The nitridoborate nitrides Mg3[BN2]N and Ca3[BN2]N – electronic structure and chemical bonding

The nitridoborates Mg-3[BN2]N (P6(3)/mmc) and Ca-3[BN2]N (P4/mmm) are electron-precise compounds with discrete linear [BN2](3-) and isolated N3- anions. Electronic structure calculations reveal pronounced B-N bonding within the [BN2](3-) units with more covalent Mg-N vs. Ca-N bonding. Total energy calculation for hexagonal normal-pressure Mg-3[BN2]N, orthorhombic high-pressure Mg-3[BN2]N and a hypothetical Ca-3[BN2]N-type tetragonal Mg-3[BN2]N modification revealed that the hexagonal modification is the ground state structure. The band structure for orthorhombic high-pressure Mg-3[BN2]N indicates a substantial metallization (delocalization in the high-pressure regime). This peculiar result calls for a reinvestigation of high-pressure Mg-3[BN2]N under different high-pressure high-temperature conditions along with physical property studies.