Intermetallic compounds which are ductile at high temperatures are of great technologica1 interest; however, purely experimenta1 searches for improved intermeta11ic materials are time consuming and expensive. Theoretical studies can shorten the experimental search by focusing on compounds with the desired properties. While current ab initio density-functional calculations cannot adequately determine materia1 properties at high temperature, it is possible to compute the staticlattice equation of state and elastic moduli of ordered binary compounds. Known correlations between equilibrium properties and high-temperature properties such as the melting temperature can then be used to point the way for experiments. We demonstrate the power of this approach by applying the linear augmented-plane-wave method to the calculation of the equation of state and all of the zero-pressure elastic moduli for SbY in the B1 (NaC1) phase, CoAl and RuZr in the B2 (CsC1) phase, and NbIr in the L 10 (Au-Cu I) phase. The calculated equilibrium lattice constants are a11 within 2% of the experimentally determined values. The only experimentally known elastic moduli in these systems are the bulk and shear moduli for polycrysta11ine SbY, CoA1, and NbIr. The predicted bulk moduli are with 7%%uo of experiment. Theory enables us to place limits on the experimen-ta1 po1ycrystalline shear modulus. The experimenta1 shear moduli of SbY and CoA1 are within our theoretica1 bounds, but the experimenta1 shear modulus of NbIr is 35% smaller than our lower bound. We stress that in the case of CoAl our calculations provided a prediction for the bulk and shear moduli that were subsequently confirmed by the experiments of Fleischer. Wt; also discuss the band structures and electronic density of states for these materials.

The multiple scattering-based theory of Gaspari and Gyorffy for the electron-ion matrix element in close packed metals is applied to Im 3m H3S, which has been predicted by Duan et al. and Bernstein et al to be the stable phase at this stoichiometry around 190 GPa, thus is the leading candidate to be the phase observed to superconduct at 190K by Drozdov, Eremets, and Troyan. The nearly perfect separation of vibrational modes into those of S and of H character provides a simplification that enables identification of contributions of the two atoms separately. The picture that arises is basically that of superconducting atomic H stabilized by strong covalent mixing with S p and d character. The reported isotope shift is much larger than the theoretical one, suggesting there is large anharmonicity in the H vibrations. Given the relative unimportance of sulfur, hydrides of lighter atoms at similarly high pressures may also lead to high temperature superconductivity.

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We present an approach to solid-state electronic-structure calculations based on the finite-element method. In this method, the basis functions are strictly local, piecewise polynomials. Because the basis is composed of polynomials, the method is completely general and its convergence can be controlled systematically. Because the basis functions are strictly local in real space, the method allows for variable resolution in real space; produces sparse, structured matrices, enabling the effective use of iterative solution methods; and is well suited to parallel implementation. The method thus combines the significant advantages of both real-space-grid and basis-oriented approaches and so promises to be particularly well suited for large, accurate ab initio calculations. We develop the theory of our approach in detail, discuss advantages and disadvantages, and report initial results, including the first fully three-dimensional electronic band structures calculated by the method. PACS 71.15-m, 02.70.Dh Typeset using REVT E X

CrN is observed to undergo a paramagnetic to antiferromagnetic transition with Néel temperature TN ∼ 280 K, accompanied by a shear distortion from cubic NaCl-type to orthorhombic Pnma structure. Our first-principles plane wave calculations, based on ultrasoft pseudopotentials, confirm that the distorted antiferromagnetic phase with spin configuration arranged in double ferromagnetic sheets along the [110] direction is energetically much more stable than the paramagnetic phase, and also slightly favored over the ferromagnetic and other examined antiferromagnetic phases. The energy gain from polarization is much larger than that from distortion; nevertheless, the distortion is decisive in resolving the competition among the antiferromagnetic phases: the anisotropy in the (100) plane arising from the ferromagnetic double-sheet magnetic order allows a small but important energy gain upon the cubic-to-orthorhombic transition. Although antiferromagnetic order leads to a large depletion of states around Fermi level, it does not open a gap. The system is metallic with occupied hole and electron pockets of Fermi surface containing ∼ 0.025 carriers of each sign per formula unit. The simultaneous occurrence of structural distortion and antiferromagnetic order, as well as the competition between antiferromagnetism and ferromagnetism, is analyzed. 71., 75., 71.15.Hx, 75.50.Ee