A density functional study of the high-pressure chemistry of MSiN(2)(M = Be, Mg, Ca): prediction of high-pressure phases and examination of pressure-induced decomposition.

Normal pressure modifications and tentative high-pressure phases of the nitridosilicates MSiN(2) with M =  Be, Mg, or Ca have been thoroughly studied by density functional methods. At ambient pressure, BeSiN(2) and MgSiN(2) exhibit an ordered wurtzite variant derived from idealized filled β-cristobalite by a C1-type distortion. At ambient pressure, the structure of CaSiN(2) can also be derived from idealized filled β-cristobalite by a different type of distortion (D1-type). Energy-volume calculations for all three compounds reveal transition into an NaCl superstructure under pressure, affording sixfold coordination for Si. At 76 GPa BeSiN(2) forms an LiFeO(2)-type structure, corresponding to the stable ambient-pressure modification of LiFeO(2), while MgSiN(2) and CaSiN(2) adopt an LiFeO(2)-type structure, corresponding to a metastable modification (24 and 60 GPa, respectively). For both BeSiN(2) and CaSiN(2) intermediate phases appear (for BeSiN(2) a chalcopyrite-type structure and for CaSiN(2) a CaGeN(2)-type structure). These two tetragonal intermediate structures are closely related, differing mainly in their c/a ratio. As a consequence, chalcopyrite-type structures exhibit tetrahedral coordination for both cations (M and Si), whereas in CaGeN(2)-type structures one cation is tetrahedrally (Si) and one bisdisphenoidally (M) coordinated. Both structure types, chalcopyrite and CaGeN(2), can also be derived from idealized filled β-cristobalite through a B1-type distortion. The group-subgroup relation of the BeSiN(2)/MgSiN(2), the CaSiN(2), the chalcopyrite, the CaGeN(2) and the idealized filled β-cristobalite structure is discussed and the displacive phase transformation pathways are illustrated. The zero-pressure bulk moduli were calculated for all phases and have been found to be comparable to compounds such as α- Si(3)N(4), CaIrO(3) and Al(4)C(3). Furthermore, the thermodynamic stability of BeSiN(2), MgSiN(2) and CaSiN(2) against phase agglomerates of the binary nitrides M(3)N(2) and Si(3)N(4) under pressure are examined.