Abstract
Atomic and electronic structure of nitridoaluminosilicate CaAlSiN~3~ (Cmc 2~1~, No. 36), a distorted AlNโbased wurtzite superstructure with Al and Si disordered on 8b site and Ca occupying 4a site, has been investigated by firstโprinciples pseudopotential method based on density functional theory. The random distribution of Al/Si atoms is treated in two ways: (1) virtual crystal approximation (VCA) with a mixed Al/Si pseudopotential, (2) several assumptions of Al/Si distribution order in the primitive unit cell. Geometry optimization based on the aliovalent VCA reproduces the experimental orthorhombic structure as well as the bond lengths. The calculations of Al/Siโordered models lead to monoclinic structures that appear sufficiently close to the experimental structure, with the averages of the optimized AlโN/SiโN bond lengths corresponding to the experimental ones. Relative stability among the Al/Siโordered models appears consistent with Pauling's second crystal rule. The random Al/Si distribution leading to the experimental determination of the system as Cmc 2~1~ can be explained by crystal symmetry, energetics among the ordered models, and configurational entropy effect. The electronic structure based on the VCA appears similar with those of the Al/Siโordered models. All the band structures indicate that the system has indirect band gap. (ยฉ 2006 WILEYโVCH Verlag GmbH & Co. KGaA, Weinheim)