We have carried out optical-absorption and reflectance measurements at room temperature in single crystals of AWO 4 tungstates (A = Ba, Ca, Cd, Cu, Pb, Sr, and Zn).

Room temperature angle-dispersive x-ray diffraction measurements on zircontype EuVO 4 , LuVO 4 , and ScVO 4 were performed up to 27 GPa. In the three compounds we found evidence of a pressure-induced structural phase transformation from zircon to a scheelite-type structure. The onset of the transition is near 8 GPa, but the transition is sluggish and the low-and high-pressure phases coexist in a pressure range of about 10 GPa. In EuVO 4 and LuVO 4 a second transition to a M-fergusonite-type phase was found near 21 GPa. The equations of state for the zircon and scheelite phases are also determined. Among the three studied compounds, we found that ScVO 4 is less compressible than EuVO 4 and LuVO 4 , being the most incompressible orthovanadate studied to date. The sequence of structural transitions and compressibilities are discussed in comparison with other zircon-type oxides.

The electronic band-structure and band-gap dependence on the d character of A 2+ cation in AWO 4 wolframitetype oxides is investigated for different compounds (A = Mg, Zn, Cd, and Mn) by means of optical-absorption spectroscopy and first-principles density-functional calculations. High pressure is used to tune their properties up to 10 GPa by changing the bonding distances establishing electronic to structural correlations. The effect of unfilled d levels is found to produce changes in the nature of the band gap as well as its pressure dependence without structural changes. Thus, whereas Mg, Zn, and Cd, with empty or filled d electron shells, give rise to direct and wide band gaps, Mn, with a half-filled d shell, is found to have an indirect band gap that is more than 1.6 eV smaller than for the other wolframites. In addition, the band gaps of MgWO 4 , ZnWO 4 , and CdWO 4 blue-shift linearly with pressure, with a pressure coefficient of approximately 13 eV/GPa. However, the band gap of multiferroic MnWO 4 red-shifts at −22 meV/GPa. Finally, in MnWO 4 , absorption bands are observed at lower energy than the band gap and followed with pressure based on the Tanabe-Sugano diagram. This study allows us to estimate the crystal-field variation with pressure for the MnO 6 complexes and how it affects their band-gap closure.

When monoclinic monazite-type LaVO 4 (space group P2 1 /n) is squeezed up to ∼12 GPa at room temperature, a phase transition to another monoclinic phase has been found. The structure of the high-pressure phase of LaVO 4 is indexed with the same space group (P2 1 /n), but with a larger unit-cell in which the number of atoms is doubled. The transition leads to an 8% increase in the density of LaVO 4 . The occurrence of such a transition has been determined by x-ray diffraction, Raman spectroscopy, and ab initio calculations. The combination of the three techniques allows us to also characterize accurately the pressure evolution of unit-cell parameters and the Raman (and IR)-active phonons of the low-and high-pressure phase. In particular, roomtemperature equations of state have been determined. The changes driven by pressure in the crystal structure induce sharp modifications in the color of LaVO 4 crystals, suggesting that behind the monoclinic-to-monoclinic transition there are important changes of the electronic properties of LaVO 4 . † Corresponding author, email: daniel.errandonea@uv.es † † Present address: