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

Abstract

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 Pb₇(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 Pb₇(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 Pb₇(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 Pb₇(OH)₃(CO₃)₃(BO₃).