Bi1-xCrxO1.5+1.5x, 0.05 ≤ x ≤ 0.15:: A new high-temperature solid solution with a three-dimensional incommensurate modulation

A new metastable solid solution Bi1-xCrxO1.5+1.5x with 0.05 less than or equal to x less than or equal to 0.15 was synthesized by quenching Bi-Cr-O melts and structurally investigated using electron microscopy and X-ray diffraction, The oxidation state of Cr was determined to be + VI by thermogravimetry, optical spectroscopy, and magnetic measurements. Infrared spectra indicate for x = 0.05 isolated CrO42- tetrahedra and also smaller fractions of Cr2O72- units. The phase exhibits an average structure of the Fm3m fluorite type and a 3D incommensurate modulation, with wave vectors q(1) = [alpha, 0, 0]*, q(2) = [0, alpha, 0]*, q(3) = [0, 0, alpha]*, a = 5.6025(3)Angstrom, and a = 0.2866(2) for x = 0.15, and with the 3 + 3D superspace group symmetry P:Fm3m:Fm3m. Atomic modulation functions and periodic nodal surfaces were deduced for observed satellite reflections and a commensurate structure model derived for x = 0.05. The modulated structure was compared with that of Bi0.78Nb0.22O1.72, with symmetry P:Fm3m:Fd3m. (C) 2001 Academic Press.