Phase stabilities of monoclinic oxoborates LaB3O6 and GdB3O6 in C121 and I12/a1 phase—Energetics and chemical bonds derived from first-principles calculations

The computational study on the relative phase stabilities between two monoclinic polymorphs of the lanthanide-containing oxoborates LaB3O6 and GdB3O6 is presented at the first-principles density functional theory gradient-corrected B3PW level. The hypothetical monoclinic α-BiB3O6-like C121 non-centrosymmetric crystal structures were assumed for LaB3O6 and GdB3O6 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 LaB3O6 and GdB3O6. 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 LaB3O6 and GdB3O6 structures with respect to the C121 non-centrosymmetric counterparts.