The elastic constant tensor of an inorganic compound provides a complete description of the response of the material to external stresses in the elastic limit. It thus provides fundamental insight into the nature of the bonding in the material, and it is known to correlate with many mechanical properties. Despite the importance of the elastic constant tensor, it has been measured for a very small fraction of all known inorganic compounds, a situation that limits the ability of materials scientists to develop new materials with targeted mechanical responses. To address this deficiency, we present here the largest database of calculated elastic properties for inorganic compounds to date. The database currently contains full elastic information for 1,181 inorganic compounds, and this number is growing steadily. The methods used to develop the database are described, as are results of tests that establish the accuracy of the data. In addition, we document the database format and describe the different ways it can be accessed and analyzed in efforts related to materials discovery and design.

The introduction of ferromagnetic order in ZnO results in a transparent piezoelectric ferromagnet and further expands its already wide range of applications into the emerging field of spintronics. Through an analysis of density functional calculations we determine the nature of magnetic interactions for transition metals doped ZnO and develop a physical picture based on hybridization, superexchange, and double exchange that captures chemical trends. We identify a crucial role of defects in the observed weak and preparation sensitive ferromagnetism in $\mathrm{ZnO}:\mathrm{Mn}$ and $\mathrm{ZnO}:\mathrm{Co}$. We predict and explain codoping of Li and Zn interstitials to both yield ferromagnetism in $\mathrm{ZnO}:\mathrm{Co}$, in contrast with earlier insights, and verify it experimentally.

A systematic method is presented for determining the ground state of absorbed species on a substrate based on a cluster expansion of the configurational energy. It is shown that the method can determine the ground state of a strongly relaxing system using a few first-principles total energy calculations of small cells only. The method is applied to a particularly challenging case, the two-sided hydrogen chemisorption on a free standing graphene sheet, where, as a function of hydrogen coverage, the carbon hybridization goes from ${\mathrm{sp}}^2$ to ${\mathrm{sp}}^3.$ The method should require still fewer calculations and yield still more accurate results in the case of physisorption where longer-ranged strain effects are less important.

The interaction of $3d$ transition metal atoms and dimers with a single-walled armchair carbon nanotube has been investigated by first-principles density functional calculations. For Fe-, Co-, and Ni-doped (4,4) nanotubes, outside adsorption sites are the most favorable. The interactions are largely ferromagnetic for Fe and Co, with the local magnetic moments of the dimers being similar to the free dimers. However, for Ni most structures are nonmagnetic. We have also examined the effects of curvature with calculations for graphene and the (8,8) nanotube. For the (8,8) nanotube, the interaction of Co becomes more favorable inside the nanotube. Doping of a single Co atom transforms the (4,4) and (8,8) nanotubes into half-metals. These results are useful for spintronics applications and could help in the development of magnetic nanostructures and metallic nanotube coatings.

Refractory high entropy alloys feature outstanding properties making them a promising materials class for next-generation high-temperature applications. At high temperatures, materials properties are strongly affected by lattice vibrations (phonons). Phonons critically influence thermal stability, thermodynamic and elastic properties, as well as thermal conductivity. In contrast to perfect crystals and ordered alloys, the inherently present mass and force constant fluctuations in multi-component random alloys (high entropy alloys) can induce significant phonon scattering and broadening. Despite their importance, phonon scattering and broadening have so far only scarcely been investigated for high entropy alloys. We tackle this challenge from a theoretical perspective and employ ab initio calculations to systematically study the impact of force constant and mass fluctuations on the phonon spectral functions of 12 body-centered cubic random alloys, from binaries up to 5-component high entropy alloys, addressing the key question of how chemical complexity impacts phonons. We find that it is crucial to include both mass and force constant fluctuations. If one or the other is neglected, qualitatively wrong results can be obtained such as artificial phonon band gaps. We analyze how the results obtained for the phonons translate into thermodynamically integrated quantities, specifically the vibrational entropy. Changes in the vibrational entropy with increasing the number of elements can be as large as changes in the configurational entropy and are thus important for phase stability considerations. The set of studied alloys includes MoTa, MoTaNb, MoTaNbW, MoTaNbWV, VW, VWNb, VWTa, VWNbTa, VTaNbTi, VWNbTaTi, HfZrNb, HfMoTaTiZr.

Activation energies for vacancy-mediated impurity diffusion in face-centered-cubic aluminum have been computed ab initio for all technologically important alloying elements, as well as for most of the lanthanides. The so-called five-frequency rate model is used to establish the limiting vacancy interchange process. Many elements were shown to be limited by Al-vacancy interchanges. For these elements we showed that the diffusion activation energy is rather close to that for Al self-diffusion, and additionally the diffusion pre-exponential factor is of the same order as that for Al self-diffusion. The diffusion activation energy is shown to exhibit a linear relation with the solute partial molar volume in Al. In contrast, transition metals are shown to deviate strongly from these generalities. Diffusion of transition-metal atoms is limited by solute-vacancy interchanges that require remarkably high activation energies. Transition-metal diffusivities in Al show strong trends with the number of $d$-valence electrons but not with partial molar volume.