Optical and calorimetric studies of K2TaF7
Mel’Nikova, Svetlana V.
Bogdanov, Evgeniy V.
Molokeev, Maxim S.
Laptash, Natalia M.
Flerov, Igor N.
Институт инженерной физики и радиоэлектроники
Базовая кафедра физики твердого тела и нанотехнологий
Journal Name:Journal of Fluorine Chemistry
Journal Quartile in Scopus:Q2
Journal Quartile in Web of Science:Q3
Bibliographic Citation:Mel’Nikova, Svetlana V. Optical and calorimetric studies of K2TaF7 [Текст] / Svetlana V. Mel’Nikova, Evgeniy V. Bogdanov, Maxim S. Molokeev, Natalia M. Laptash, Igor N. Flerov // Journal of Fluorine Chemistry. — 2019.
Optical and calorimetric experiments on K2TaF7 are performed in a wide temperature range. No features were found in the behavior of the birefringence Δnb(T), the angle of rotation of the indicatrix ϕ(T) and the heat capacity ΔCp(T) except for those associated with the Pnma↔P21/c phase transition. Structural transformation was characterized as strong first order “proper” ferroelastic accompanied by a huge angle ϕ ≈40° and strong pretransition phenomena in Δnb(T). Two contributions to the anomalies of the optical properties were found associated with the photoelastic effect and the transition parameter related linearly to the spontaneous deformation. Thermal treatments cause correlated changes in temperature and enthalpy of the phase transition, which leads to the invariance of the large magnitude of the corresponding entropy ΔS=22 J/mol·K which does not match the model with the absence of structural disorder in the Pnma phase.