5d Orbital Covalency Controls the High-Pressure Polymorphism of BaO.

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.