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(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.

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