The tetragonal to orthorhombic ferroelastic phase transition between rutile-and CaCl 2 -type SiO 2 at high pressures is studied using first-principles calculations and the Landau free-energy expansion. The phase transition is systematically investigated in terms of characteristic phonon modes with B 1g and A g symmetries, shear moduli, transverse-acoustic mode, rotation angle of the SiO 6 octahedra, spontaneous symmetry-breaking and volume strains, and enthalpy. The results show that these physical behaviors at the transition are well described using the Landau free-energy expansion parametrized by the first-principles calculations.

First-principles calculations based on hybrid Hartree-Fock density functionals provide a clear picture of the defect energetics and electronic structure in ZnO. Among the donorlike defects, the oxygen vacancy and hydrogen impurity, which are deep and shallow donors, respectively, are likely to form with a substantial concentration in n-type ZnO. The zinc interstitial and zinc antisite, which are both shallow donors, are energetically much less favorable. A strong preference for the oxygen vacancy and hydrogen impurity over the acceptorlike zinc vacancy is found under oxygen-poor conditions, suggesting that the oxygen vacancy contributes to nonstoichiometry and that hydrogen acts as a donor, both of which are without significant compensation by the zinc vacancy. The present results show consistency with the relevant experimental observations.

The formation energies and electronic structure of native defects in tin monoxide are investigated by first-principles calculations. Equilibrium defect concentrations, which are obtained using the calculated formation energies and charge neutrality, indicate that the tin vacancy is the dominant defect under oxygen-rich conditions. It forms shallow acceptor states, suggesting that the p-type conductivity of tin monoxide originates from the tin vacancy. The equilibrium concentration of the oxygen interstitial is comparable with the tin vacancy at elevated temperatures. However, it is hardly ionized and therefore not expected to contribute to the conductivity. The concentrations of donorlike defects such as the tin interstitial and the oxygen vacancy are low enough not to compensate holes generated by the tin vacancy.

Systematic and automatic calculations of the electronic band structure are a crucial component of computationally-driven high-throughput materials screening. An algorithm, for any crystal, to derive a unique description of the crystal structure together with a recommended band path is indispensable for this task. The electronic band structure is typically sampled along a path within the first Brillouin zone including the surface in reciprocal space. Some points in reciprocal space have higher site symmetries and/or have higher constraints than other points regarding the electronic band structure and therefore are likely to be more important than other points. This work categorizes 2 points in reciprocal space according to their symmetry and provides recommended band paths that cover all special wavevector ( k -vector) points and lines necessarily and sufficiently. Points in reciprocal space are labeled such that there is no conflict with the crystallographic convention. The k -vector coefficients of labeled points, which are located at Brillouin zone face and edge centers as well as vertices, are derived based on a primitive cell compatible with the crystallographic convention, including those with axial ratio-dependent coordinates. Furthermore, we provide an open-source implementation of the algorithms within our SeeK-path python code, to allow researchers to obtain k -vector coefficients and recommended band paths in an automated fashion. Finally, we created a free online service to compute and visualise Brillouin Zone, labeled k -points and suggested band paths for any crystal structure, that we made available at http://www.materialscloud.org/tools/seekpath/ .