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 temperature between 8 and 380 K, using four single crystal neutron diffraction, five single crystal X-ray diffraction and two neutron powder diffraction data sets. Third-order anharmonic Gram-Charlier terms have been determined in all the neutron single crystal and two of the X-ray refinements. Results are consistent with a predominantly ionic compound K-2(+)(PtCl6)(2-) Th, dimensions of the octahedral (PtCl6)(2-) ion hardly vary with temperature. During thermal expansion, these rigid ions move farther apart. The temperature dependence of the atomic mean-square displacements agrees well with that of ensemble-averaged quantized harmonic oscillators. The thermal motion of (PtCl6)(2-) is dominated by rigid-body libration and translation, but contributions from internal 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.