Investigation of thermal properties and structure of complex fluoride K3ZrF7

Gorev, Mikhail, V; Molokeev, Maxim S.; Kartashev, Andrey, V; Pogoreltsev, Evgeniy, I; Mel'nikova, Svetlana, V; et al. Journal Of Fluorine Chemistry.

X-ray, calorimetric and dilatometric studies of K3ZrF7 revealed the existence of the phase transition Fm-3m ↔ R-3m at T0 = 320 K. The structural model assumes a disorder of a pentagonal bipyramid ZrF7 with the following ratio of equivalent orientation positions in the initial and distorted phases: 12 to 6. A good agreement was found between the experimental and model-calculated changes in strain and entropy during the phase transition. A comparative analysis of entropy and structural parameters of related fluorides K3ZrF7 - (NH4)2KZrF7 - (NH4)3ZrF7 was performed. The anomalous behavior of thermodynamic properties in the range 140−230 K is not typical for phase transitions and is accompanied by a significant change in the entropy of the crystal lattice.