Anisotropy of Ionic Conductivity of TbF 3 Crystals
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ICAL PROPERTIES OF CRYSTALS
Anisotropy of Ionic Conductivity of TbF3 Crystals N. I. Sorokina,*, D. N. Karimova, and B. P. Soboleva a
Shubnikov Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia *e-mail: [email protected] Received September 25, 2017; revised September 25, 2017; accepted December 7, 2017
Abstract—Temperature measurements (in the range of 375–830 K) of the ionic conductivity of terbium trifluoride single crystals (structure type β-YF3) along three unit-cell crystallographic axes (a, b, and c) have been performed. It is found that TbF3 crystals (orthorhombic system, sp. gr. Pnma) have a weak anisotropy of electrical conductivity: σ||b/σ||a = σ||b/σ||c ≈ 2. The conductivity along the b axis is σ||b = 8 × 10–6 S/cm at 500 K. The anisotropy of fluorine-ion conductivity in rare-earth fluorides with β-YF3, LaF3 (tysonite), and β-BaTm2F8 structures is discussed in the contest of the specific features of their atomic structure. DOI: 10.1134/S1063774519040217
INTRODUCTION The anisotropy of ionic conductivity in the fluorides containing rare-earth elements (REEs) is of great interest for studying the mechanism of ion transport in fluoride-conducting solid electrolytes (FSEs). Experimental data on the FSE electrical conductivity, measured along different crystallographic directions, make it possible to reveal structural paths of ion transport and study the relationship between the ionic conductivity and the specific features of crystal structure. However, the investigations of ion transport anisotropy in fluoride crystals are scarce and hindered by a number of circumstances. To carry out these experiments, one needs large (more than 3–5 mm in size) single crystals of high quality. Note that the growth of fluoride crystals is hindered by their high reactivity and pyrohydrolysis. Electrical measurements for determining the static dc conductivity σdc should be performed using impedance spectroscopy in a wide frequency range. Particular attention should be paid to the separation of the bulk impedance and impedance of the crystal/electrode interface. To date, the anisotropy of ionic conductivity (based on F– ions) has been studied for only few oriented single crystals of REE-containing fluorides: LaF3 [1–4], La1 – yBayF3 – y [5, 6], R1 – ySryF3 – y (R = Ce, Pr) [7], HoF3 [8], Gd0.5Y0.5F3 [9], and BaR2F8 (R = Er0.945Tm0.05Ho0.005) [10]. Terbium trifluoride belongs to the group of REE trifluorides (TbF3, DyF3, and HoF3) with a β-YF3type structure, which do not exhibit any polymorphic transformations up to the melting point. TbF3 crystals are considered as promising media for magneto-opti-
cal devices [11]. Green lasing was implemented in a diode-pumped TbF3 crystal in [12]. In this paper, we report the results of studying the temperature dependences of the ionic conductivity of TbF3 crystals along different crystallographic directions and comparing the effect of ion transport anisotropy in REE-containing fluorides in
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