Change in the crystal structure of germanium-containing lithiophosphate upon heating
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E DYNAMICS AND PHASE TRANSITIONS
Change in the Crystal Structure of GermaniumContaining Lithiophosphate upon Heating D. A. Ksenofontova, Yu. K. Kabalova, and L. N. Dem’yanetsb a
Moscow State University, Moscow, 119992 Russia email: [email protected] b Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiі pr. 59, Moscow, 119333 Russia Received January 26, 2009
Abstract—The crystal structure of lithiophosphate with phosphorus atoms partially replaced by germanium, Li3.17(P0.69Ge0.24Mo0.07)O4 , at temperatures of 25, 150, 300, 450, and 600°C has been refined by the Rietveld method using powder data. New additional Bragg reflections are observed at T = 600°C, which indicate a change in the crystal structure of this compound. DOI: 10.1134/S106377451002015X
INTRODUCTION The compounds that are derivatives of γLi3PO4 are of particular interest for the following reason: the par tial isomorphic substitution of certain ions makes it possible to change the localization of alkali cations, the occupancies of these positions, and possible ways for the transport of ion with a change in temperature conditions, as well as to reveal new superionic conduc tors. The structures of Li3.31(P0.69Ge0.31)O4 [1] and Li3.17(P0.69Ge0.24Mo0.07)O4 [2] were previously studied on single crystals. Both phases have a crystal structure close to γLi3PO4 (orthorhombic system, sp. gr. Pnma, a = 10.690(1) Å, b = 6.1946(7) Å, c = 5.0266(6) Å). Structures of this type are based on a 3D framework composed of [PO4] and [LiO4] tetrahedra with shared vertices. The two main crystallographically nonequiv alent positions of Li atoms in γLi3PO4, which are characterized by partial occupancy, are supplemented with four (for Li3.31P0.69Ge0.31O4) and three (for Li3.17(P0.69Ge0.24Mo0.07)O4 (LPGM)) positions of Li cations, which have a relatively low occupancy. These solid solutions are superionic conductors with lithium conductivity. The purpose of this study was to analyze the behavior of the crystal structure of the other superi onic conductor with a structure type of γLi3PO4 upon heating. As an object of study, we chose the singlecrystal solid solution based on lithium ortho phosphate, Li3.17(P0.69Ge0.24Mo0.07)O4, synthesized under peculiar conditions (with an external electric current applied) [3]. A hightemperature (in the range of 25–700°С) Xray study of the obtained Li3.17(P0.69Ge0.24Mo0.07)O4 sample was performed to determine the stability of this compound, refine the phasetransition temperature, and calculate the ther
mal expansion coefficients. A series of hightempera ture measurements and subsequent Rietveld refine ment of the structure makes it possible to trace the possible ways of motion of Li interstitials (conducting ions) in the sublattice of the material studied. EXPERIMENTAL A single crystal of germaniumsubstituted lithio phosphate was grown in the Li3PO4–Li4GeO4– Li2MoO4–LiF system by flux crystallization in an electric field, which leads to a change in the compo sition of the final crystallization
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