19]: Γ = 2A 1g + A 2g + 4B 1g + B 2g + 5E g + A 1u + 4A 2u + B 1u + 2B 2u + 5E u . In the Raman spectra, the active vibrational modes are Γ
RS
= 2A 1g + 4B 1g + B 2g + 5E g .
Among them, 2A1g + B2g + 2B1g + 2Eg are the inter nal vibrations of the tetrahedral anion. PHYSICS OF THE SOLID STATE
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In [16], in the Raman spectra of yttrium and gado linium orthovadanates, 10 and 9 lines, respectively, from 12 expected lines were detected. Our experi ments performed on perfect and welloriented YVO4 and GdVO4 single crystals in polarized light allowed us to identify the whole set of 12 lines in the Raman spec tra of these crystals. Figure 3 shows the regions in which missing lines were detected. The missing bands have a very low intensity and were observed at the slopes of the more intense lines. The results obtained are listed in the table along with the data from [16].
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(b) GdVO4
390 ∆ν, cm–1
GdVO4
3
380 370
2
IB/IA
YVO4 1
∆ν, cm–1
400
0 1 CaWO4
YVO4
390 380 370
0 0
200
400
600
800
0
T, K
200
400
600
800
T, K
Fig. 5. Temperature dependences of (a) the ratios of the integrated intensities of components B and A of the A1g(ν2) vibration lines in the Raman spectra of the YVO4 and GdVO4 crystals and the Ag(ν1) vibration line in the Raman spectrum of the CaWO4 crystal and (b) frequency shift of components B and A of the A1g(ν2) vibration lines in the Raman spectra of the YVO4 and GdVO4 crystals.
The study of the Raman spectra of ZrSiO4, YVO4, and GdVO4 with a high resolution over the tempera ture range 14–800 K allowed us to detect an interest ing behavior of the profile of the A1g(II)vibration line as a result of the splitting of the ν2(E) internal vibration of the free tetrahedral anion into the components A1g + B2g in the crystal field (Fig. 4). It is seen from Fig. 4 that the A1g(II) vibration in ZrSiO4 shows up over entire temperature range studied as a line that is well approximated by a profile of the Lorentzian shape. At the same time, this line has an asymmetric profile in the Raman spectra of YVO4 and GdVO4. The decomposition of this profile shows that it consists of two lines A and B with the Lorentzian shapes. The polarization studies in various scattering geometries show that components A and B correspond to the A1g symmetry and both components are A1g(II) vibrations of the zircon structure. The intensity of the highfre quency B component in the Raman spectrum of YVO4 is close to zero over the range 14–77 K and increases substantially with temperature (Fig. 4a). Identical temperature behavior of components A and B of the A1g(II) vibration is also characteristic of the Raman spectrum of GdVO4 (Fig. 4b). The difference is that the intensity of the B line in the Raman spectrum of GdVO4 does not tend to zero at low temperatures and the ratio of the integrated intensities IB/IA remains sig nificant in the range 14–77 K (Fig. 4b). At high tem
peratures, the integrated intensities of components A and B become equal in the Raman spectrum of YVO4, while IB substantially exceeds IA in the Raman spec trum of GdVO4 (Fig. 4b). The aforementioned asymmetry of the shape of A1g(II)vibration line in the Raman spectra of YVO4 and GdVO4 crystals cannot be related to the presence of defects due to oxygen loss during the synthesis and transition of the vanadium to other valence state. According to [20], these processes occur in the vana date systems at temperatures above 1600 K in inert atmospheres and require a prolonged time exposure, and prolonged (several hours) annealing in an oxygen atmosphere is necessary to recover the initial valence state of the vanadium. In our experiments, the revers ible temperature effect of the redistribution of compo nents A and B of the A1g(II) line in the Raman spectra of YVO4 and GdVO4 was observed at room and lower temperatures during exposures for several tens of min utes. The phenomenon that we detected may be explained by a possible existence, in molecular crys tals, apart from the potentialenergy minimum of a crystal lattice with regularly oriented molecular groups, intermediate minima provided by rotation of the molecular units at a certain angle with respect to their regular sites [13, 14]. This concept was used to explain the temperature changes in the shape of the asymmetric line of the stretching ν1 vibration of the
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(b) 300 K
300 K
150 K
150 K
Intensity
Intensity
A
77 K
77 K
14 K
900
910 ∆ν, cm–1
14 K
920
920
925 ∆ν, cm–1
930
Fig. 6. (a) Decomposition of the Ag(ν1) vibration line in the Raman spectra of the CaWO4 crystal into components A and B and (b) the region of the Raman spectrum of the corresponding vibration in BaWO4 at various temperatures.
trigonal [NO3] anion in the Raman spectra of nitrates of alkali and alkalineearth metals [13, 14]. Thus, the effect of the inhomogeneous broadening of the A1g(II) line may be ascribed to the existence of irregular sites of the tetrahedral anion [VO4] in the structures of YVO4 and GdVO4. As the surroundings of such an anion in regular and irregular lattice sites are different, the spectral lines of its vibration spectrum are shifted with respect to one another, which manifests itself in the existence of the A and B profiles for the A1g(II) vibration. Taking into account that the frequency shift between the A and B lines is 7–20 cm–1, we may con clude that the local fields for a [VO4] anion in YVO4 and GdVO4 in regular and irregular sites differ insig nificantly and the anion reorientation does not require large thermal activation. In this connection, the pro PHYSICS OF THE SOLID STATE
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cesses of occupying of the irregular sites are detected even at temperatures below 77 K and are illustrated by an increase in the relative intensity of the B compo nent with temperature. Correspondingly, the intensity of the A component is an indicator of the relative con centration of [VO4] anions in regular sites. At a tem perature of below 77 K, the B component is com pletely frozen in the Raman spectrum of YVO4, which demonstrates the transition of the crystal to the ordered state (Fig. 4a). The thermal activation results in a monotonic increase in the ratio of the intensities IB/IA and disordering of the YVO4 crystal lattice (Fig. 4a). The temperature dependence of IB/IA in the Raman spectrum of GdVO4 is more complex (Fig. 4b). First, IB/IA does not become zero even at 14 K, and the IB/IA(T) dependence in the Raman spectrum of
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GdVO4 is substantially stepper in the range 14–400 K as compared to the case of YVO4 (Fig. 5a). Above 400 K, the increase in IB/IA in the Raman spectrum of GdVO4 becomes markedly slower. These factors are indicative first of a substantially lower potential barrier ∆U for reorientation of a [VO4] anion in GdVO4 than that in YVO4. The nonmonotonic IB/IA(T) depen dence in the Raman spectrum of GdVO4 may be explained by the fact that the quantity ∆U in the GdVO4, unlike YVO4, is temperaturedependent. The explanation is supported by the temperature depen dence of the frequency shifts of components A and B of the spectra considered in Fig. 5b. In the Raman spectrum of YVO4, the temperature dependences of the positions of components A and B are practically identical; because of this, their difference (about 20 cm–1) is constant over entire temperature range studied. At the same time, in the Raman spectrum of GdVO4, the difference substantially decreases from 20 to 5 cm–1 as temperature decreases (Fig. 5b). The absence of additional splitting of the Raman line for the A1g(II) vibration in ZrSiO4 at least to a temperature of 1000 K testifies that the energy of reorientation of the [SiO4] anion in the zircon lattice is significant. This is likely due to a substantial fraction of covalency of the Zr–O bond, which hampers the reorientation of the [SiO4] anion in the crystal under consideration. The observation of the disordering effects in the crystals with the zircon structure is possible owing to anomalously strong influence of the local crystal field on the splitting of the ν2(E) internal vibration of the tetrahedral anion (120–180 cm–1), which provides a high sensitivity of the Raman spectrum in the fre quency range of this vibration to a distortion of the structure near the tetrahedral anion. In the crystals with the scheelite structure, such a splitting is several cm–1 and cannot be an indicator of disordering in this structure using the Raman spectra. In the scheelite crystals, the indicator of distortion of surroundings of the tetrahedral anion is the highest frequency Ag vibration, which is a result of the Davy dov splitting of the ν1(A1) free tetrahedral anion (Fig. 2). In a series of tungstates of alkalineearth met als, the maximum Davydov splitting is characteristic of CaWO4 (60 cm–1) and the minimum splitting is characteristic of BaWO4 (0–1 cm–1) [2]. In this con nection, the effect of asymmetry of the profile of the totally symmetric Ag(ν1) vibration was detected in the Raman spectrum just of CaWO4 and was not observed in barium tungstate. This phenomenon is illustrated in Fig. 6, which shows the Raman spectra of CaWO4 and BaWO4 in the highfrequency range over the tempera ture range 14–300 K. Similar to the inhomogeneously split profile of the A1g(II)(ν2)vibration line in the zir con structure, the Ag(ν1) line in the Raman spectrum of CaWO4 consists of two components A and B; in this
case, as temperature increases, the integrated intensi ties are redistributed to the side of IB (Fig. 5a). We were able to built the IB/IA(T) dependence for CaWO4 crys tals only to 300 K, since a further increase in temper ature led to a large broadening of the Ag(ν1) line whose profile became symmetric, and its decomposition into two components became impossible. 4. CONCLUSIONS Thus, our studies show that the effects of disorien tation of the molecular fragments under the action of thermally activated processes are inherent not only in lowmelting crystals as nitrates of alkali and alkaline earth metals but also in a number of highmelting van adates and calcium tungstate. In GdVO4 and CaWO4, they manifest themselves even at 77 K and lower tem peratures, while the process of disordering of YVO4 becomes significant at a temperature above 600 K. As these materials are used in the quantum elec tronics, the processes of disordering in them should be taken into account when interpreting their optical and laser properties. ACKNOWLEDGMENTS This study was supported by the Presidium of the Russian Academy of Science in the framework of the program for Support of Young Scientists and the Rus sian Foundation for Basic Research (project no. 07 0200375). REFERENCES 1. H.D. Jiang, H.J. Zhang, J.Y. Wang, H.R. Xia, X.B. Hu, B. Teng, and Ch.Q. Zhang, Opt. Commun. 198, 447 (2001). 2. T. T. Basiev, S. V. Vassiliev, V. A. Konjushkin, V. V. Osiko, A. I. Zagumennyi, Y. D. Zavartsev, S. A. Kutovoi, and I. A. Shcherbakov, Laser Phys. Lett. 1, 237 (2004). 3. S. A. Miller, H. H. Caspers, and H. E. Rast, Phys. Rev. 168, 964 (1968). 4. P. G. Zverev, T. T. Basiev, A. A. Sobol’, V. V. Skornyakov, L. I. Ivleva, N. M. Polozkov, and V. V. Osiko, Kvan tovaya Élektron. (Moscow) 30 (1), 55 (2000) [Quantum Electron. 30 (1), 55 (2000)]. 5. T. T. Basiev, A. A. Sobol, Yu. K. Voronko, and P. G. Zverev, Opt. Mater. 15, 205 (2000). 6. T. T. Basiev, P. G. Zverev, A. Ya. Karasik, S. V. Vassiliev, A. A. Sobol, D. S. Chunaev, V. A. Konjushkin, A. I. Zagumennyi, Y. D. Zavartsev, S. A. Kutovoi, V. V. Osiko, and I. A. Shcherbakov, Trends Opt. Photo nics 94, 298 (2004). 7. I. A. Bondar’, N. V. Vinogradova, L. N. Dem’yanets, Zh. A. Ezhova, V. V. Ilyukhin, V. Yu. KaraUshanov, L. N. Komisarova, E. V. Lazarevskiі, B. N. Litvin, P. P. Mel’nikov, D. A. Murashov, V. P. Orlovskiі, K. K. Palkina, M. A. Petrova, I. A. Rozanov, N. N. Chu dinova, and A. A. Fotiev, Compounds of RareEarth Ele
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Translated by Yu. Ryzhkov
