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

Structural, electronic properties and Fermi surface of ThCr2Si2-type charge-balanced KFe2AsSe phase as a parent system for a new group of "mixed" pnictide-chalcogenide superconductors

The ThCr2Si2-type arsenide-selenide phase KFe2AsSe is proposed as a parent system for a new "intermediate" group of Fe-based superconducting materials bridging the known families of Fe-pnictides (such as BaFe2As2) and Fe-chalcogenides (such as KxFe2-ySe2) superconductors. The characterization of the proposed charge-balanced phase by means of FLAPW-GGA approach covers the crystal structure, As/Se atomic ordering, stability, electronic bands, Fermi surface, and density of electronic states.Comment: 8 pages, 4 figure

Elastic properties of mono- and polycrystalline hexagonal AlB2-like diborides of s, p and d metals from first-principles calculations

We have performed accurate ab initio total energy calculations using the full-potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange-correlation potential to systematically investigate elastic properties of 18 stable, meta-stable and hypothetical hexagonal (AlB2-like) metal diborides MB2, where M = Na, Be, Mg, Ca, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ag and Au. For monocrystalline MB2 the optimized lattice parameters, independent elastic constants (Cij), bulk modules (B), shear modules (G) are obtained and analyzed in comparison with the available theoretical and experimental data. For the first time numerical estimates of a set of elastic parameters of the polycrystalline MB2 ceramics (in the framework of the Voigt-Reuss-Hill approximation), namely bulk and shear modules, compressibility, Young's modules, Poisson's ratio, Lame's coefficients are performed.Comment: 24 pages, 3 figure

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