Vacancies and substitutional defects in multicomponent diboride Ti0.25Zr0.25Hf0.25Ta0.25B2: First-principle study

We study the hard and electrically conductive multicomponent diboride Ti0.25Zr0.25Hf0.25Ta0.25B2 with high thermal stability by ab initio calculations. We focus on the effect of defects (either vacancies or C atoms, both relevant for numerous experiments including our own) on material characteristics. Different types, concentrations and distributions of defects were investigated, and the configurations leading to the lowest formation energies were identified. We show that the replacement of B by C is more unfavorable than the formation of B vacancies. We show that vacancies prefer to coalesce into a larger planar void, minimizing the number of broken B B bonds and the volume per atom, while carbon substitutions at boron sites do not prefer coalescence and tend to minimize the number of C-C bonds. We show the effect of vacancies on mechanical and electronic properties, and use the results to explain experimental data.

The Electronic Structural and Defect-Induced Absorption Properties of a Ca2B10O14F6 Crystal

Comprehensive ab initio electronic structure calculations were performed for a newly developed deep-ultraviolet (DUV) non-linear optical (NLO) crystal Ca2B10O14F6 (CBOF) using the first principle method. Fifteen point defects including interstitial, vacancy, antisite, Frenkel, and Schottky of Ca, O, F, and B atoms in CBOF were thoroughly investigated as well as their effects on the optical absorption properties. Their formation energies and the equilibrium concentrations were also calculated by ab initio total energy calculations. The growth morphology was quantitatively analyzed using the Hartman–Perdok approach. The formation energy of interstitial F (Fi) and antisite defect OF were calculated to be approximately 0.33 eV and 0.83 eV, suggesting that they might be the dominant defects in the CBOF material. The absorption centers might be induced by the O and F vacancies (VF, VO), interstitial B and O (Oi, Bi), and the antisite defect O substitute of F (OF), which might be responsible for lowering the damage threshold of CBOF. The ionic conductivity might be increased by the Ca vacancy (Vca), and, therefore, the laser-induced damage threshold decreases.

Surface Structure and Electronic Properties of Lu3Al5O12

Lu3Al5O12 (LuAG) is a famous scintillator that has the advantages of high efficiency, high light yield, and fast decay after being doped with active ions. F centers (oxygen vacancies with two electrons) and antisite defects are the most important defects and can greatly affect the scintillation performance in the bulk materials. However, the surface defects that strongly affect the spectrum of a single crystal (SC) and single crystal film (SCF) and the effect on the electronic properties have not been investigated. In this context, we investigate the surface structural and electronic properties of Lu3Al5O12 using first-principles calculations. The Lu atoms are six-fold and seven-fold coordinated with the O atoms on the S1 and S2 surfaces. The surface oxygen vacancies and antisites have considerably lower formation energies than for the bulk. The oxygen vacancies in the bulk introduce the occupied states in the band gap. The surface electronic states are mainly located on the oxygen atoms and can be eliminated via oxygen vacancies.

Regulation of Electrical Properties of ZrOxNy by Oxygen Doping and Zirconium Vacancies

Transition metal oxynitrides are important materials in electronic devices, electrocatalysis, machinery industry and other fields, according to their excellent properties, such as high sensitivity to temperature and high electron transport characteristics. Especially in sensor and MOS applications, transition metal oxynitrides with semiconductor properties play an important role in the sensitivity and frequency response of sensors. Here, we study the effects of different concentrations of zirconium vacancy (VZr) and oxygen doping on the ZrN structure, and calculate the formation energies and density of states of ZrOxNy in different element ratios by density functional theory. The results show that the introduction of VZr and oxygen doping promote the Fermi level of ZrOxNy to move towards the valence band and conduction band, respectively. The structure of the non-degenerate semiconductor ZrOxNy can be constructed at Zr0.425N0.569O0.006. Taking ZrOxNy as an example, this work proposes a method to regulate the electrical properties of transition metal oxynitrides by introducing zirconium vacancy/oxygen doping, which greatly promotes the rapid discovery of novel transition metal oxynitrides semiconductor materials.

Iron in Hydroxyapatite: Interstitial or Substitution Sites?

Iron-doped hydroxyapatite (Fe-HAp) is regarded as a promising magnetic material with innate biocompatibility. Despite the many studies reported in the literature, a detailed theoretical description of Fe inclusions is still missing. There is even no consensual view on what kind of Fe defects take place in Fe-HAp—iron interstitial or calcium substitutions? In order to address these questions, we employ modern first-principles methodologies, including hybrid density functional theory, to find the geometry, electronic, magnetic and thermodynamic properties of iron impurities in Fe-HAp. We consider a total of 26 defect configurations, including substitutional (phosphorus and calcium sites) and interstitial defects. Formation energies are estimated considering the boundaries of chemical potentials in stable hydroxyapatite. We show that the most probable defect configurations are: Fe3+ and Fe2+ substitutions of Ca(I) and Ca(II) sites under Ca-poor conditions. Conversely, Fe interstitials near the edge of the hydroxyl channel are favored in Ca-rich material. Substitutional Fe on the P site is also a probable defect, and unlike the other forms of Fe, it adopts a low-spin state. The analysis of Fe K-XANES spectra available in the literature shows that Fe-HAp usually contains iron in different configurations.

Application of new method in quantum-mechanical calculations of the structural properties and the formation energy of the ordered Mu - phase in the Fe - Mo system

A new technique of finding the minimum total energy, calculated by quantum mechanical calculations, for hexagonal Mu- (μ-) phases of stoichiometric compositions taking into account relaxation, as well as optimized crystal lattice parameters for different compositions in the Fe-Mo system, has been applied. The convergence of the total energy is investigated as a function of the number of plane waves (k) in the range 2000 ÷ 105. Differences in the structural energies between bcc, C14 Laves and μ- phases of pure components, as well as the formation energies of μ- phases of stoichiometric compositions Fe9Mo4, Fe7Mo6 and Fe6Mo7 are calculated.

Doping two boron atoms in germanene nanoribbons in an external electric field

Germanene is a two-dimensional system made of germanium atoms, its configuration is hexagonal honeycomb. Germanene nanoribbons (GNRs) are one-dimensional systems made from germanene with hydrogen-modified edges. The GNRs configuration studied here consists of 12 germanium atoms and 4 hydrogen atoms per unit cell. This work investigated the doping of two boron atoms into the unit cell of GNRs. Changing the different doping sites produces different configurations, the configurations been studied as meta-configuration, para-configuration, and ortho-configuration. By using density functional theory (DFT), the formation energies, energy band structures, and density of states of the configurations are studied. The ortho-configuration for the formation energy is the smallest, so this configuration is the most stable. The appearance of an external electric field changes the band gap and the energy band structure of the system.

Embrittlement Analysis of $$\sum {{{{5}\left[ {{21}0} \right]} \mathord{\left/ {\vphantom {{{5}\left[ {{21}0} \right]} {\left( {\overline{1}\overline{2}0} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {-{1}-{2}0} \right)}}}$$ FeAl Grain Boundary in Presence of Defects: An Ab Initio Study

Iron aluminide (FeAl) inter-metallic compounds are potential candidates for structural applications at high temperatures owing to their superior corrosion resistance, high temperature oxidation, low density and inexpensive material cost. However, the presence of defects can lead to reduction in the strength and ductility of FeAl-based materials. Here we present a density functional theory (DFT) study of the effect of the presence of defects including Fe and Al vacancies as well as H dopants at the substitutional and interstitial sites at a $$\sum {{{{5}\left[ {{21}0} \right]} \mathord{\left/ {\vphantom {{{5}\left[ {{21}0} \right]} {\left( {\overline{1}\overline{2}0} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {\overline{1}\overline{2}0} \right)}}}$$ ∑ 5 210 / 1 ¯ 2 ¯ 0 FeAl grain boundary focusing on the energetics. The plane wave pseudopotential code Vienna Ab initio Simulation Package (VASP) in the generalized gradient approximation (GGA) is used to carry out the computations. The formation energy calculations showed that intrinsic defects such as Fe and Al vacancies probably form at the GB, indicated by their negative formation energies. These vacancies can further form defect complexes with H impurities, indicated by lowered formation energies, compact bonds and charge gain of H atoms. Electronic structure analysis showed stronger hybridization of 1s orbitals of H with Fe and Al atoms, which leads to the stabilization of these defects resulting in degradation of material strength.