Structure and Physical Properties of Mg-Containing Oxymolybdates La 2 MoO 6
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CTURE OF INORGANIC COMPOUNDS
Structure and Physical Properties of Mg-Containing Oxymolybdates La2MoO6 E. I. Orlovaa, E. P. Kharitonovaa, N. I. Sorokinab, T. A. Sorokinb, A. M. Antipinb, and V. I. Voronkovaa,* a
b
Moscow State University, Moscow, 119991 Russia Shubnikov Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia *e-mail: [email protected] Received March 4, 2020; revised April 22, 2020; accepted April 24, 2020
Abstract—Oxymolybdates La2MoO6, both pure and doped with magnesium, have been obtained by the flux crystallization method and solid-phase synthesis as single crystals and polycrystals, respectively. The compounds are described by X-ray diffraction analysis, thermogravimetry, and differential scanning calorimetry; their conducting properties have been investigated. A model of the structure of magnesium-doped La2MoO6 single crystals is proposed. Incorporation of divalent magnesium atoms into the sites of hexavalent molybdenum atoms in the La2MoO6 structure leads to the formation of oxygen vacancies and a slight increase in the conductivity of doped samples in comparison with pure crystals. The compounds under study have hygroscopic properties. DOI: 10.1134/S1063774520050168
INTRODUCTION Binary Ln2O3–MoO3 systems (Ln = La, Pr, or Nd) in the range of 25–50 mol % Ln2O3 are promising for researchers, because they provide formation of several compounds with interesting properties. Compounds with the composition of 1 : 3 (ratio of the molar concentrations of oxides) have excellent luminescence properties and high refractive indices; improper ferroelectrics were found for the first time among these compounds [1]. The vast LAMOX family based on molybdate La2Mo2O9, discovered in 2000 [2], is distinguished by a high oxygen conductivity (10–2 S/cm at 800°C) of vacancy character. The compounds with a composition of 5 : 6 have similar values of conductivity, although its character is different (interstitial) [3]. Finally, compounds Ln2MoO6 with a composition of 1 : 1 (the so-called oxymolybdates) have complex polymorphism, which depends on the size of the ionic radius of the rare-earth cation and the synthesis temperature [4–6]. X-ray diffraction analysis of pure oxymolybdates revealed two types of polymorphs in dependence of the ionic radius [4–7]. In the case of large (La–Nd) cations, the compounds were crystallized in a tetragonal phase at a synthesis temperature of 1200°C. The only exception was Nd2MoO6, which existed in a monoclinic phase up to 1000°C; all Ln2MoO6 compounds with rare-earth cations with smaller ionic radii existed in the same phase at low and high tempera-
tures. Above 1010°C, the Nd2MoO6 compound irreversibly passed to a tetragonal phase [4–8]. The structure of oxymolybdates with large cations (La, Pr, and Nd) was investigated in a number of studies [9–12]. The structure of La2MoO6 single crystals obtained at a temperature of 800°C was analyzed for the first time in [9]; they were describe
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