Etude par correlation angulaire γ-γ perturbee des aspects statiques et dynamiques de la structure du heptafluorohafniate de sodium

Abstract

We have studied the temperature dependence of the perturbation of the differential γ-γ angular correlation of the 181Ta 133–482 keV cascade in sodium heptafluorohafnate. In the 20–300°C temperature region, we observed a drastic variation of the quadrupole interaction parameters when raising the temperature from 100 to 180°C. Such a variation is due to the existence of a phase transition corresponding to an orthorhombic-quadratic transformation of the lattice. In the low temperature phase ( T < 120°C) the perturbation is static and the nuclear probe is located at two equally populated sites. In the high temperature phase ( T > 180°C) the perturbation arises from the combination of static and dynamic interactions. The dynamic part of the interaction could be analysed within the isotropic perturbation model and was ascribed to jumps of the fluorine atoms onto the vertices of the cubic coordination polyhedra of the hafnium in the HfF 7 3− ion. The activation energy of the process is 0.13 ±0.03 eV. The idea of an order-disorder phenomena occuring in the course of the phase transition is consistently supported by these observations. Our results yield also valuable informations about the nature of the hafnium-fluor bond in this compound.