Phonon dynamics simulation of TlH 2 PO 4 crystal

Lattice dynamics of TlH 2 PO 4 crystal was simulated within the atomistic semi‐empirical approach in high‐temperature Pbcn and ferroelastic P 2 1 / b phases. Phonon dispersion relations, partial density of phonon states, dispersion of mean square displacements and isotropic temperature factors were calculated. Using both the lattice dynamics simulation and phenomenological Landau–Ginzburg analysis it was shown that the ferroelastic phase transition occurs due to the bilinear interaction between the soft B 3g optical and transverse acoustic TA YZ phonon modes. The oxygen–hydrogen interactions within the shorter O 1 H 1 ···O 2 hydrogen bonds play the key role at the ferroelastic phase transition.