Motional Averaging of the Electric Field Gradient at Chlorine Nuclear Sites in ${\mathrm{K}}_2$Pt${\mathrm{Cl}}_6$ and ${\mathrm{K}}_2$Pd${\mathrm{Cl}}_6$ by the Lattice Vibrations

Temperature variations of the ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ nuclear-quadrupole-resonance (NQR) frequencies in ${\mathrm{K}}_2$Pt${\mathrm{Cl}}_6$ and ${\mathrm{K}}_2$Pd${\mathrm{Cl}}_6$ are discussed. The dominant thermal motions of the lattice responsible for the averaging of the electric field gradient at a nuclear site are assumed to take place in the plane perpendicular to the symmetry axis of the field gradient. Contributions to the NQR frequency shifts due to the internal modes $Q_3$, $Q_4$, $Q_5$, and $Q_6$ of the ${\left[M{\mathrm{Cl}}_6\right]}^{=}$ octahedra are calculated using the results of the harmonic theory of lattice dynamics and the Bayer-Kushida theory of NQR frequencies. Differences between the experimental and calculated frequency shifts are assumed to be due to the rotary lattice modes; the frequencies of these modes are deduced in ${\mathrm{K}}_2$Pt${\mathrm{Cl}}_6$ and ${\mathrm{K}}_2$Pd${\mathrm{Cl}}_6$ for temperatures in the range $100°\mathrm{K}<T<\mathrm{}340\mathrm{}°\mathrm{K}$. In each case the torsional oscillation frequency is found to be relatively insensitive to the temperature.