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hesis, Crystal Structure and Thermodynamic Properties of LuGaTi2O7 L. T. Denisovaa, *, M. S. Molokeeva, b, L. G. Chumilinaa, Yu. F. Karginc, V. M. Denisova, and V. V. Ryabovd aSiberian

Federal University, Krasnoyarsk, 660041 Russia Kirensky Institute of Physics, Krasnoyarsk Scientific Center (Federal Research Center), Siberian Branch, Russian Academy of Sciences, Krasnoyarsk, 660036 Russia cBaikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, 119991 Russia d Institute of Metallurgy, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620016 Russia *e-mail: [email protected] b

Received June 14, 2020; revised August 6, 2020; accepted August 10, 2020

Abstract—Single-phase LuGaTi2O7 samples have been prepared by solid-state reaction in a starting mixture of Lu2O3, Ga2O3, and TiO2 via sequential firing in air at temperatures of 1273 and 1573 K. The crystal structure of the lutetium gallium dititanate has been determined by the Rietveld method (profile analysis of X-ray diffraction patterns of polycrystalline powders): sp. gr. Pcnb; a = 9.75033(13) Å, b = 13.41425(17) Å, c = 7.29215(9) Å, V = 957.32(2) Å3, d = 6.28 g/cm3. The heat capacity of LuGaTi2O7 has been determined as a function of temperature by differential scanning calorimetry in the range 320–1000 K. The Cp(T) data thus obtained have been used to calculate the principal thermodynamic functions of the oxide compound. Keywords: lutetium gallium titanate, mixed oxide compounds, high-temperature heat capacity, thermodynamic properties DOI: 10.1134/S0020168520120055

INTRODUCTION Unfailing interest of researchers and practicians in rare-earth titanates is aroused by their potential practical applications [1–5]. The best studied among them are rare-earth dititanates: R2Ti2O7 (R = rare-earth element). Data on their crystal structure have been presented in many reports [1, 6–10]. The R2Ti2O7 (R = Sm–Lu, Y) compounds have a cubic face-centered pyrochlore structure (sp. gr. Fd3m) [6–10], whereas La2Ti2O7, Pr2Ti2O7, and Nd2Ti2O7 crystals have a monoclinic structure (sp. gr. P21) [1, 11, 12]. There are also published data on their magnetic [1, 12, 13], electrical [1, 14], and dielectric [15] properties. At the same time, many properties of the R2Ti2O7 titanates (especially their thermophysical properties) have not yet been studied in sufficient detail. Moreover, RMTi2O7 (M = Ga, Fe) substituted titanates are essentially unexplored. Such compounds were first obtained by Genkina et al. [16]. They were shown to exist in the systems with R = Sm–Lu and M = Ga and Fe, but not in stannate or zirconate systems and not at M = Cr or Al. In addition, Genkina et al. [16] reported that, in the synthesized series of substituted rare-earth titanates, crystal structure was only determined for GdGaTi2O7. Note that there are such data for the

RMGe2O7 (M = Al, Ga, In, Fe) substituted germanates [17–19]. The objectives of this work were to determine the crystal structure of the LuGaTi2O7 titanate, measure the heat capacity of the synthesized s