Jian Hua Xu

Cohesive properties, electronic structure, and bonding characteristics of RuAl—A comparison to NiAl

Recent experiments on promising high-temperature aluminide intermetallic compounds discovered that, in contrast to NiAl, RuAl has critical 〈111〉 slip systems that facilitate ductility under compression at room temperature. In order to understand this different mechanical property from a microscopic point of view, the cohesive and electronic properties of NiAl and RuAl have been studied by means of the first principles local density all-electron self-consistent linearized muffin-tin orbital (LMTO) and full-potential linearized augmented plane wave (FLAPW) methods. The ground state cohesive properties calculated by the LMTO method (including equilibrium lattice constant, bulk modulus, and formation energy) are found to be in good agreement with experiment. The analysis of the band structure and density of states shows that the transition metal (Ni or Ru) d -Al p hybridization provides the major contribution to the cohesive energy in both compounds. The anti-phase boundary (APB) energy of RuAl associated with the ½〈111〉 {110} superdislocation (580 mJ/m 2 ) is found to be only 65% that of NiAl. Moreover, the charge density near the Fermi energy reveals that (i) the strong Ni d -Al p hybridization at E F for NiAl causes a directional charge distribution along the 〈111〉 direction which may affect its dislocation dissociation; (ii) for RuAl, a mostly Ru- d electron charge distribution shows only d-d bonding along the 〈100〉 direction between Ru atoms.

Electronic structure and magnetism of modulated structures

Abstract Results of all-electron self-consistent spin-polarized band structure studies on CuNi and PdAu coherent modulated structures are presented. For CuNi, modulations along [111] and [100] for different wavelengths and supercells up to 12 layers predict Ni moments to be reduced vs. fcc Ni, but no ‘dead” layers even in the most dilute system. Adjacent Cu layers acquire small magnetic moments, which may be measured by NMR. Increased exchange enhancements are found and a possible ferromagnetic instability is discussed in relation to the strongly enhanced susceptibility observed in PdAu sandwiches by Brodsky and Freeman. Shear and distortion effects are investigated with separate calculations on tetragonally distorted bulk Ni and Pd.

Magnetism and electronic structure of bimetallic superlattices: Cr/Fe

Abstract The electronic and magnetic properties of Cr/Fe(110) coherent modulated superlattices (CMS) with various layer thicknesses were studied using the local spin density linear muffin-tin orbital method. In agreement with experiment, we find that the magnetic moments of the Fe layers are not significantly affected by the Cr layers and to have almost the same magnitude as in bulk bcc Fe, even in the case of an Fe monolayer in the Cr/Fe(110) CMS. Only a small moment (≤0.4μ B ) is found for the Cr layers aligned antiparallel to the n.n. Fe moments. The Cr moment alignment is determined by a delicate balance between the different magnetic interactions (Fe-Fe, Fe-Cr and Cr-Cr); the Cr magnetic moment always prefers to align antiparallel to the n.n. Fe and Cr moments.

Phase Stability, and Cohesive, Electronic and Mechanical Properties of Intermetallic Compounds

Current sophisticated electronic structure simulations are at the forefront of understanding and predicting a variety of materials properties of intermetallic compounds. Several examples are given here that illustrate how first principles total energy local density methods have addressed the problems of (i) phase stability and the effect of ternary additions, (ii) anti-phase boundaries (APB’s) in B2 NiAl, FeAl and RuAl aluminides and other faults in determining their structural and bonding character. A key objective has been to attempt to understand, at the electronic level, fundamental quantities that may be related to the crucial ductility issue in high temperature intermetallics. Differences between observed ductility properties of related systems may relate to their differing electronic and bonding properties, particularly the nature of p-d hybridization and the directional charge distributions of the states near the Fermi energy.