Phase stabilities of monoclinic oxoborates LaB 3O 6 and GdB 3O 6 in C121 and I12 /a1 phase—Energetics and chemical bonds derived from first-principles calculations

Abstract

The computational study on the relative phase stabilities between two monoclinic polymorphs of the lanthanide-containing oxoborates LaB 3O 6 and GdB 3O 6 is presented at the first-principles density functional theory gradient-corrected B3PW level. The hypothetical monoclinic α-BiB 3O 6-like C121 non-centrosymmetric crystal structures were assumed for LaB 3O 6 and GdB 3O 6 and the corresponding geometries were calculated and compared with their monoclinic I12 /a1 centrosymmetric structures. The enthalpy–pressure correlations were calculated and the first-principles chemical bonds based on the crystal orbital overlapping population were quantitatively analyzed for LaB 3O 6 and GdB 3O 6. The chemical bonds between the central cation (La (III)/Gd (III)) and coordinated oxygen atoms rather than the B–O bonds in the borate units are found to stabilize the I12 /a1 centrosymmetric LaB 3O 6 and GdB 3O 6 structures with respect to the C121 non-centrosymmetric counterparts.