Motional Averaging of the Electric Field Gradient at Chlorine Nuclear Sites inK2PtCl6andK2PdCl6by the Lattice Vibrations

Abstract

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 ${[M{\mathrm{Cl}}_{6}]}^{=}$ 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 $100\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}lTl340\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}$. In each case the torsional oscillation frequency is found to be relatively insensitive to the temperature.