BI2LA4O9 - A MONOCLINIC PHASE IN THE SYSTEM BI2O3-LA2O3

The binary system Bi2O3-La2O3 with La2O3 greater than or equal to 50 m ole% has been investigated at 950-1000 degrees C, The results revealed three compounds: Bi8La10O27, Bi2.4La3.6O9, and Bi2La4O9. Bi2.4La3.6O9 has a rhombohedral subcell with hexagonal lattice parameters a = 3.96 017(5) Angstrom, c = 9.9691(2) Angstrom, and space group R3m. It is pr obably the same phase which was reported previously as Bi2La4O9 (Refs. 5 and 6). Bi2La4O9 has a monoclinic structure with the subcell lattic e parameters a = 6.8290(3) Angstrom, b = 3.9887(1) Angstrom, c = 4.052 4(1) Angstrom and beta = 125.094(3)degrees and subcell space group C2/ m. Its average structure has been determined by X-ray powder diffracti on and refined by the Rietveld method. The average structure can be de scribed as an oxygen deficient fluorite structure of the delta-Bi2O3 t ype with La and Bi randomly distributed on the cation site. In additio n, a superstructure has been observed by TEM. (C) 1996 Academic Press, Inc.