Single-crystal structure of vanadium-doped La 2 Mo 2 O 9
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CTURE OF INORGANIC COMPOUNDS On the 70th Anniversary of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences
SingleCrystal Structure of VanadiumDoped La2Mo2O9 O. A. Alekseevaa, A. M. Antipina, A. Gagorb, A. Pietraszkob, N. E. Novikovaa, N. I. Sorokinaa, E. P. Kharitonovac, and V. I. Voronkovac a
Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia email: [email protected] b Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wroc l aw, 50422 Poland c Moscow State University, Moscow, 119992 Russia Received September 11, 2012
Abstract—A highprecision Xray diffraction study of single crystals of two compositions— La2Mo1.78V0.22O8.89 and La2Mo1.64V0.36O8.82—was performed. In the vanadiumdoped compounds, as in the structure of the metastable βms phase of pure La2Mo2O9, the La and Mo atoms and one of the three oxygen atoms are displaced from the threefold axis, on which they are located in the hightemperature β phase. The structure contains two partially occupied oxygen sites. It was shown that molybdenum atoms are partially replaced by vanadium atoms, which are not involved in the disordering, are located on the threefold axis, and are shifted toward one of the oxygen atoms. This is consistent with the temperatureinduced changes in the structure of La2Mo2O9 and the changes in the properties of these crystals caused by the introduction of vana dium atoms into the structure. DOI: 10.1134/S1063774513060035
INTRODUCTION Crystalline materials having high electrical con ductivity associated with anomalous mobility of oxide anions are of both fundamental and practical interest, in particular, owing to the prospects of their use as oxygenexchange membranes, sensors, and other devices. High oxideion conductivity (0.06 S/cm at 800°С) of a compound having its own oxygen vacan cies and the composition La2Mo2O9 (LM) in the La2O3–MoO3 system was discovered in 2000 by Lacorre et al. [1]. At 580°С, this compound exhibits a firstorder phase transition. Thus, the lowtempera ture monoclinic α phase (P21) undergoes a tempera tureinduced transition to the hightemperature cubic β phase (Р213) accompanied by an increase in the conductivity by two orders of magnitude [1, 2]. Depending on the rate of cooling of the samples after their preparation, they can exist at room temperature as the stable monoclinic α phase, the metastable cubic βms phase, or a mixture of these phases [3, 4]. During slow cooling, LM exhibits the dynamic disorder (β phase)–static order (α phase) phase transition. After the quenching, the LM sample exists as the metastable βms phase with statically disordered oxygen atoms. During subsequent heating, this phase usually undergoes a transition to the α phase followed by the transition to the β phase at 580°С [4]. The existence of the metastable cubic βms phase at room temperature is associated with the fact that it is difficult to arrange a large number of oxygen atoms, which ar
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