ISSN 1063-7745, Crystallography Reports, 2016, Vol. 61, No. 3, pp. 466–468. © Pleiades Publishing, Inc., 2016. Original Russian Text © N.I. Sorokin, 2016, published in Kristallografiya, 2016, Vol. 61, No. 3, pp. 449–451.

PHYSICAL PROPERTIES OF CRYSTALS

Electrical Conductivity of Cs2CuCl4 Crystals N. I. Sorokin Shubnikov Institute of Crystallography, Russian Academy of Sciences, Moscow, 119333 Russia e-mail: [email protected] Received May 5, 2015

Abstract―The electrical conductivity of Cs2CuCl4 single crystals, synthesized by crystallization from aqueous solutions in the CsCl–CuCl2–H2O system, has been investigated. The temperature dependence of the electrical conductivity of crystals in a temperature range of 338–584 K exhibits no anomalies. The electrical transfer activation enthalpy is ΔHσ = 0.72 ± 0.05 eV and the conductivity is σ = 3 × 10–4 S/cm at 584 K. The most likely carriers in Cs2CuCl4 are Cs+ cations, which transfer electric charge according to the vacancy mechanism. DOI: 10.1134/S1063774516020255

INTRODUCTION Cs2CuCl4 is crystallized into the orthorhombic system (sp. gr. Pnam, number of formula units per unit cell Z = 4) [1–3]. Cu2+ cations in the structure are surrounded by four Cl– ions (coordination number 4) and form complex [CuCl4]2– anions. Chains of {CuCl4} tetrahedra are oriented along the crystallographic b axis, the direction in which the lattice parameter is maximal (a = 0.976, b = 1.2397 and c = 0.7609 nm [2]). According to [4], phase transitions are not observed in the range from room temperature to the fusion point (Tfus = 505°C). A technique for growing bulk Cs2CuCl4 single crystals of optical quality was developed and their structure was certified in [5, 6]. Some of their physical properties, including electrical conductivity, were investigated in [7]. The magnetic properties of Cs2CuCl4 chloride (1D antiferromagnet) were especially actively studied in [8, 9]. There are no data on the conductivity of Cs2CuCl4 crystals at temperatures above room temperature, because the electrical measurements in [7] were performed only at low temperatures (250–300 K). In this paper we report the results of an experimental investigation of the electrical conductivity of Cs2CuCl4 crystals and the relationship between the electrical transfer in these crystals with the specific features of their atomic structure. EXPERIMENTAL AND DISCUSSION OF RESULTS Cs2CuCl4 single crystals were grown at the Institute of Crystallography of the Russian Academy of Sciences by crystallization from aqueous solutions in the

CsCl–CuCl2–H2O system using cooling modes or isothermal evaporation at 18–50°C [5, 6]. The crystals were orange because of the presence of divalent Cu2+ ions entering the composition of [CuCl4]2– complex anions. The single-crystal sample for electrical measurements was a plane-parallel plate with a thickness of h = 2 mm and an area of S = 6 mm2. The sample was oriented with allowance for the habit of the grown Cs2CuCl4 crystal. Electrical measurements were performed along the crystallographic b axis. It was shown in [7] that the electrical transfer anisotropy in Cs2CuCl4 crystals is small. The electrical conductivity σ of crystal was determined at a frequency of f = 1 kHz using an ac E8-2 bridge in combination with a external G3-36 generator and a distant F582 zero indicator. The experimental setup was described in [10]. The temperature dependence σ(T) was measured in the range of 338–584 K in vacuum (residual pressure ~1 Pa). Graphite paste Dag-580 was used to form conducting contacts. Electrical conductivity activation enthalpy ΔHσ was found from the Arrhenius–Frenkel equation: σT = Aexp(–ΔHσ/kT), where A is the preexponential factor, k is the Boltzmann constant, and T is temperature. The electrical conductivity of Cs2CuCl4 crystal has an activation character; its temperature dependence σ(T) is shown in Fig. 1. There are no anomalies (jumps or kinks) on the σ(T) dependence in the temperatures range under study, a fact indicating that the mechanism of electrical conductivity does not change. The conductivity activation enthalpy is ΔHσ = 0.72 ± 0.05 eV and the conductivity reaches a value of σ = 3 ×

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logσ, S/cm 3 (а) 4 5 6

1

7

b

8 c

2

9

(b) 10 1.5

1.7

1.9

2.1

2.3

2.5 2.7 103/T, K1 a

Fig. 1. Temperature dependences of electrical conductivity σ(T) of (1) Cs2CuCl4 and (2) CsCl [11] single crystals.

c

10–4 S/cm at 584 K. For comparison, Fig. 1 shows data on conductivity σ [11] for a CsCl crystal. The conductivity of Cs2CuCl4 single crystal exceeds that of CsCl by about two orders of magnitude (at 400 K). The Cs2CuCl4 crystal structure is shown in Fig. 2. One specific feature of the atomic structure of this compound is the presence of tetrahedral [CuCl4]2– complexes in which the Cu–Cl distance is 2.25 Å [1]. The unit cell contains four [CuCl4]2– complex anions and eight Cs+ cations. {CuCl4} tetrahedra do not share directly any vertices, edges, or faces. Linear–Cu–Cl– Cl–Cu– chains are formed along the crystallographic b axis, in which the least distance between Cu2+ ions in neighboring {CuCl4} tetrahedra is ~6 Å. Cs+ cations occupy two nonequivalent sites: Cs1 and Cs2 [2]. A Cs1 atom is coordinated by 11 Cl atoms located at an average distance of 3.79 Å, while a Cs2 atom is coordinated by nine Cl atoms (at an average distance of 3.59 Å). Cu2+ cations (ionic radius 1.57 Å for a coordination number of 4 in the system of “effective” Shannon radii [12]) and Cl– anions (1.81 Å for a coordination number of 6) are bound and form a tetrahedral group [CuCl4]2–. The formation of [CuCl4]2– complex anions does not facilitate the translational mobility of Cu2+ and Cl– ions in the Cs2CuCl4 structure. Hence, one can conclude that the electric charge carriers in the compound under study are Cs+ cations (ionic radius 1.78 Å for a coordination number of 9 and 1.85 Å for a coordination number of 11). Diffusion CRYSTALLOGRAPHY REPORTS

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Fig. 2. Projections of the Cs2CuCl4 crystal structure [3] onto the (a) (100) and (b) (010) planes.

mobility of Cs+ ions in Cs2CuCl4 was found when studying the dynamic behavior of the cesium sublattice [13] by 133Cs nuclear magnetic resonance. The value of conductivity activation enthalpy (ΔHσ = 0.7 eV) for Cs2CuCl4 crystals is in agreement with the data of [7] for this compound at 250–300 K (ΔHσ ~ 0.6 eV), as well as with the data on the migration enthalpy of VCs− cesium vacancies in CsCl crystals (ΔHm, v = 0.60 [14], 0.62 eV [15]). The migration activation enthalpy for VCl+ chlorine vacancies in cesiumcontaining chlorides is lower by a factor of 2–4: ΔHm, v = 0.34 [14], 0.2–0.3 eV [15] for CsCl, ΔHm, v = 0.29 eV for CsPbCl3 [16], and ΔHm, v = 0.15 eV for CsSnCl3 [17]. CONCLUSIONS The electrical conductivity of Cs2CuCl4 crystals reaches a value of σ = 3 × 10–4 S/cm at 584 K at an electrical transfer activation enthalpy of ΔHσ = 0.7 eV. The crystallochemical analysis of the Cs2CuCl4 structure showed that the most likely carriers in it are Cs+ cations, which transfer electric charge according to the vacancy mechanism.

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ACKNOWLEDGMENTS I am grateful to L.V. Soboleva for supplying a crystal for study. REFERENCES 1. L. Helmholz and R. F. Kruh, J. Am. Chem. Soc. 74, 1176 (1952). 2. J. A. McGinnety, J. Am. Chem. Soc. 94, 8406 (1972). 3. Y. Xu, S. Carlson, K. Soderberg, et al., J. Solid State Chem. 153, 212 (2000). 4. L. V. Soboleva and M. G. Vasil’eva, Zh. Neorg. Khim. 22 (5), 1293 (1977). 5. M. G. Vasil’eva and L. V. Soboleva, Zh. Neorg. Khim. 21 (11), 2795 (1976). 6. L. V. Soboleva, L. M. Belyaev, V. V. Ogadzhanova, et al., Kristallografiya 26 (4), 817 (1981). 7. Z. Tylczynski, P. Piskunowicz, A. N. Nasyrov, et al., Phys. Status Solidi A 133, 33 (1992).

8. R. Coldea, D. A. Tennant, R. A. Cowley, et al., J. Phys.: Condens. Matter 8, 7473 (1996). 9. M. A. Vachon, G. Koutroulakis, V. F. Mitrovic, et al., New J. Phys. 13, 093029 (2011). 10. A. K. Ivanov-Shits, N. I. Sorokin, P. P. Fedorov, et al., Fiz. Tverd. Tela 25, 1748 (1983). 11. G. A. Samara, Solid State Phys. 38, 1 (1980). 12. R. D. Shannon, Acta Crystallogr. A 32, 751 (1976). 13. A. R. Lim, K. S. Hong, and S. Y. Jeong, J. Phys. Chem. Solids 65, 1373 (2004). 14. L. W. Barr and A. B. Lidiard, Physical Chemistry. An Advanced Treatise, Ed. by H. Eyring, D. Henderson, and W. Jost (Academic, New York, 1970), Vol. 10, p. 151. 15. G. A. Samara, Phys. Rev. B 22, 6476 (1980). 16. J. Mizusaki, K. Agai, and K. Fueki, Solid State Ionics 11, 203 (1983). 17. N. A. Mel’nikova, O. V. Glumov, A. V. Glumov, et al., Russ. J. Appl. Chem. 72 (4), 624 (1999).

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Translated by A. Grudtsov

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