Barium oxide forms unique structures under pressure because of Ba—O bonding through the 5d orbitals. DFT bonding analysis shows that these bonds outweigh electrostatic interactions in BaO, but not CaO or SrO.

Abstract

The influence of metal d orbitals on the high-pressure structures of CaO, SrO, and BaO are investigated with DFT calculations and chemical bonding analysis. CaO and SrO undergo the B1-B2 transition, from the rock salt structure to the caesium chloride structure, while BaO undergoes a sequence of transitions from B1-B8, the NiAs structure, and then B8 − dB2, a distorted form of B2. DFT calculations of bond strengths show that the B8 and dB2 structures are stabilized relative to B1 and B2 by metal–oxygen covalency through the metal d orbitals. In BaO covalency outweighs electrostatics because of the large 5d orbitals of Ba, so the unique B8 and dB2 structures form. This marks an important expansion of the importance of d orbitals in group II chemistry.