Angle-dispersive x -ray diffraction (ADXRD) and x -ray absorption near-edge structure (XANES) measurements have been performed on CaWO 4 and SrWO 4 up to pressures of approximately 20 GPa. Both materials display similar behavior in the range of pressures investigated in our experiments. As in the previously reported case of CaWO 4 , under hydrostatic conditions SrWO 4 undergoes a pressure-induced scheelite-to-fergusonite transition around 10 GPa. Our experimental results are compared to those found in the literature and are further supported by ab initio total energy calculations, from which we also predict the instability at larger pressures of the fergusonite phases against an orthorhombic structure with space group Cmca. Finally, a linear relationship between the charge density in the AO 8 polyhedra of ABO 4 scheelite-related structures and their bulk modulus is discussed and used to predict the bulk modulus of other materials, like hafnon.

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.

Abstract:We report the results of both angle-dispersive x -ray diffraction and x -ray absorption near-edge structure studies in BaWO 4 and PbWO 4 at pressures of up to 56 GPa and 24 GPa, respectively. BaWO 4 is found to undergo a pressure-driven phase transition at 7.1 GPa from the tetragonal scheelite structure (which is stable under normal conditions) to the monoclinic fergusonite structure whereas the same transition takes place in PbWO 4 at 9 GPa. We observe a second transition to another monoclinic structure which we identify as that of the isostructural phases BaWO 4 -II and PbWO 4 -III (space group P2 1 /n). We have also performed ab initio total-energy calculations which support the stability of this structure at high pressures in both compounds. The theoretical calculations further find that upon increase of pressure the scheelite phases become locally unstable and transform displacively into the fergusonite structure. The fergusonite structure is however metastable and can only occur if the transition to the P2 1 /n phases were kinetically inhibited. Our experiments in BaWO 4 indicate that it becomes amorphous beyond 47 GPa.

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Room-temperature Raman scattering has been measured in lead tungstate up to 17 GPa. We report the pressure dependence of all the Raman modes of the tetragonal scheelite phase (PbWO 4 -I, space group I4 1 /a), which is stable at ambient conditions. Upon compression the Raman spectrum undergoes significant changes around 6.2 GPa due to the onset of a partial structural phase transition to the monoclinic PbWO 4 -III phase (space group P2 1 /n). Further changes in the spectrum occur at 7.9 GPa, related to a scheelite-to-fergusonite transition. This transition is observed due to the sluggishness and kinetic hindrance of the I→III transition. Consequently, we found the coexistence of the scheelite, PbWO 4 -III, and fergusonite phases from 7.9 to 9 GPa, and of the last two phases up to 14.6 GPa. Further to the experiments, we have performed ab initio lattice dynamics calculations which have greatly helped us in assigning the Raman modes of the three phases and discussing their pressure dependence. Tel.: + 34 96 387 52 87, Fax: + 34 96 387 71 89 was shown that in BaWO 4 the onset of a partial scheelite-to-BaWO 4 -II phase transition occurs at 6.9 GPa; i.e., at lower pressure than the scheelite-to-fergusonite transition, which was observed at 7.5 GPa. These results are in good agreement with previous ab initio total-energy calculations and ADXRD and XANES measurements [12]. Furthermore, previous high-pressure Raman spectra in BaWO 4 [8] were interpreted on the basis of the results of Ref. 10. Similarly, we will show in this work that PbWO 4 suffers the same phase transitions than BaWO 4 and that the frequencies of the Raman modes in the high-pressure phases of PbWO 4 previously reported [9] can be completely understood on the light of the present work. Our results allow us to develop a picture of the structural behavior of PbWO 4 that solves apparent discrepancies among earlier experiments and theory. II. Experimental detailsThe PbWO 4 samples used in this study were obtained from scheelite-type bulk single crystals which were grown with the Czochralski method starting from raw powders having 5N purity [15,16]. Small platelets (100µm x 100µm x 30µm) were cleaved from these crystals along the {101} natural cleavage plane [17] and inserted in a diamond-anvil cell (DAC). Silicone oil was used as pressure-transmitting medium [18] and the pressure was determined by calibration with the ruby photoluminescence [19].Raman measurements at RT were performed in backscattering geometry using the 488 Å line of an Ar + -ion laser with a power of less than 100 mW before the DAC. Dispersed light was analyzed with a Jobin-Yvon T64000 triple spectrometer equipped with a confocal microscope in combination with a liquid nitrogen (LN)-cooled multi-channel CCD detector. Spectral resolution was better than 1 cm -1 and Ar and He plasma lines were used to calibrate the Raman and photoluminescence spectra.

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The optical-absorption edge of single crystals of CaWO 4 , SrWO 4 , BaWO 4 , and PbWO 4 has been measured under high pressure up to ~20 GPa at room temperature.From the measurements we have obtained the evolution of the band-gap energy with pressure. We found a low-pressure range (up to 7-10 GPa) where alkaline-earth tungstates present a very small E g pressure dependence (-2.1 < dE g /dP < 8.9 meV/GPa).In contrast, in the same pressure range, PbWO 4 has a pressure coefficient of -62 meV/GPa. The high-pressure range is characterized in the four compounds by an abrupt decrease of E g followed by changes in dE g /dP. The band-gap collapse is larger than 1.2 eV in BaWO 4 . We also calculated the electronic-band structures and their pressure evolution. Calculations allow us to interpret experiments considering the different electronic configuration of divalent metals. Changes in the pressure evolution of E g are correlated with the occurrence of pressure-induced phase transitions. The band structures for the low-and high-pressure phases are also reported. No metallization of any of the compounds is detected in experiments nor is predicted by calculations.