Phase transformation in some hexahydrated metal(II) fluorogermanates and fluorotitanates: Optical and differential scanning calorimetry studies

https://doi.org/10.1016/j.jfluchem.2022.110048

Large single crystals of hydrated fluorides CoGeF6∙6H2O, FeGeF6∙6H2O, CoTiF6∙6H2O, and ZnTiF6∙6H2O were obtained and their birefringence and heat capacities have been studied. The compounds undergo a phase transition (PT) of the first order, which is accompanied by a change in optical anisotropy, twinning, and a decrease in the compound symmetry from trigonal (m, ) to monoclinic (Р21/c) in accordance with the rotation of the optical indicatrix around only one direction. The value of the entropy change during the PT is small for the first two compounds (0.36R and 0.34R) and large for the last two (0.76R and 0.95R).