A density functional study of the high-pressure chemistry ofMSiN2(M = Be, Mg, Ca): prediction of high-pressure phases and examination of pressure-induced decomposition

Abstract

Normal pressure modifications and tentative high-pressure phases of the nitridosilicatesMSiN2 withM = Be, Mg, or Ca have been thoroughly studied by density functional methods. At ambient pressure,BeSiN2 andMgSiN2 exhibit an ordered wurtzite variant derived from idealized filledβ-cristobalite by a C1-type distortion. At ambient pressure, the structure ofCaSiN2 can also be derivedfrom idealized filled β-cristobalite by a different type of distortion (D1-type). Energy–volumecalculations for all three compounds reveal transition into an NaClsuperstructure under pressure, affording sixfold coordination for Si. At 76 GPaBeSiN2 formsan LiFeO2-type structure, corresponding to the stable ambient-pressure modification ofLiFeO2, whileMgSiN2 andCaSiN2 adoptan LiFeO2-type structure, corresponding to a metastable modification (24 and 60 GPa, respectively). For bothBeSiN2 andCaSiN2 intermediate phases appear(for BeSiN2 a chalcopyrite-typestructure and for CaSiN2 a CaGeN2-type structure). These two tetragonal intermediate structures are closely related, differing mainly intheir c/a ratio. As a consequence, chalcopyrite-type structures exhibittetrahedral coordination for both cations (M and Si), whereas inCaGeN2-type structures one cation is tetrahedrally (Si) and onebisdisphenoidally (M) coordinated. Both structure types, chalcopyrite andCaGeN2, can also be derivedfrom idealized filled β-cristobalite through a B1-type distortion. The group–subgroup relation of theBeSiN2/MgSiN2, theCaSiN2, the chalcopyrite,the CaGeN2 and theidealized filled β-cristobalite structure is discussed and the displacive phase transformationpathways are illustrated. The zero-pressure bulk moduli were calculated forall phases and have been found to be comparable to compounds such asα-Si3N4,CaIrO3 andAl4C3. Furthermore, the thermodynamic stability ofBeSiN2,MgSiN2 andCaSiN2 against phase agglomerates of the binary nitridesM3N2 andSi3N4 under pressure are examined.