Electronic Structure of Superconducting MgB₂ and Related Binary and Ternary Borides

First-principles full potential linear muffin-tin orbital-generalized gradient approximation electronic structure calculations of the new medium-Tc superconductor (MTSC) MgB2 and related diborides indicate that superconductivity in these compounds is related to the existence of Px,y-band holes at the γ point. Based on these calculations, we explain the absence of medium-Tc superconductivity for BeB2, AlB2, ScB2, and YB2. The simulation of a number of MgB2-based ternary systems using a supercell approach demonstrates that (i) the electron doping of MgB2 (i.e., MgB2-yXy with X=Be, C, N, O) and the creation of defects in the boron sublattice (nonstoichiometric MgB2-y) are not favorable for superconductivity, and (ii) a possible way of searching for similar or higher MTSC should be via hole doping of MgB2 (CaB2) or isoelectronic substitution of Mg (i.e., Mg1-xMxB2 with M = Be, Ca, Li, Na, Cu, Zn) or creating layered superstructures of the MgB2/CaB2 type

Quantum-Chemical Analysis of the Chemical Stability and Cohesive Properties of Hexagonal TiB₂, VB₂, ZrB₂ and NbB₂

The cohesive properties and chemical stability of diborides MB2 have been analysed using the results of full-potential LMTO calculations. A comparison of interatomic M-M, M-B and B-B interactions in MB2 phases (M = Ti, V, Zr, Nb) shows that the changes in the cohesive properties are mainly controlled by the strength of the covalent M-B bonds

Electric Field Gradients in S-, P-, and D-Metal Diborides and the Effect of Pressure on the Band Structure and T\u3csub\u3ec\u3c/sub\u3e in MgB₂

Results of full-potential linear muffin-tin orbital generalized gradient approximation calculations of the band structure and boron electric field gradients (EFG\u27s) for the new medium-Tc superconductor MgB2 and related diborides MB2, M = Be, Al, Sc, Ti, V, Cr, Mo, and Ta are reported. The boron EFG variations are found to be related to specific features of their band structure and particularly to the M-B hybridization. The strong charge anisotropy at the B site in MgB2 is completely defined by the valence electrons - a property which sets MgB2 apart from other diborides. The boron EFG in MgB2 is weakly dependent on applied pressure: the B p-electron anisotropy increases with pressure, but it is partly compensated by the increase of core charge asymmetry. The concentration of holes in bonding or bands is found to decrease slightly from 0.067 to 0.062 holes/B under a pressure of 10 GPa. Despite a small decrease of N(EF), the Hopfield parameter increases with pressure and we believe that the main reason for the reduction under pressure of the superconducting transition temperature Tc is the strong pressure dependence of phonon frequencies, which is sufficient to compensate for the electronic effects

Band Structure of the Superconducting MgB₂ Compound and Modeling of Related Ternary Systems

Band structure of a novel superconductor - magnesium diboride - is studied by the self-consistent FP-LMTO method. Density of states near the Fermi level of MgB2 and its electronic properties are governed by the metal-like boron 2p orbitals in the planar network of boron atoms. The modification of the band structure of MgB2 upon doping the boron (with Be, C, N, and O substitutional impurities) and the magnesium (with Be, Ca, Li, and Na substitutional impurities) sublattices or upon the introduction of structural vacancies (boron nonstoichiomety) is analyzed. The electronic structures of MgB2 and hypothetical CaB2 are also studied as functions of pressure

New Solid Electrolyte Na 9 Al(MoO 4 ) 6 : Structure and Na + Ion Conductivity

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New Solid Electrolyte Na₉Al(MoO₄)₆: Structure and Na⁺ Ion Conductivity

Solid electrolytes are important materials with a wide range of technological applications. This work reports the crystal structure and electrical properties of a new solid electrolyte Na₉Al­(MoO₄)₆. The monoclinic Na₉Al­(MoO₄)₆ consists of isolated polyhedral [Al­(MoO₄)₆]^9– clusters composed of a central AlO₆ octahedron sharing vertices with six MoO₄ tetrahedra to form a three-dimensional framework. The AlO₆ octahedron also shares edges with one Na1O₆ octahedron and two Na2O₆ octahedra. Na3–Na5 atoms are located in the framework cavities. The structure is related to that of sodium ion conductor II-Na₃Fe₂(AsO₄)₃. High-temperature conductivity measurements revealed that the conductivity (σ) of Na₉Al­(MoO₄)₆ at 803 K equals 1.63 × 10^–2 S cm^–1. The temperature behavior of the ²³Na and ²⁷Al nuclear magnetic resonance spectra and the spin-lattice relaxation rates of the ²³Na nuclei indicate the presence of fast Na⁺ ion diffusion in the studied compound. At <i>T</i><490 K, diffusion occurs by means of Na⁺ ion jumps exclusively through the sublattice of Na3–Na5 positions, whereas Na1 and Na2 become involved in the diffusion processes (through chemical exchange with the Na3–Na5 sublattice) only at higher temperatures

Virtual Herbarium ALTB: collection of vascular plants of the Altai Mountain Country

Background The herbarium of the South-Siberian Botanical Garden of Altai State University (ALTB) houses the largest collection of plants from the Altai Mountain Country (AMC), an area that extends across Russia, Kazakhstan, Mongolia and China. The collection of ALTB includes more than 450,00 specimens, making it the seventh largest in Russia and the fourth largest amongst Russian university herbaria. Altai State University (ASU), the home of ALTB, is one of the most important centres of academic education and research in Siberia and the Russian Far East. It is a sociocultural centre that provides a distinguished learning environment for undergraduate and graduate students in many scholarly and professional fields, meeting the needs of today's knowledge-based post-industrial society and contributing to regional development. It actively promotes international cooperation and strategic collaboration amongst countries of the AMC in the fields of science, education and culture. In particular, the activities of the South-Siberian Botanical Garden include: development of measures to protect rare and endangered plant species, research on the flora and vegetation of the AMC, preparation and publication of a multi-volume work "Flora Altaica", monographic study of individual plant groups, conducting laboratory classes, summer practicals and special courses. The main purpose of this article is to attract the attention of the scientific community to the botanical research of transboundary territory of the Altai Mountain Country (Russia, Kazakhstan, China and Mongolia) and to the future development of digital plant collections in partnership with Global Biodiversity Information Facility (GBIF). New information The Virtual Herbarium ALTB (Russian interface - altb.asu.ru) is the largest digital collection of plants from the transboundary territory of the Altai Mountain Country and the main source of primary material for the "Flora Altaica" project (http://altaiflora.asu.ru/en/). Since 2017, when Altai State University became a GBIF data publisher, data from the Virtual Herbarium ALTB has been exported to the dataset "Virtual Herbarium ALTB (South-Siberian Botanical Garden)" in GBIF. Currently, it includes images and data from 22,466 vascular plants, of which 67% have geographic coordinates (accessed on 30.03.2021). Most of the specimens have been collected since 1977, with the most intensive collecting years being 1995-2008. In 2019, the label-data table of the Virtual Herbarium ALTB was modified to bring it into conformity with the Darwin Core specification (http://altb.asu.ru/). This effectively solved the major impediment to sharing plant diversity data from the AMC and adjacent regions in a multilingual environment