Elastic properties of potential superhard phases of RuO2

First-principles plane-wave pseudopotential and full-potential linearized-augmented plane-wave methods have been used to study the elastic and electronic properties of several potential superhard RuO2 phases. The structures, relative stabilities, and the elastic constants and bulk moduli of these phases have been calculated within local-density approximation (LDA) and generalized gradient approximation (GGA). In RuO2, the LDA and GGA approximations yield smaller and larger lattice constants, respectively, for the Pa (3) over bar-RuO2 structure. The internal structural parameter for oxygen atoms in the Pa (3) over bar structure has a volume dependence that differs from the experimental result and therefore implies a significantly different compression mechanism. The calculated bulk moduli are very similar for the fluorite and Pa (3) over bar structures and therefore apparently independent of the internal structural parameter. The structure and stability of a hypothetical orthorhombic RuO2 phase is investigated.