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,x,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 CrO2-4 tetrahedra and also smaller fractions of Cr2O-27 units. The phase exhibits an average structure of the Fm3m fluorite type and a 3D incommensurate modulation, with wave vectors q1=[alpha, 0, 0]*, q2=[0, alpha, 0]*, q3=[0, 0, alpha]*, a=5.6025(3) ?, and alpha=0.2866(2) for x=0.15, and with the 3+3D superspace group symmetry P:Fm m:Fm m. 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:Fm m:Fd m.