Electronic structure calculation of cohesive properties of some Si6-zAlzOzN8-z spinels

The structures of a spinel form of Si6-zAlzOzN8-z are investigated using techniques of ab initio density functional plane wave electronic structure theory with soft pseudopotentials. Four spinel configurations are considered corresponding to z=0, 1, 2, and 4. In the case of z=2 (Si2AlON3 spinels), that has now been synthesized, a normal or inversed configuration is considered. Very small energy differences are found suggesting that a mixed random atomic structure is very likely for the Si2AlON3 spinels. Results across the other range of spinels show that incompressible structures are associated with larger concentrations of N. These structures also have the larger cohesive energies. All spinels have direct energy band gaps varying between 3 and 4 eV in the spinel 56-atom unit cell depending upon oxygen concentration