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

Abstract New evidence supporting the ferroelastic nature of the pressure-induced scheelite-to-fergusonite phase transition in ABO 4 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 B g symmetry in the scheelite phase softens up to the transition pressure and then becomes a hard A g 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 B g 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 PbWO 4 , BaWO 4 , EuWO 4 , SrWO 4 , and CaWO 4 and also in the series PbMoO 4 , BaMoO 4 , SrMoO 4 , and CaMoO 4 . 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.