Pressure-induced irreversible phase transition in KSc(MoO₄)₂

We have studied the layered double molybdate KSc(MoO₄)₂ under high pressure through Raman scattering experiments. At about 2.0 GPa we observed drastic changes in the Raman spectra associated with a structural phase transition. The high-pressure phase is characterized by the presence of a large number of Raman modes, thus indicating that the high-pressure phase has a lower symmetry than the starting trigonal phase. By performing lattice-dynamics calculations for the trigonal phase we have been able to make a definite assignment of the various Raman modes observed in the experiments, and to get fundamental insights about the mechanism driving the structural modifications in molybdates that are associated with the electrostatic repulsion between the oxygen atoms due to the decreasing of Mo–O bonds and O–O distances. Upon releasing pressure the spectrum of the starting trigonal phase was not recovered, thereby indicating that pressure induced an irreversible phase transition in the KSc(MoO₄)₂ crystal.