Several types of defect structures in PbWO4 and CaMoO4 were studied within the framework of density functional theory. While PbWO4 was currently of greater technological interest, it was possible to carry out more extensive calculations for CaMoO4, including lattice relaxation, large simulation cells, and more complicated defects. The structural and chemical similarity of the two materials suggested that their defect properties may also be similar. The electronic structure of isolated O vacancies, O and Pb or Ca double vacancies, and substitutional Y were modeled by using a super-cell approximation. It was found that the main effect of O vacancies in PbWO4 and CaMoO4 was the introduction of states of W or Mo d character into the band gap. The energies of these defect states were very sensitive to their occupancy. An isolated O vacancy produced a doubly occupied defect state below the conduction band. Removing charge from this defect state lowers its energy and caused additional states of W or Mo d character to move into the band gap. Large super-cell simulations for the Ca and O double vacancy in an unrelaxed or slightly relaxed structure produced an unstable electronic structure suggesting the possibility of more extensive lattice distortion. In addition, the preliminary results of simulations of interstitial O atoms in CaMoO4 were also presented. This revealed a relatively stable configuration with the interstitial O forming a weak bond between two MoO4 clusters.

Electronic Structure of Oxygen-Related Defects in PbWO4 and CaMoO4 Crystals. Y.B.Abraham, N.A.W.Holzwarth, R.T.Williams, G.E.Matthews, A.R.Tackett: Physical Review B, 2001, 64[24], 245109 (10pp)