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hesis, Structure, and Thermophysical Properties of EuGaGe2O7 L. T. Denisovaa, *, M. S. Molokeevb, c, L. A. Irtyugoa, V. V. Beletskiia, Yu. F. Kargind, and V. M. Denisova a

Institute of Nonferrous Metals and Materials Science, Siberian Federal University, Svobodnyi pr. 79, Krasnoyarsk, 660041 Russia b Kirensky Institute of Physics, Krasnoyarsk Scientific Center Federal Research Center, Siberian Branch, Russian Academy of Sciences, Akademgorodok 50/38, Krasnoyarsk, 660036 Russia cInstitute of Engineering Physics and Radio Electronics, Siberian Federal University, ul. Akademika Kirenskogo 28/12(B), Krasnoyarsk, 660041 Russia dBaikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii pr. 49, Moscow, 119991 Russia *e-mail: [email protected] Received December 16, 2019; revised January 23, 2020; accepted February 4, 2020

Abstract—The europium gallium germanate EuGaGe2O7 has been prepared by solid-state reaction in air in the temperature range 1273–1473 K using a stoichiometric mixture of Eu2O3, Ga2O3, and GeO2. Its crystal structure has been determined by X-ray diffraction (sp. gr. P21/c, a = 7.1693(7) Å, b = 6.57008(6) Å, c = 12.7699(1) Å, β = 117.4522(5)°, V = 533.768(8) Å3). The heat capacity of polycrystalline samples has been determined by differential scanning calorimetry in the temperature range 350–1053 K and the experimental data have been used to calculate the thermodynamic properties (enthalpy increment, entropy change, and reduced Gibbs energy change) of EuGaGe2O7. Keywords: europium gallium germanate, solid-state synthesis, differential scanning calorimetry, heat capacity, structure, thermodynamic properties DOI: 10.1134/S002016852008004X

INTRODUCTION Researchers’ interest in the mixed oxide compounds with the general formula RMGe2O7 (R = rareearth metal; M = Al, Ga, In, Fe) is aroused by their potential practical applications [1–8]. The RMGe2O7 (M = Al, Ga) germanates have a monoclinic structure (sp. gr. P21/c), whereas the space group of the RFeGe2O7 compounds depends on the ionic radius of the rare-earth ions: P21/c for La–Gd and P21/m for Tb–Lu [7–9]. The least studied germanates include the RGaGe2O7 compounds. There are only fragmentary data on their crystal structure [2, 6, 9] and optical properties [2, 4]. Data on their heat capacity or thermodynamic properties are not available in the literature. It is worth noting that, among the RMGe2O7 germanates, the high-temperature heat capacity and thermodynamic properties have been measured only for YInGe2O7 and TmInGe2O7 [11]. The phase diagram of the Eu2O3–Ga2O3–GeO2 system has not yet been constructed. Thermodynamic modeling requires data on the thermodynamic properties of the compounds existing in this system, but such data are as yet not available.

The purpose of this work was to study the structure and thermophysical properties of EuGaGe2O7. EXPERIMENTAL Europium gallium germanate was prepared by solid-state reaction using Eu2O3 (reagent-grade), Ga2O3 (extrapure-grade), and GeO2 (99.999%) as starting chemicals. After ca