The elastic properties of rutile transition metal dioxides XO$_{2}$ (X=Ru, Rh, Os, and Ir) are investigated using first-principles calculations based on density functional theory. Elastic constants, bulk modulus, shear modulus, and Youngs modulus as
well as Possion ratio are given. OsO$_2$ and IrO$_2$ show strong incompressibility. The hardness estimated for these dioxides shows that they are not superhard solids. The obtained Debye temperatures are comparative to those of transition metal dinitrides or diborides.
We study the electronic and lattice dynamical properties of compressed solid germane in the pressure range up to 200 GPa with density functional theory. A stable metallic structure, Aba2, with a base-centered orthorhombic symmetry was found to be the
lowest enthalpy phase for pressure from 23 to 177 GPa, suggesting an insulator to metal phase transition around 23 GPa. The Aba2 structure is predicted to have higher superconducting transition temperature than SiH4 reported recently, thus presenting new possibilities for exploring high temperature superconductivity in this hydrogen-rich system.
The structural, elastic and electronic properties of ReN are investigated by first-principles calculations based on density functional theory. Two competing structures, i.e., CsCl-like and NiAs-like structures, are found and the most stable structure
, NiAs-like, has a hexagonal symmetry which belongs to space group P63/mmc with a=2.7472 and c=5.8180 AA. ReN with hexagonal symmetry is a metal ultra-incompressible solid and has less elastic anisotropy. The ultra-incompressibility of ReN is attributed to its high valence electron density and strong covalence bondings. Calculations of density of states and charge density distribution, together with Mulliken atomic population analysis, show that the bondings of ReN should be a mixture of metallic, covalent, and ionic bondings. Our results indicate that ReN can be used as a potential ultra-incompressible conductor. In particular, we obtain a superconducting transition temperature T$_c$=4.8 K for ReN.
