Raman scattering and phase transitions in (NH4)(3)TiF7

Raman spectra of (NH₄)₃TiF₇ within spectral range 10–3,400 cm⁻¹ at temperatures 8–370 K are recorded, which revealed 2 phase transitions at temperatures T₁ = 355 K and T₂ = 283 K. First principles calculations of lattice dynamics and assignment of the observed lines to specific vibrational types are performed. The first phase transition is classified as an “order–disorder” one. Manifestation of 97 cm⁻¹ vibration (mixed translational [NH₄ + F7]) evidences the creation of the hydrogen bond between ammonium group and free fluorine atom, indicating to reconstructive type of phase transition an T₂ = 283 K. The emergence of a large number of vibrational lines in Raman spectra at low temperatures proves symmetry lowering of NH₄ tetrahedra with the simultaneous ordering of TiF₆ octahedra.