ISSN 1070-4272, Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 12, pp. 1835−1841. © Pleiades Publishing, Ltd., 2013. Original Russian Text © N.M. Kompanichenko, A.A. Omel’chuk, A.P. Ivanenko, R.N. Pshenichnyi, E.V. Timukhin, 2013, published in Zhurnal Prikladnoi Khimii, 2013, Vol. 86, No. 12, pp. 1887−1893.
TECHNOLOGIES OF ELECTROCHEMICAL INDUSTRY
Conductivity of Nonstoichiometric Fluorides LnF2+x (Ln = Sm, Eu, Yb) N. M. Kompanichenkoa, A. A. Omel’chuka, A. P. Ivanenkoa, R. N. Pshenichnyia, and E. V. Timukhinb a
Vernadskii Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukraine b Bogatskii Physicochemical Institute, National Academy of Sciences of Ukraine, Kiev, Ukraine e-mail:
[email protected] Received December 19, 2013
Abstract—Nonstoichiometric fluorides LnF2+x (Ln = Sm, Eu, Yb) were synthesized by reduction of trifluorides with the above rare-earth elements. The resulting phases were identified by chemical and X-ray phase analyses, their composition and structure were determined, and their lattice constants were found. The ac bridge method at a frequency of 70 kHz was used to study the conductivity of the synthesized compounds and starting trifluorides in the temperature range 773–298 K. The temperature dependence of the conductivity of these compounds is satisfactorily approximated by the Arrhenius–Frenkel equation. A kink is observed on the plots of the electrical conductivity against temperature for all compounds. With decreasing content of fluorine, this kink shifts to higher temperatures. The highest conductivity is observed for the phases with low crystal packing density. With increasing content of fluorine, the conductivity of all nonstoichiometric phases not belonging to substitution solid solutions approaches the conductivity of the corresponding trifluorides. DOI: 10.1134/S1070427213120069
It is known [1–5] that, because of the disordered anion sublattice, nonstoichiometric fluorides of rare-earth elements (REEs), LnF2±x, possess a rather high unipolar anionic conductivity and can be recommended for use in various electrochemical devices [4]. The conductivity by the fluorine ion is directly associated with the disturbed stoichiometry of these compounds and can be obtained in binary systems of REE fluorides, LnF2–LnF3, in different oxidation states [1]. A distinctive feature of the interaction between the components of the systems of this kind is their ability to form nonstoichiometric compounds with structural distortions of the anion sublattice of the crystals, with nearly full occupancy of the cation sublattice being retained [1–3]. As a result, defects providing the mobility of fluorine ions appear in the anion sublattice. The following phases are among the interaction products formed in the given systems: phases indexed as individual nonstoichiometric compounds LnmF2m+5, where m takes values of 13, 14, 15, and (13+14), depending on the synthesis conditions, and heterovalent-
substitution solid solutions LnF2+x, where 0 ≤ x ≤ 0.25 [6–8]. Depending on the synthesis conditions and composition, nonstoichiometric compounds can form crystals of cubic, tetragonal, hexagonal, or rhombic systems. The phases corresponding to the substitution solid solutions form cubic crystals. The low packing density of fluorite-like crystals, complex nature of isomorphous substitution, and significant intrinsic disorder at elevated temperatures make it possible to obtain solid solutions with a wide homogeneity range in MF3–MF2 systems [9]. Despite the considerable accumulated amount of information about the transport properties of nonstoichiometric fluorides of rare-earth elements [1–5, 9], the relationship between their composition, structure, and properties has not been definitively established. Data on how the conductivity varies at different temperature with the nature of rare-earth element are scarce. In this context, determining the relationship between the composition, structure, and transport properties of
1835
1836
KOMPANICHENKO et al.
fluorine-conducting phases of rare-earth elements is of not only scientific, but also practical interest. In this communication, we report the results of a study of the conductivity of phases with various compositions and structures in the systems LnF3–LnF2, where Ln = Sm, Eu, Yb, to determine the relationship between the conductivity, composition, and structure of nonstoichiometric compounds of samarium, europium, and ytterbium. EXPERIMENTAL Fluorides of rare-earth elements in lower oxidation states (LnF2+x) were produced by reduction of chemically pure LnF3 with the corresponding metal by the reaction (2+x)LnF3 + Ln = 3LnF2+x at various component ratios. Metallic samarium and ytterbium (of M-1 grade) were ground to a particle size of 0.1–0.15 mm with a broach file in a dry box. After that the metal was treated with 0.1 N HCl, washed with acetone, and dried in a vacuum. Europium was cut with scissors to particles 0.20–0.30 cm in size, which were treated with hexane or decane. The mixture of a metal and trifluoride were placed in a quartz ampule and heated in a vacuum until air was completely removed and then were sealed. The ampule with the mixture was heated in a shaft furnace and kept at 700–850°C for 30–80 h, depending on the mass of the reaction mixture. Because the lanthanides evaporated when the ampule was heated and a metallic “mirror” formed [10] on its inner surface, the reaction mixture components did not react with quartz. The thus synthesized phases were identified by chemical and X-ray phase (XPA) analyses, IR spectroscopy, and diffuse reflectance electron spectroscopy. The rare-earth metals were determined by gravimetry in the form of an oxide, and fluorine, by potentiometry in a solution produced by water leaching of an alloy of the compound under study with sodium peroxide (Tfus = 480°C) or with potassium-sodium carbonate (Tfus = 1000°C) with an I-160MI ion meter and ELIS-131F fluorine-selective electrode. The X-ray phase analysis was made with a DRON3M diffractometer (CuKα radiation) by the powder method. The phases were identified against the ASTM database. The IR spectroscopy was performed with a Specord M-80 spectrophotometer in the range from 4000 to 200 cm–1 with samples pelletized with
potassium bromide or cesium iodide. Diffuse reflectance spectra were obtained with a Perkin–Elmer Lambda 9 spectrophotometer in the range 200–2500 nm. The electrical conductivity was measured with an R5083 ac bridge at a frequency of 70 kHz in the temperature range 25–500°C in the atmosphere of argon. Samples were compacted into pellets with a diameter of 10 mm and thickness of 2.0–2.5 mm at 15–20 MPa. Silver was deposited onto their surface to diminish the transition resistance and provide a reliable contact with the current leads. The electrical conductivity was calculated from the Arrhenius–Frenkel equation: σ = l/SR, where l is the pellet thickness; S, contact area; and R, pellet resistance. DISCUSSION OF RESULTS The charge transport in stoichiometric fluorides with fluorite structure in the absence of impurities and electromagnetic radiation is governed by the Frenkel mechanism [11]. The intrinsic electronic conductivity of these compounds is very low (10–7–10–9 S cm–1) because of the large degree of ionicity of the Ln–F bond and large energy gap width. The conductivity in compounds of this kind can be controlled by changing the number of defects in nonstoichiometric phases. It is known that compounds of samarium, europium, and ytterbium in lower oxidation states form both basic structures and superstructures [6–8]. An analysis of the results of X-ray phase studies with Match and Index programs determined the crystal systems of the crystal lattices of the synthesized compounds LnF2+x and the lattice constants. For example, the fluorine-deficient phases SmF1.82, EuF1.94, and YbF1.91 are mixtures of LnF2.00 compounds and a metal present in the amorphous state in the crystal lattice. Therefore, these compounds are indexed as stoichiometric compounds with cubic lattice constants: 5.8482 Å for EuF1.95 and 5.6078 Å for YbF1.91. The phases formed in the temperature range 700– 750°C have the composition SmF2.07, EuF2.11, and YbF2.01–2.07 and cubic structure with lattice contact a (Å): 5.8115 (SmF2.07), 5.8245 (EuF2.11), and 5.5886– 5.5949 (YbF2.01–2.07) (see table). All these compounds can be classed as substitution solid solutions. The YbF2.37 and EuF2.37 phases crystallize in cubic structures, basic structure and superstructure, with
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 12 2013
CONDUCTIVITY OF NONSTOICHIOMETRIC FLUORIDES
ν, cm–1 Fig. 1. IR spectra of the nonstoichiometric compounds (1) SmF2.07, (2) EuF2.11, and (3) YbF2.03. (ν) Wave number.
lattice constants a = 5.5737 and 17.3686 Å, respectively. However, they are indexed as separate compounds not belonging to solid solutions [6–8]. The necessary condition for phases of this kind to be formed is higher temperature (800–850°C) and longer synthesis duration (>30 h). The IR spectra of the synthesized compounds LnF2+x are shown in Fig. 1. The peaks of the absorption bands in the IR spectra are observed in the range 260–290 cm–1. These bands can be related to deformation vibrations of the Ln–F bond. The spectra of the compounds forming superstructures are similar to those of trifluorides [12], with the intensities of the main absorption bands decreasing and the bands shifted to longer wavelengths.
Two regions can be distinguished in the electronic spectra of the compounds LnF2+x (Fig. 2): the absorption bands in the range 200–400 nm are due to the 4f–5d electron transitions in Ln2+ ions [13], and those at 800– 2200 nm, to 4f–4f electron transitions characteristic of Ln3+ ions [14]. Figure 3 shows plots the dependences of the conductivity of the compounds synthesized in the systems SnF2–SnF3, EuF2–EuF3, and YbF2–YbF3 against temperature in the coordinates of the Arrhenius Frenkel equation lnσ = const – ΔEa/kT, where ΔEa is the activation energy of conductivity. The calculated values of activation energy and electrical conductivities at various temperatures are listed in the table. The identical trend of these plots and nearly the same order of magnitudes of the activation energies for both basic structures and superstructures give suggest that the conduction is effected by ions of the same kind, fluoride ions. The bend observed in the plots of the conductivity against temperature is in all probability due to the release of fluorine ions into large interstitial voids of the fluorite structure. The insignificant contribution of electronic conductivity to the total conductivity of LnF2+x compounds forming superstructures is evidenced by the ordered arrangement of lanthanide cations in the oxidation state +3 in these compounds. Figure 4 shows a fragment of the cationic skeleton in crystals of fluorides LnF2+x of the superstructural type. It can be seen that the motion of an electron is only possible via Ln2+ → Ln3+ trans(b)
(a)
2.0
1837
(c)
1.2
0.4 200 400 400
800 1000 λ, nm
1500
1900
λ, nm
λ, nm
Fig. 2. Electronic spectra of the nonstoichiometric compounds (1) SmF2.07, (2) EuF2.11, and (3) YbF2.03. (λ) Wavelength. RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 12 2013
1838
KOMPANICHENKO et al.
Characteristics of the temperature dependences of the conductivity of samarium, europium, and ytterbium fluorides Composition of compound and its Temperature range, K crystal lattice constants, Å 299–433 SmF2.00 а = 17.4069
477–773
300–429
Activation energy, eV
0.10 ± 0.01
0.78 ± 0.01
0.11 ± 0.01
SmF2.07 а = 5.8115 472–773
290–438
0.79 ± 0.01
0.10 ± 0.01
SmF2.23 а = 4.0976 с = 5.9589
SmF3.00 (tysonite) а = 6.97 с = 7.19
YbF1.91 а = 5.6078
YbF2.01 а = 5.5886
YbF2.03 а = 5.5978
478–775
410–774
0.77 ± 0.02
0.49 ± 0.01
369–595
0.24 ± 0.01
618–773
0.68 ± 0.02
396–549
0.14 ± 0.004
572–773
0.63 ± 0.02
358–479
0.20 ± 0.01
499–773
0.62 ± 0.01
σ × 107, S cm–1
Т, K
1.3
299
3.5
433
5.7
453
8800.0
630
10000.0
773
4.0
300
9.4
429
25.0
472
2000.0
616
30000.0
773
1.4
290
1.7
438
8.3
468
380.0
626
7000.0
774
32.0
410
4500.0
773
1.2
369
12.0
595
15.0
618
140.0
773
0.4
396
1.1
549
1.3
572
22.0
773
1.4
358
4.4
479
5.2
499
530.0
773
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 12 2013
CONDUCTIVITY OF NONSTOICHIOMETRIC FLUORIDES
1839
Table (Contd.) Composition of compound and its Temperature range, K crystal lattice constants, Å
YbF2.07 а = 5.5949
YbF2.37 а = 5.5737
YbF3 (tysonite) а = 6.99 с = 8.32
EuF1.95 а = 5.8482
EuF2.11 а = 5.8240
Eu27F64 а = 17.3686
EuF2.337 а = 5.7557
EuF3 (tysonite) а = 6.916 с = 7.091
343–533
Activation energy, eV 0.23 ± 0.005
573–773
0.63 ± 0.01
353–565
0.25 ± 0.007
587–773
0.65 ± 0.01
353–553
0.19 ± 0.01
593–773
0.94 ± 0.02
343–603
0.30 ± 0.01
653–773
1.15 ± 0.02
363–557
0.26 ± 0.01
573–773
0.93 ± 0.02
383–560
0.35 ± 0.01
580–773
0.83 ± 0.01
326–478
0.15 ± 0.02
478–773
0.72 ± 0.03
348–503
0.20 ± 0.01
523–773
1.08 ± 0.02
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 12 2013
σ × 107, S cm–1
Т, K
0.6
343
4.9
533
6.4
573
110.0
773
0.4
353
4.7
565
6.3
587
65.0
773
0.1
353
0.6
553
1.3
593
46.0
773
0.4
343
13.0
603
28.0
653
490.0
773
0.4
363
4.4
537
5.0
557
340.0
773
0.5
383
10.0
560
14.0
580
560.0
773
0.3
326
1.5
478
830.0
773
0.1
348
1.0
503
1.2
523
1100.0
773
1840
KOMPANICHENKO et al. (a)
log σ, S cm–1
log σ, S cm–1
(b)
103/T, K
103/T, K
log σ, S cm–1
(c)
103/T, K Fig. 3. Conductivity σ of the nonstoichiometric phase of fluorides of rare-earth elements vs. temperature T. (a) SmF2+x, (b) EuF2+x, and (c) YBF2+x.
Fig. 4. Fragment of the cation skeleton in crystals of fluorides LnF2+x of superstructural type. RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 12 2013
CONDUCTIVITY OF NONSTOICHIOMETRIC FLUORIDES
lations. The encounter of two Ln3+ ions terminate the electron motion. An analysis of the results obtained (see table) demonstrated that the conductivity of the phases under study strongly depends on their structure. The highest conductivity is observed for the cubic phases, which belong to substitution solid solutions with low crystal packing density. For example, the phases SmF2.00 and SmF2.07 have at 773 K a higher conductivity than the SmF2.23 phase of hexagonal crystallographic system. The same pattern is observed for the phases in the YbF2–YbF3 system. In the EuF2–EuF3 system, the cubic phases indexed as separate compounds (EuF2.337 and EuF2.37) and phases belonging to substitution solid solutions (EuF2.11) have conductivities of the same order of magnitude. It should be noted that the conductivities of all the nonstoichiometric compounds under study with high fluorine content (x ≥ 0.23) approach those of the corresponding trifluorides. CONCLUSIONS (1) The highest anionic conductivity among the nonstoichiometric samarium, europium, and ytterbium fluorides under study is observed for phases of the cubic system with low crystal packing density, which belong to substitution solid solutions LnF2+x, where 0 ≤ x ≤ 0.20. (2) The electrical conductivity of the trifluorides of the rare-earth elements under study (tysonite structure) is lower, with the exception of SmF3, than that of the corresponding nonstoichiometric phases and increases in the order YbF3 < EuF3 < SmF3. (3) All plots of the conductivity against temperature in the log σ–1/T coordinates of the investigated phases, show a bend due to the Faraday phase transition. The transition temperature increases in the order EuF2+x
