Selected optical-absorption spectra for different pressures in each of the materials studied.
Experimental evolution of the bandgap with pressure. solid symbols represent experimental data collected upon pressure increase, while open symbols correspond to experimental data collected upon pressure release. The solid lines are the fitting curves and the vertical dotted lines represent transition pressures. The different optical transitions related to the fundamental gap are also indicated according to the interpretation of the theoretical calculations. Error bars for BaWO4 data are smaller than symbols size.
Electronic band-structure dispersion curves. (a) For CaWO4, SrWO4, PbWO4, and BaWO4 in the scheelite structure at ambient pressure. (b) For fergusonite CaWO4 and SrWO4 at 12.9 and 14 GPa, respectively, for BaWO4-II at 11.5 GPa, and for PbWO4-III at 12.8 GPa.
Diagram of the Brillouin zone for the scheelite, fergusonite, and monoclinic P21/n structures.
(Color online) Density of states of the scheelite phase at (a) ambient pressure, and (b) 5.5 GPa.
Calculated evolution of the bandgap with pressure for the stable phase in each pressure range. The lines are fitting curves and the symbols represent the points of the Brillouin zone where the optical transition takes place. For the fergusonite phases of CaWO4 and SrWO4, the evolution of the Γ→Γ and the Γ→Y gaps is shown to denote the band-crossing present in these materials. For the scheelite phase of BaWO4 and PbWO4, we show the direct and indirect gaps.
Experimental and theoretical values of the bandgaps at ambient pressure, Eg(0) in eV, and pressure coefficients, dEg/dP, in meV/GPa. Values are obtained from linear fits at the different pressure ranges from Fig. 2 (experiment) and Fig. 6 (theory). We also listed the k-points for the top of the valence band and bottom of the conduction band. Pressure is given in GPa.
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