Ferroelastic phase transition in the family of double fluoride crystals by Raman spectroscopy

In order to clarify ordering mechanisms of ferroic phase transitions in double fluoride salts (NH₄)₃SnF₇, (NH₄)₃GeF₇ and (NH₄)₃TiF₇, their Raman scattering spectra have been studied in wide ranges of frequencies (20–3400 cm⁻¹) and temperatures (8–410 K). Substitution of the central ion in the octahedron groups was found to change not only the phase transition types and sequences, but to modify the types of structural disorder as well, which is a special feature of these high-symmetry fluoride crystals.