Oxygen atom thermal vibration anisotropy in Ba0.87K0.13BiO3

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Abstract
Crystals of Ba1−xKxBi2O3 were obtained from a melt of a mixture of Ba(OH)2⋅8H2O, Bi2O3 (0.6Ba:1Bi), and excess KOH heated at 360 °C in air. Single‐crystal x‐ray diffraction data show that the crystals are cubic a=4.3223(5) Å, space group Pm3m. The 55 unique reflections resulting from averaging 1364 measured intensities were used to refine the structure by least squares to R=0.0089, wR=0.0076. The atomic positional parameters are fixed by symmetry; the refinement of the Ba occupancy factor yields the value 0.872(6). The thermal vibration parameters for the cations are constrained to be spherical; the oblate vibration ellipsoid for oxygen yields rms displacements of 0.258(5) Å perpendicular to the Bi‐Bi distance and 0.12(1) Å parallel to it. A prolate ellipsoid for the oxygen anisotropic thermal displacement parameters would have indicated a softening of ‘‘breathing’’ modes at the bismuth atoms. Instead, an oblate spheroid locks the oxygen atoms at mid‐bismuth positions with enhanced vibrations perpendicular to the Bi‐Bi distance. The potassium substitution suppresses the Bi(III)‐Bi(V) disproportionation observed in BaBiO3.