Structural properties, electronic structure and bonding of Bam(PO4)6X2 (X = F, Cl and Br)

A foundation with a thorough analysis of the structural and electronic properties of barium apatites Ba-10(PO4)(6)X-2 (X = F, Cl and Br), including the band structure, density of states and bonding, was laid by the use of four exchange-correlation functionals (PBE, PBE0, B3LYP, and sX-LDA). All four functionals give values for lattice parameters that are in fairly good agreement with experiment, although the sX-LDA hybrid functional is found to be the most accurate one. The order of stability of these barium apatites is Ba-10(PO4)(6)F-2 > Ba-10(PO4)(6)Cl-2 > Ba-10(PO4)(6)Br-2. Band gaps are found to be, in general, different and increase for each barium apatite when the proportion of Hatree-Fock (HF) exchange contained in the exchange-correlation functional increases. The bonding results show that barium apatites are dominated by PO4 groups and Ba channels populated by X ions. The Mulliken population analysis and charge density are also performed to investigate the bonding characters. Overall, due to its mixing of 100% HF exchange, the sX-LDA functional seems to be the best choice as it provides coincident lattice parameters and bond lengths with experiment, and extremely reasonable cohesive energies and formation enthalpies. (C) 2016 Elsevier B.V. All rights reserved.