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A new allotropic superhard and superdense structure of sp3‐bonded boron nitride (hP3‐BN) is proposed. Geometric, electronic, elastic, and optical properties of the structure are investigated by density functional theory‐generalized gradient approximation (DFT‐GGA) calculations. It is shown that the structure is an insulator with a band gap of 5.06 eV; its density is 5.03% higher than that of cubic boron nitride (c‐BN) and its bulk modulus is 2.25% higher than that of c‐BN. The calculated hP3‐BN refractive index in the range 200–800 nm is significantly higher than the indexes of diamond and all known carbon allotropes, except for the analogous carbon h‐P3 structure, and it has a large optical dispersion. In the range above 800 nm the refractive index changes from 2.60 to 2.35.

We use density functional theory to study the structure and the band structure of the monolayer FeSe deposited on the SrTiO 3 substrate with the additional layer of Se between them. Top of the SrTiO 3 is formed by the double TiO layer with and without oxygen vacancies. Several structures with different arrangements of the additional Se atoms above the double TiO layer is considered. Equilibrium structures were found and the band structures for them were obtained. Near the Γ = (0, 0, 0) point of the Brillouin zone, the hole Fermi surface pockets persist and, additionally, an electron pocket appears. Thus neither the presence of the additional Se layer nor the oxygen vacancies in the double TiO layer leads to the sinking of hole bands below the Fermi level near the Γ point. Necessity to include the strong electronic correlations into account is discussed.Keywords Fe-based superconductors · Monolayer FeSe · SrTiO 3 · density functional theory 1 IntroductionIron-based materials represents a class of high-T c superconductors that is based on the conducting layer

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The features of anomalous behavior of silica glass doped with titanium oxide are discussed. The low thermal expansion of silica glass is considered on the base of the idea about polymorphous transformations in meta-stable glass structures. The analogy of these transformations to structural transitions of martensite type in metallic alloys is assumed. The effect of TiO2-doping on the structure and the intrinsic strength of silica glass fibers is studied. The structural (intrinsic) strength has been found to decrease by about 15% compared to that of non-doped silica glass. This decrease of strength is assumed to be accounted for the incorporation of a part of titanium ions into glass structure with six-fold coordination resulting in weakening the connectivity of a glass anionic network.