Crystal structure of La 2 Mo 2 O 9 single crystals doped with bismuth
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URE OF INORGANIC COMPOUNDS
Crystal Structure of La2Mo2O9 Single Crystals Doped with Bismuth O. A. Alekseevaa*, I. A. Verina, N. I. Sorokinaa, A. E. Krasil’nikovaa, and V. I. Voronkovab a
Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiі pr. 59, Moscow, 119333 Russia b Faculty of Physics, Moscow State University, Moscow, 119992 Russia *email: [email protected] Received October 15, 2009
Abstract—Precision Xray diffraction studies of La2 – xBixMo2O9 (x = 0.04, 0.06, and 0.18) single crystals are performed. It is found that in the compounds doped with bismuth, analogously with the structure of the metastable βms phase of pure La2Mo2O9 (LM), the La, Mo1, and O1 atoms deviate from the threefold axis on which they are located in the hightemperature β phase. It is shown that bismuth atoms substitute for part of lanthanum atoms and occupy a position at the threefold axis in the neighborhood of the split lanthanum position. The implantation of bismuth atoms in the LM structure results in the return of a part of the molyb denum atoms to the position at the threefold axis. The occupancy of this position is equal to the occupancy of the bismuth atomic position. DOI: 10.1134/S1063774510040085
INTRODUCTION Materials that can be used as oxygenconducting solid electrolytes in fuel elements have received much attention in connection with the development of new energetics. Structural types exhibiting oxygen conduc tion are few in number and are reduced to fluorites, perovskites, pyrochlores, apatites, Aurivillius phases, and the LAMOX family, which is obtained by the dop ing of lanthanum molybdate La2Mo2O9 (LM). The phase transition in LM from the monoclinic α phase (space group Р21) to the cubic β phase (space group Р213) is accompanied by an increase in the oxygen conductivity by two orders of magnitude up to 0.06 S/cm at 800°С [1]. This value exceeds the con ductivity in widely known zirconium oxide stabilized with yttrium or calcium at lower temperatures and attracts attention to lanthanum molybdate. Cooling of the LM specimens synthesized down to the room tem perature affords crystals of the stable monoclinic α phase or the metastable cubic βms phase (Fig. 1) or mixedphase crystals, depending on the cooling rate and additive content. In the last few years, the structure and properties of doped LM have been the objective of much research. The substitution of sodium, potassium, rubidium, cal cium, strontium, barium, bismuth, and rareearth ele ments for lanthanum, as well as vanadium, tungsten, chromium, niobium, and tantalum for molybdenum, has been reported [2–5]. At a certain concentration, many of these additives, especially those substituting for lanthanum, suppress the β → α transition and sta bilize the cubic phase at room temperature. It is
known that, in Bicontaining LM, the transition is suppressed with 5–15% of lanthanum replaced by bis muth [6, 7]. An effort to follow the dynamics of
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a Fig. 1. Crystal structure of the metastable βms phase of La2Mo2O9. The O1 a
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