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

Wignacourt, J. P.; Swinnea, J. S.; Steinfink, H.; Goodenough, J. B.
October 1988
Applied Physics Letters;10/31/1988, Vol. 53 Issue 18, p1753
Academic Journal
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.


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