Nuclear-Quadrupole-Resonance Study of Lattice Dynamics in $K_{2}$ Pt ${Cl}_{6}$

10 October 1968

journal article
research article
Published by American Physical Society (APS) in Physical Review B

Vol. 174 (2), 359-369
https://doi.org/10.1103/physrev.174.359

Abstract

A pulsed magnetic-resonance spectrometer is used to measure pure NQR frequencies and relaxation times of the

{}^{35}{Cl}

and

{}^{37}{Cl}

nuclei in powdered samples of

K_{2}

{Cl}_{6}

over the temperature range

6 < T < 490 °

K. The ratio of frequencies is independent of temperature and equal to 1.2688±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 ∼38

{cm}^{- 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

{[P t {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 μsec at 490°K to 20 min at 6°K. For

T < 320 °

\frac{T_{1} ({}^{37}{Cl})}{T_{1} ({}^{35}{Cl})} ≃ 1.6

; for

T > 320 °

T_{1} ({}^{35}{Cl}) = T_{1} ({}^{37}{Cl})

. For

6 < T < 90 °

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 ∼33

{cm}^{- 1}

, that is, by the rotary lattice mode. For

90 < T < 320 °

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

320 < T < 490 °

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

Keywords

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Cited by 58 articles

Nuclear-Quadrupole-Resonance Study of Lattice Dynamics inK2PtCl6

Abstract

Keywords

Nuclear-Quadrupole-Resonance Study of Lattice Dynamics in $K_{2}$ Pt ${Cl}_{6}$