High-pressure crystal chemistry of scheelite-type tungstates and molybdates

Unit-cell parameters and crystal structures of CaWO₄(scheelite) and CaMoO₄ (powellite) have been determined at several pressures to 5 8 GPa, and unit-cell parameters of PbMoO₄ (wulfemte), PbWO₄ (stolzite) and CdMoO₄ have been measured at pressures to 6 0 GPa All five tetragonal scheelite-type compounds compress anisotropically, with the (itc) axis 1 2 to 1 9 times more compressible than a. In both CaWO₄ and CaMoO₄ the cation tetrahedra (with W⁶⁺ or Mo⁶⁺) behave as rigid structural elements with no observed cation-oxygen compression (W-O and Mo-O bond compression < 0001 GPa^?1) Compression of the eight-coordinated calcium polyhedron, on the other hand, is comparable to bulk compression of the compounds (Ca-O bond compression = 0005 ± 0 001 GPa^?1) Amsotropies in the pressure response of the calcium polyhedron, which is more compressible parallel to c than perpendicular to c, result in the amsotropic unit-cell compression Bulk moduli of the five compounds (with K' assumed to be 4) are CaWO₄ (68 ± 9 GPa), CaMoO₄ (81 5 ± 0 7 GPa), PbWO₄ (64 ± 2 GPa), PbMoO₄ (64 ± 2 GPa), and CdMoO₄ (104 ± 2 GPa) No reversible transitions to the monoclinic (fergusomte) distortion of scheelite were observed in these compounds Pressure-volume data for PbWO₄, however, display strong positive curvature (K'_calc) = 23 ± 2) up to about 5 GPa, at which pressure crystals appear to undergo a first-order phase transition The relatively large curvature may be a premonitory effect pnor to a reconstructive transition Structural changes in these compounds with increasing pressure are qualitatively similar to changes that result from isobanc cooling or substitution of a smaller cation in the eight-coordinated site.