NEUTRON AND X-RAY-DIFFRACTION STUDY OF THE THERMAL MOTION IN K2PTCL6 AS A FUNCTION OF TEMPERATURE

The crystal structure and thermal displacement parameters of potassium hexachloroplatinate, K2PtCl6, have been studied as functions of tempe rature between 8 and 380 K, using four single crystal neutron diffract ion, five single crystal X-ray diffraction and two neutron powder diff raction data sets. Third-order anharmonic Gram-Charlier terms have bee n determined in all the neutron single crystal and two of the X-ray re finements. Results are consistent with a predominantly ionic compound K-2(+)(PtCl6)(2-) Th, dimensions of the octahedral (PtCl6)(2-) ion har dly vary with temperature. During thermal expansion, these rigid ions move farther apart. The temperature dependence of the atomic mean-squa re displacements agrees well with that of ensemble-averaged quantized harmonic oscillators. The thermal motion of (PtCl6)(2-) is dominated b y rigid-body libration and translation, but contributions from interna l vibrations are also identified. The anharmonic coefficients of Cl at high temperature agree with curvilinear motion due to the rigid-body libration. The motion of K+ is also anharmonic.