Influence of an oxygen vacancy on the electronic structure of the asymmetric mixed borate-carbonate Pb7O(OH)3(CO3)3(BO3)

The influence of an oxygen vacancy on the electronic properties of a mixed borate-carbonate compound Pb7O(OH)(3)(CO3)(3)(BO3) is studied. We report calculations of the electronic band structure, the angular momentum resolved projected density of states and the valence electronic charge density distribution. The full-potential method within the generalized gradient approximation (PBE-GGA) and a recently modified Becke-Johnson potential (mBJ) shows an indirect band gap of 3.34 eV (PBE-GGA) and 3.56 eV (mBJ) for Pb7O(OH)(3)(CO3)(3)(BO3), while it is a direct gap of 1.10 eV (PBE-GGA) and 1.61 eV (mBJ) for the compound Pb-7(OH)(3)(CO3)(3)(BO3) which has one O vacancy. Thus, the O vacancy causes a significant reduction in the band gap and also changes it from indirect to direct. The band gap reduction in Pb-7(OH)(3)(CO3)(3)(BO3) is attributed to the appearance of new energy bands inside the energy gap of Pb7O(OH)(3)(CO3)(3)(BO3). The angular momentum resolved projected density of states for Pb7O(OH)(3)(CO3)(3)(BO3) and Pb-7(OH)(3)(CO3)(3)(BO3) show that there exists a strong hybridization between B and O of the BO3 group and between C and O of the CO3 group. The valence electronic charge density for both compounds is presented. It reveals the origin of chemical bonding characteristics and the influence of the O vacancy. After a careful comparison, it is found that the crystal structure of Pb7O(OH)(3)(CO3)(3)(BO3) without an O vacancy can be considered as a parent phase of defect Pb-7(OH)(3)(CO3)(3)(BO3).