Nuclear-Quadrupole-Resonance Study of Lattice Dynamics inK2PtCl6

Abstract

A pulsed magnetic-resonance spectrometer is used to measure pure NQR frequencies and relaxation times of the $^{35}\mathrm{Cl}$ and $^{37}\mathrm{Cl}$ nuclei in powdered samples of ${\mathrm{K}}_{2}$Pt${\mathrm{Cl}}_{6}$ over the temperature range $6lTl490\ifmmode^\circ\else\textdegree\fi{}$K. The ratio of frequencies is independent of temperature and equal to 1.2688\ifmmode\pm\else\textpm\fi{}0.001, in agreement with the ratio of quadrupole coupling constants as found for free Cl atoms. It is shown that a single low-frequency vibrational mode of frequency \ensuremath{\sim}38 ${\mathrm{cm}}^{\ensuremath{-}1}$ dominates the motional averaging of the electric field gradient at a Cl site. This mode is identified as the rotary lattice mode which corresponds to the threefold degenerate torsional oscillations of the ${[\mathrm{P}\mathrm{t}{\mathrm{Cl}}_{6}]}^{=}$ ion. A realistic model of the field gradient is proposed, and the data are discussed in terms of this model. The ${T}_{1}$ data span seven orders of magnitude from 100 \ensuremath{\mu}sec at 490\ifmmode^\circ\else\textdegree\fi{}K to 20 min at 6\ifmmode^\circ\else\textdegree\fi{}K. For $Tl320\ifmmode^\circ\else\textdegree\fi{}$K, $\frac{{T}_{1}(^{37}\mathrm{Cl})}{{T}_{1}(^{35}\mathrm{Cl})}\ensuremath{\simeq}1.6$; for $Tg320\ifmmode^\circ\else\textdegree\fi{}$K, ${T}_{1}(^{35}\mathrm{Cl})={T}_{1}(^{37}\mathrm{Cl})$. For $6lTl90\ifmmode^\circ\else\textdegree\fi{}$K, the temperature dependence of ${T}_{1}$ is consistent with the Van Kranendonk theory, assuming that the relaxation is dominated by a single optical mode of frequency \ensuremath{\sim}33 ${\mathrm{cm}}^{\ensuremath{-}1}$, that is, by the rotary lattice mode. For $90lTl320\ifmmode^\circ\else\textdegree\fi{}$K, the ${T}_{1}$ data increase more slowly with temperature than can be accounted for by the Van Kranendonk theory. This is thought to be because the amplitude of the torsional oscillations has become so large as to invalidate the assumptions of a harmonic theory. For $320lTl490\ifmmode^\circ\else\textdegree\fi{}$K the ${T}_{1}$ data suggest that the energy of the torsional oscillations has become sufficient for the onset of a hindered rotational motion. This would lead to a nonresonant relaxation process and a ${T}_{1}$ determined solely by the residence time of a ${[\mathrm{P}\mathrm{T}{\mathrm{Cl}}_{6}]}^{=}$ ion in one of its equilibrium orientations. An order-of-magnitude calculation gives support to such an interpretation. The ${T}_{2}$ data are dominated by a magnetic dipolar spin-spin interaction. This is verified experimentally by observing the magnetic field dependence of the spin-echo beats.