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 ABO(4) orthotungstates and orthomolybdates (A = Ca, Sr, Ba, Pb, Eu and B = W, Mo) has been obtained from the analysis of Rarnan 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. (C) 2008 Elsevier Ltd. All rights reserved.