Highly coordinated metal dioxides in the cotunnite structure

Plane-wave pseudopotential calculations are employed to study various metal dioxides in the cotunnite-type phase. This geometry has the highest cation coordination number observed for any dioxide and is very likely to be a structure for the hardest form of high-pressure synthesized metal-oxide-based materials. Optimized equilibrium crystal structures are, in most cases, much denser than the ambient phases observed experimentally. A correlation is found between the d occupancy of the metal ion and the bulk modulus of its dioxide. We attribute these maxima to the structural symmetry of the cotunnite structure and the optimal way that the metal fits in between the oxygen ions. There is also a correlation of the bulk modulus and electronic states near the Fermi surface. Based on the numerical evidence presented in this work, suggestions are made for further experimental and theoretical investigations of metal dioxides under pressure.