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−. The 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.