Calculating the Lattice Dynamics in the RFe3(BO3)(4) Crystals in the Quasi-Harmonic Approximation

Pavlovskii, M. S.; Andryushin, N. D. PHYSICS OF THE SOLID STATE Том: 61 Выпуск: 11 Стр.: 2019-2025, 10.1134/S10 6378341911026X

 The frequencies of lattice vibrations in the RFe3(BO3)4 (R = Pr, Nd, Tb, Dy, or Ho) crystals in the high-temperature R32 phase and their temperature dependence have been calculated using the quasi-harmonic approximation. It has been found that, at the boundary point Λ of the Brillouin zone, the frequency of the unstable vibration mode the structural phase transition R32 → P3121 is related to strong changes with temperature in the TbFe3(BO3)4, DyFe3(BO3)4, and HoFe3(BO3)4 crystals. With increasing temperature, the frequency of the soft mode stabilizes and takes a real value. No significant changes in the phonon spectra, including the boundary point Λ, with increasing temperature for the PrFe3(BO3)4 and NdFe3(BO3)4 crystals have been observed.