High pressure theoretical and experimental analysis of the bandgap of BaMoO4, PbMoO4, and CdMoO4

Abstract

We have investigated the origin of the bandgap of BaMoO4, PbMoO4, and CdMoO4 crystals on the basis of optical absorption spectroscopy experiments and ab initio electronic band structure, density of states, and electronic localization function calculations under high pressure. Our study provides an accurate determination of the bandgaps Eg and their pressure derivatives d E g / dP for BaMoO4 (4.43 eV, -4.4 meV/GPa), PbMoO4 (3.45 eV, -53.8 meV/GPa), and CdMoO4 (3.71 eV, -3.3 meV/GPa). The absorption edges were fitted with the Urbach exponential model which we demonstrate to be the most appropriate for thick crystals with direct bandgaps. So far, the narrowing of the bandgap of distinct PbMoO4 had been qualitatively explained considering only the presence of the Pb 6s levels at the top of its valence band. Its fast pressure dependent redshift and the occurrence of its direct bandgap away from Γ in contrast to the other scheelites had remained unsolved. Here, we show that in contrast to what had been proposed and different from the other scheelites, in PbMoO4, the bandgap takes place between the Pb 6s levels at the top of the valence band and the antibonding O 2p levels at the bottom of the conduction band. For this reason, the direct bandgap is pushed away from the zone center in order to allow s-p mixing. Its pressure dependence is one order of magnitude faster than that in the other scheelites due to two effects: its delocalized character and the higher compressibility of dodecahedral units, PbO8, compared to tetrahedral units, MoO4.