Electric field gradient tensor temperature dependence

Spin-Orbit Coupling and Temperature-Dependent Electric Field Gradient Tensor... 

It has been found that fluctuations in the orientation of the electric field gradient tensor due to torsional vibrations of the CIO3 group of KCIO3 account for the temperature dependence of the NQR frequency at low temperatures through changes in the values of the qa. Above 80 K, however, this does not adequately account for the variation since an expansion of the lattice occurs and the molecular vibrations cannot be approximated by harmonic oscillators. As the lattice expands, the distance between the ions increases, causing an additional decrease of qa and an increase in the sensitivity of the thermometer. 

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