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We present detailed investigations of the structural, elastic, dielectric, and piezoelectric properties of scandium aluminum nitride (ScxAl1-xN) with the wurtzite crystal structure by means of first-principles calculations based on density functional theory in order to enable a detailed comparison to the corresponding physical properties of GaAlN and InAlN. The goal of our approach is to use atomistic simulations to extract the novel solid state characteristics of ScxAl1-xN crystals by the determination of complete sets of coefficients for the elastic, compliance, and piezoelectric tensor and to confirm the theoretical predictions by experimental measurements of selected tensor coefficients. The calculation of the tensor components is accompanied by a detailed analysis of the crystal structures, e.g., average bond length, bond angles, lattice parameters, and mass density in dependence on alloy composition of ScxAl1-xN. If an increasing number of Al atoms of up to x = 0.5 are replaced by Sc atoms, we observe a nonlinear change of the ratio of lattice parameter c(x)a(x) and average bond angles of about 10% and 5%, respectively, which give an indication of an increasing deviation of the crystal structure of ScxAl1-xN from an ideal hexagonal lattice. As a consequence of the deformed crystal structure and the iconicity of the ScN bond, we predict a change in value of the elastic coefficient CScAlN33(x), the piezoelectric coefficient eScAlN33(x), and the value of spontaneous polarization PScAlNSP(x) of up to 65%, 150%, and 230%, respectively. Based, on these simulation results, physical features of practical use are derived, like the average bulk, shear, and the Young modulus as well as the reciprocal Young's modulus and Poisson ratio. Furthermore, the spontaneous polarization of ScxAl1-xN is approximated, taking nonlinear effects into account as well as the piezoelectric polarization caused by uniaxial, biaxial, and hydrostatic stresses in dependency on alloy composition and strain. A detailed comparison of the structural and polarization related properties of GaAlN and InAlN allows us to point out the peculiarity of wurtzite ScAlN crystals within the group III-nitrides.

We examine the ionic migration of Li in LATP [ Li1+xAlxTi2-x(PO4)3 ] solid electrolytes from an atomistic viewpoint by means of density functional theory calculations. We vary the Al content and investigate its effects on the crystal structure of LATP and on the migration energy landscape of interstitial Li ions. The energy profiles governing the Li diffusion are found to be systematically influenced by the position of Al ions in direct vicinity of the migration path, and we derive a simplified classification scheme of three universal energy profile shapes. The overall influence of the Al/Ti-ratio on the Li migration is analyzed by a separation into chemical and geometrical aspects. This work provides a solid basis for a resource-efficient computational examination of the ionic conductivity of Li in LATP with varying Al/Ti concentrations.

We present a density-functional theory analysis of nitrogen-vacancy (NV) centers in diamond, which are located in the vicinity of extended defects, namely, intrinsic stacking faults, extrinsic stacking faults, and coherent twin boundaries on {111} planes in diamond crystals. Several sites for NV centers close to the extended defects are energetically preferred with respect to the bulk crystal. This indicates that NV centers may be enriched at extended defects. We report the hyperfine structure and zero-field splitting parameters of the NV centers at the extended defects, which typically deviate by about 10% but in some cases up to 90% from their bulk values. Furthermore, we find that the influence of the extended defects on the NV centers is of short range: NV centers that are about three double layers (corresponding to ≈6Å) away from defect planes already show bulklike behavior.

We report on theoretical investigations of ferromagnetic rare-earth-transition-metal phases with the structure of YNi9In2 and with additional atoms X at two interstitial positions. By a high-throughput-screening (HTS) approach based on density functional theory the intrinsic key properties of hard magnets, namely the magnetization M, energy product (BH)max and uniaxial magnetocrystalline anisotropy constant K 1 are estimated. The HTS identifies several promising phases NdFe10AX with A=Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Si, P, and X=B, C, N. These phases partially outperform Nd2Fe14B in terms of (BH)max and K 1 values, and they contain about 35% less rare-earth atoms.

A density functional theory study of atomic defects and dopants in ternary Z-phase transition-metal nitrides CrMN with M=V, Nb, or Ta is presented. Various defect formation energies of native point defects and of substitutional atoms of other metal elements which are abundant in the steel as well are evaluated. The dependence thereof on the thermodynamic environment, i.e., the chemical conditions of a growing Z-phase precipitate, is studied, and different growth scenarios are compared. The results obtained may help to relate results of experimental atomic-scale analysis by atom probe tomography or transmission electron microscopy to the theoretical modeling of the formation process of the Z phase from binary transition-metal nitrides.

The possibilities of turning intrinsically n-type oxide semiconductors like ZnO and Zn1-xMgxO into p-type materials are investigated. Motivated by recent experiments on Zn1-xMgxO doped with nitrogen, we analyze the electronic defect levels of point defects NO,vZn, and NO-vZn pairs in ZnO and Zn1-xMgxO by means of self-interaction-corrected density functional theory calculations. We show how the interplay of defects can lead to shallow acceptor defect levels, although the levels of individual point defects NO are too deep in the band gap to be responsible for p-type conduction. We relate our results to p-type conduction paths at grain boundaries seen in polycrystalline ZnO and develop an understanding of a p-type mechanism which is common to ZnO, Zn1-xMgxO, and related materials.

We present a microscopic interpretation for the appearance and behaviour of subgap states in stoichiometric and oxygen-deficient, amorphous In-Zn-O (a-IZO) and In-Sn-O (a-ITO) derived from a density functional theory analysis using a self-interaction-correction scheme. Our findings concerning the defect structures and the resulting deep levels are qualitatively similar to earlier results on a-IGZO and a-ZTO and in agreement with recent experimental results. Based on our extensive set of DFT results for In-, Sn-, Zn- based oxides we develop a general concept of the subgap states which is applicable to these systems. Electronic defect levels in the lower half of the band gap are created by undercoordinated oxygen atoms while local oxygen deficiencies cause defect levels in the upper part of the band gap.

We present a density-functional-theory analysis of crystalline and amorphous Zn- and Sn-based oxide systems which focuses on the electronic defect states within the band gap. A comparison of these electronic levels reveals that the hybrid functionals PBE0, HSE06, or B3LYP agree with a self-interaction corrected (SIC) local-density-approximation functional on occupied defect levels when similar treatments of the self-interaction are considered. However, for unoccupied levels, the hybrid functionals and the SIC approach lead to very different predictions. We show that a prerequisite for the determination of the energetic position of subgap states in these oxides is that a functional needs to predict correctly the electronic band structure over a wide energy range and not just close to the band gap. We conclude that for accurate defect levels, an adequate treatment of the self-interaction problem is required especially in the presence of nearby metal-metal interactions.