On the ferroelastic nature of the scheelite-to-fergusonite phase transition in orthotungstates and orthomolybdates

New evidence supporting the ferroelastic nature of the pressure-induced scheelite-to-fergusonite phase transition in ABO4 orthotungstates and orthomolybdates (A = Ca, Sr, Ba, Pb, Eu and B = W, Mo) has been obtained from the analysis of Raman spectroscopy data. In the studied scheelite-type compounds, one external translational mode of Bg symmetry in the scheelite phase softens up to the transition pressure and then becomes a hard Ag mode in the fergusonite high-pressure phase. However, other scheelite-type compounds not undergoing the scheelite-to-fergusonite transition do not show softening of the Bg mode. The reported results have allowed us to establish a relationship between the square of the soft-mode frequency at ambient pressure and the transition pressure in the series PbWO4, BaWO4, EuWO4, SrWO4, and CaWO4 and also in the series PbMoO4, BaMoO4, SrMoO4, and CaMoO4. This fact is in accordance with the soft-mode theory of second-order phase transitions. The conclusions drawn confirm a previous characterization of the phase transition obtained from the Landau theory.