ELECTRONIC BAND STRUCTURES OF THE SCHEELITE MATERIALS CAMOO4, CAWO4, PBMOO4, AND PBWO4

Density-functional calculations using the linearized-augmented-plane-w ave method were carried out for the scheelite materials CaMoO4, CaWO4, PbMoO4, and PbWO4 in order to determine their ground-state electronic properties. The results indicate that CaMoO4 and CaWO4 have direct ba nd gaps at the center of the Brillouin zone, while PbMoO4 and PbWO4 ha ve band extrema at wave vectors away from the zone center with possibl y indirect band gaps. The magnitudes of the band gaps increase in the order PbMoO4<PbWO4 <CaMoO4<CaWO4. The valence and conduction bands nea r the band gap are dominated by molecular orbitals associated with the MoO4-alpha and WO4(-alpha) ions, where alpha approximate to 2. The va lence-band widths are 5 and 5.5 eV for the Ca and Pb materials, respec tively. In the Pb materials, the Pb 6s states form narrow bands 1 eV b elow the bottom of the valence bands, and also hybridize with states t hroughout the valence bands, while the Pb 6p states hybridize with sta tes throughout the conduction bands. In the Ca materials, the Ca 3d st ates contribute to a high density of states 3-4 eV above the bottom of the conduction bands.