Time-differential perturbed-angular-correlation study of phase transitions and molecular motions in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">K</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>(Hf,Zr)<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">F</mml:mi></mml:mrow><mml:mrow><mml:mn>7</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

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.