Time-differential perturbed-angular-correlation study of phase transitions and molecular motions in K3(Hf,Zr)F7.

Abstract

Using time-differential perturbed-angular correlation, two thermal reversible phase transitions have been observed in both isostructural compounds ${\mathrm{K}}_{3}$${\mathrm{HfF}}_{7}$ and ${\mathrm{K}}_{3}$${\mathrm{ZrF}}_{7}$: the orthorhombic-to-tetragonal transition, occurring in ${\mathrm{K}}_{3}$${\mathrm{HfF}}_{7}$ around 275 K, and the tetragonal-to-disordered cubic transition, which developed from room temperature (RT) over a wide thermal range. It has been inferred that ionic complexes M${\mathrm{F}}_{7}^{3\mathrm{\ensuremath{-}}}$ in the tetragonal and cubic phases have the same structure, and that the cubic phase appears due to the onset of their reorientational motion. The detailed investigation performed on ${\mathrm{K}}_{3}$${\mathrm{HfF}}_{7}$ has allowed the determination of a two-stage mechanism for this movement: a slow diffusional reorientation near RT and an isotropic fast relaxation at higher temperatures. Activation energies for different thermal ranges have been calculated.