EXAFS ANALYSIS AND REINVESTIGATION OF THE STRUCTURE OF A DEFECT FLUORITE-TYPE COMPOUND, Y3TAO7

The compositional and positional disorder of constituent atoms in Y3TaO7 crystals with the defect-fluorite-type structure have been elucidated through analysis of the extended X-ray absorption fine structure (EXAFS) and reinvestigation of the crystal structure with single crystal X-ray diffraction data. EXAFS analysis based on the single shell model gave mean Y-O and Ta-O bond distances of 2.29 and 1.98 Angstrom, respectively. X-ray diffraction study revealed that the structure is essentially of the fluorite type, though Y atoms are displaced along [111] by 0.219(5) Angstrom from the position of Ca in fluorite, and half of the oxygen atoms are also displaced along [001] by 0.54(4) Angstrom from the position of F. The Ta atom is surrounded by at most eight oxygen atoms with Ta-O bond distance of 2.01(2) Angstrom, while the Y atom is surrounded by at most seven oxygen atoms with mean Y-O bond distance of 2.277(7) Angstrom. The mean bond distances and the coordination numbers of metal atoms obtained by X-ray diffraction are generally consistent with those obtained by EXAFS analysis. Most oxygen vacancies in Y3TaO7 appear to concentrate around Y atoms. The local structure around Y atoms in Y3TaO7 is similar to that in Y2O3. Crystal data for Y3TaO7 are as follows; cubic, Fm (3) over bar m, a = 5.2553(3) Angstrom at 24 degrees C, Z = 1, and D-x = 6.41 g/cm(3). (C) 1995 Academic Press, Inc.