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Metal fluorides are ionic compounds with intriguing properties and a wide range of industrial and technological applications. In this paper, we present the results of an extensive first-principles study of the electronic band structure of nineteen mono-, di-, and tri-fluorides. Four approaches are employed in this study: LDA, pseudopotential and all-electron modified Becke–Johnson (mBJLDA@PP and mBJLDA@AE) potentials, and Yukawa Screened Hybrid Functional (YS-PBE0). The mBJLDA and YS-PBE0 band gaps are in good overall agreement with the available G 0 W 0 results and experimental data, which ensures the reliability of the provided predictions for systems where accurate band gaps are lacking. The LDA and YS-PBE0 upper valence band (UVB) widths are very close to each other, and these results are, in general, slightly smaller than the G 0 W 0 results and experimental data. It is shown that the band gap and UVB width vary with F F distance, and distinct trends are identified. The LDA density of states of the UVB for the alkali and alkaline-earth fluorides are analyzed and compared to the available XPS and XES spectra. It is found that the LDA approach provides accurate results for peak intensities as well as satellite peak positions relative to the main peak. A strong correlation is demonstrated between the adopted crystal structures of the considered fluorides and their bond ionicity. • Four approaches are used to study the electronic structure of 19 metal-fluorides. • For several of the fluorides considered, reliable predictions have been made. • Band gaps and upper valence band widths are found to exhibit clear trends. • The bonding mechanisms of fluorides are discussed in detail. • A correlation is established between bond ionicity and crystal structure. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Accurate determination of the phase diagram of MgO is proved to be a challenging task, mainly because of the very high B1–B2 transition pressures ( P t ) and melting temperatures. Only very recently experimental observations of the B1–B2 and solid-liquid transitions of MgO have been achieved. In spite of the extensive experimental and theoretical studies, a satisfactory agreement about the phase diagram of MgO has not yet been reached. In this work, we will thoroughly investigate the various factors that may affect the calculated B1–B2 phase boundary, using first-principle techniques. The structural, vibrational and thermal properties of the B1 and B2 phases of MgO are also investigated. It has been found that P t is quite sensitive to the set of pseudopotentials (PPs) used in the PP plane-wave approach, and volume (or pressure, P) range over which the Helmholtz free energy is directly calculated. An interplay between such a P range and the anharmonic effects is demonstrated, which allows (by a suitable choice of the P range) for a high reduction in these effects (i.e., highly extends the validity of the temperature range of the quasiharmonic approximation). The very recently proposed optimized norm-conserving Vanderbilt (ONCV) PPs are found to yield results in very good agreement with those obtained by the all-electron projected augmented wave calculations, contrary to other norm-conserving PPs. As for the exchange-correlation energy, the local density approximation is not only found to provide a good description of P t of the above transition, but also its temperature dependence. The obtained results are discussed in comparison with the most recent theoretical results and experimental data. In particular, our results suggest that the estimated temperature of the B1–B2 transition directly observed by using the laser-driven ramp loading technique is highly exaggerated. [ABSTRACT FROM AUTHOR]

The modified Becke–Johnson exchange potential combined with local-density approximation correlation (mBJLDA) has recently attracted interest because it provides highly improved band gaps at a very low computational cost. In this work we performed an extensive investigation of the performance of the mBJLDA potential employing a norm-conserving pseudopotential plane-waves approach (mBJLDA@PP), as implemented in the abinit code, using a test set of 83 solids representing a wide range of semiconductors and insulators. Our results confirm the conclusion of our previous study that the number of electrons treated as valence in the pseudopotentials of the cations can have a significant impact on the calculated mBJLDA@PP band gaps. More specifically, while the use of typical pseudopotentials leads to accurate band gaps of certain systems, it yields significantly underestimated band gaps for other systems compared to experiment and to those of the all-electron mBJLDA (mBJLDA@AE) approach. The classes of the latter systems are identified, and this problem is resolved by including some outer core states as valence. The resulting mean absolute error in the calculated band gaps (compared to experiment) is of 0.46 eV, which is comparable to that of the mBJLDA@AE band gaps, reflecting the accuracy and reliability of the mBJLDA@PP approach for the band gap calculations. [Display omitted] • Benchmarking mBJLDA band gaps, using pseudopotential (PP) plane-waves method. • Typical PP's can lead to underestimated mBJLDA@PP band gaps. • Treating upper core states as valence significantly improves the mBJLDA@PP band gaps. • The overall accuracy of the mBJLDA@PP approach matches that of the all-electron one. • The accuracy of the mBJLDA band gaps is compared to that of GW and HSE06 methods. [ABSTRACT FROM AUTHOR]

Dissipative two-level systems (TLS) have been a long-standing problem in glassy solids over the last fifty years, and have recently gained new relevance as sources of decoherence in quantum computing. Resonant absorption by TLSs in the dielectric poses a serious limitation to the performance of superconducting qubits; however, the microscopic nature of these systems has yet to be established. Based on first-principles calculations, we propose that hydrogen impurities in Al2O3 are the main source of TLS resonant absorption. Hydrogen is an ubiquitous impurity and can easily incorporate in Al2O3. We find that interstitial H in Al2O3 forms a hydrogen bond (O-H...O). At specific O-O distances, consistent with bond lengths found in amorphous Al2O3 or near Al2O3 surfaces or interfaces, the H atom feels a double well. Tunneling between two symmetric positions gives rise to resonant absorption in the range of 10 GHz, explaining the experimental observations. We also calculate the expected qubit-TLS coupling and find it to lie between 16 and 20 MHz, consistent with experimental measurements. [ABSTRACT FROM AUTHOR]

S-vacancy in WS 2 is the most stable anchoring site for single TM atoms adsorption, tune the electronic propeties of pristine WS 2 and exhibit excellent catalytic performance in CO 2 reduction reaction [Display omitted] • S-vacancy in WS 2 is the most stable anchoring site for single TM atoms adsorption. • S-vacancy in WS 2 tune the electronic properties of pristine WS 2. • CO 2 adsorbed strongly on TMs surface compared to other site and reaction performed on single atom surface. • Cu- and Co/S-vacancy WS 2 shows excellent performance in CO 2 reduction reaction compared to other studied system. Under mild conditions, carbon dioxide reduction reactions (CO 2 RRs) retain a tremendous complexity in effective CO 2 activation on metal based catalysts. Single-atom catalysts are only effective if all the isolated metal centers on the support interact in the same way. However, the support often has different topologies and defects, which can lead to a lack of homogeneity. In this study, we anchored Co, Cu, Ni, Pt, and Pd on a WS 2 nanosheet as a single-atom catalyst support for the activation and reduction of CO 2. The results showed that S-vacancies in WS 2 nanosheets are good sites for depositing single metal atoms and that WS 2 nanosheets with a well-distributed distribution of S-vacancies are good substrates for anchoring single metal atoms. The addition of S-vacancies and single metal atoms to WS 2 changes its electronic structure and reduces the band gap value from 1.98 eV to 1.17, 1.19, 1.45 and 1.42 eV respectively. Additionally, the Cu- and Co/S-vacancy WS 2 system showed good activity for the reduction of CO 2 to formic acid and CO, making it effective single-atom catalyst with low energy consumption and high stability for the activation and reduction of CO 2 to valuable products. [ABSTRACT FROM AUTHOR]

[Display omitted] • Boron nitride nanocage (B 12 N 12) is explored as an efficient adsorbent for the removal of chromate ions from water using DFT simulations. • The structural optimization reveals that the chromates ions are strongly anchored onto the surface of B 12 N 12 nanocages which leads to stronger adsorption energies. • The thermodynamic parameters i.e. ΔH and ΔG shows that the adsorption process is spontaneous and feasible at ordinary temperature. • The atom in molecule, reduced density gradient and non-covalent interaction analysis evidenced the formation of covalent O B and N H hydrogen bonds between the adsorbate and adsorbent. In present study, we for the first time investigated boron nitride (B 12 N 12) nanocage as adsorbent material for removal of chromate species Cr(VI) from water using density functional theory simulations. The geometry optimization reveals that Cr(VI) strongly interact with the surface of B 12 N 12 nanocage. The bonding analysis shows that Cr(VI) species make strong intermolecular hydrogen bonds and chemical bonds with the exterior surface of B 12 N 12 nanocage. This stronger interaction leads to larger adsorption energies for different complexes ranging from −27.17 to −168.96 kcal/mol. Moreover, the computed thermodynamic values i.e. ΔH and ΔG are negative (ranging from −55.54 to −156.42 kcal/mol and from −47.31 to −151.21 kcal/mol, respectively) which reflects their feasible and spontaneous adsorption. Furthermore, a large amount of charge transfer between Cr(VI) and B 12 N 12 nanocage, resulting strong electrostatic interactions. The HOMO-LUMO gap is strongly reduced after the adsorption of chromate ions on the B 12 N 12 nanocage surface indicating a strong sensitivity of adsorbent. The AIM, RDG and NCI analyses evince the existence of stronger intermolecular hydrogen and chemical bond between the B 12 N 12 surface and chromate ions. The outcomes of the present study demonstrate that the B 12 N 12 nanocage could be used as an efficient adsorbent for purification of water from toxic pollutants. [ABSTRACT FROM AUTHOR]