Electronic structure of oxygen-related defects inPbWO4andCaMoO4crystals

Abstract

Several types of defect structures in ${\mathrm{PbWO}}_4$ and ${\mathrm{CaMoO}}_4$ are studied within the framework of density functional theory. While ${\mathrm{PbWO}}_4$ is currently of greater technological interest, we were able to carry out more extensive calculations for ${\mathrm{CaMoO}}_4,$ including lattice relaxation, large simulation cells, and more complicated defects. The structural and chemical similarity of the two materials suggests that their defect properties may also be similar. The electronic structure of isolated oxygen vacancies, oxygen and Pb or Ca double vacancies, and substitutional Y are modeled using a supercell approximation. We find that the main effect of oxygen vacancies in ${\mathrm{PbWO}}_4$ and ${\mathrm{CaMoO}}_4$ is the introduction of states of W or Mo d character into the band gap. The energies of these defect states are very sensitive to their occupancy. An isolated O vacancy produces a doubly occupied defect state below the conduction band. Removing charge from this defect state lowers its energy and causes additional states of W or Mo d character to move into the band gap. Large supercell simulations for the Ca and O double vacancy in an unrelaxed or slightly relaxed structure produce an unstable electronic structure suggesting the possibility of more extensive lattice distortion. In addition, we also present preliminary results of simulations of interstitial oxygen atoms in ${\mathrm{CaMoO}}_4,$ finding a relatively stable configuration with the interstitial O forming a weak bond between two ${\mathrm{MoO}}_4$ clusters.