Masato Aoki

Electronic structure of the ordered ternary compound Y(M0.75Al0.25)2 (M=Co and Fe) with C15-type Laves phase structure

Abstract The electronic structures of the binary compound YM 2 and the ordered ternary compound Y(M 0.75 Al 0.25 ) 2 (M = Co and Fe) with the C15-type Laves phase structure are calculated by the self-consistent APW method. It is shown that the calculated density-of-states (DOS) curves for YM 2 are very similar to those obtained previously, being characterised by two sharp peaks mainly of 3d-states of M atoms near the Fermi level. For ordered Y(M 0.75 Al 0.25 ) 2 , some 3d-states of the M atom hybridise strongly with 3p-states of the Al atom and only a single sharp peak is shown to appear in the DOS near the Fermi level. Moreover, the position of the Fermi level is shown to shift toward the lower energy side by the substitution of Al atoms. The anomalous magnetic properties observed in the pseudobinary compound Y(M,Al) 2 are discussed on the basis of these calculated electronic structures.

Defect modelling: the need for angularly dependent potentials

Abstract The reliable atomistic simulation of defects in intermetallics requires the development of angularly dependent interatomic potentials. Ab initio calculations predict, for example, that the nearest competing phases to the L10 ground state of TiAl and the B2 ground state of NiAl are B33 (CrB) and B20 (FeSi) respectively, whose structural stability is determined by directional pd bonding. This is well described by the Tight Binding model, within which approximation angularly dependent many-atom interatomic potentials have recently been derived. The nature of these so-called Bond Order Potentials (BOPs) and their application to structural prediction in elemental d-valent transition metals and sp-valent semiconductors are discussed.

Electronic structure and magnetism of , and

Dependences of the magnetic moment on the lattice constant for binary and pseudo-binary compounds , and with a hexagonal -type structure are studied by an ab initio calculation on a self-consistent linear muffin-tin orbital method within the atomic sphere approximation. It is found that Co moments in decrease abruptly at a critical lattice constant smaller than the observed one. On the other hand, Ni moments in are shown to appear at a lattice constant a little larger than the observed one. Similar calculations are carried out for the ordered compound , where Co and Ni atoms occupy the 3g and 2c sites, respectively. It is shown that Co and Ni atoms in this compound lose their moments at a lattice constant slightly smaller than the observed one. It is also found that the Co atom on the 3g site in and has two magnetic states, a high-moment state and a low-moment or non-magnetic state, depending on the lattice constant. The present results explain the concentration dependence of the the bulk moment observed for and .

First-principles calculations on bcc-hcp transition of titanium

Abstract Bcc–hcp transition of titanium was investigated by the first-principles calculations. Very accurate calculations using the gradient-corrected full-potential linear muffin-tin orbital correctly estimated the hcp structure slightly lower in energy than the ω structure. The adiabatic potential energy surfaces of the distortions related to the structural transformations showed similar with those of Zr. The curvatures of the potential surfaces at the equilibrium bcc point are very small and negative, which suggests that Ti has strong anisotropic characters of bondings. The electronic thermal contribution stabilizes the bcc structure over the hcp structure above 3000 K.

Electronic structure and magnetic properties of CoS2

Abstract Electronic structures of pyrite-type compounds CoS 2 and CoSe 2 are calculated by the linear muffin-tin orbital method and the atomic sphere approximation. It is shown that the bandwidth of the anti-bonding 3p-states of S in CoS 2 is rather narrower than that of the 4p-states of Se in CoSe 2 . This fact causes CoS 2 to be ferromagnetic, while CoSe 2 to be paramagnetic. By a spin-polarized band calculation, it is found for CoS 2 that the ferromagnetic ground state becomes unstable at smaller lattice constants, which is consistent with the recent results observed under high pressure and high magnetic field.

A simple environment-dependent overlap potential and Cauchy violation in solid argon

We develop an analytic and environment-dependent interatomic potential for the overlap repulsion in solid argon, based on an approximate treatment of the non-orthogonal tight-binding theory for closed-shell systems. The present model can reproduce the observed elastic properties of solid argon well, including the Cauchy violation at high pressures, yet it is very simple. A useful and novel analysis is given to show how the elastic properties are related to the environment dependence incorporated into a generic pairwise potential. The present study has a close link to the broad field of computational materials science, in which the inclusion of environment dependence in the short-ranged repulsive part of a potential model is sometimes crucial in predicting the elastic properties correctly.

Isomer shift of Fe in the intermetallic compound Y(Fe1−xAlx)2 with C15-type Laves phase structure

Abstract The isomer shift of Fe nucleus in intermetallic compound Y(Fe 1− x Al x ) 2 with the C15-type Laves phase structure is discussed by calculating the charge density of s electrons in the local density approximation and the self-consistent APW method. The calculated charge density of the 4selectrons at the Fe nucleus position is shown to decrease with increasing Al content, which is consistent with the observed isomer shift. The atomic environment effect on isomer shift of the Fe nucleus surrounded by the Al atoms are discussed.

Tight-binding bond order potential a forces for atomistic simulations

The formalism of a new bond order potential is described in detail, including very recent developments-most notably the implementation of a numerically stable recursive algorithm for obtaining the coefficients of the many-atom expansion. This tight-binding Green’s function method is efficient, the computer time scaling linearly with number of atoms. Technical notes on the proof of the central results and on handling the integration of Green’s functions are attached.

Bond order potentials and sum rules

The many-atom potentials for the bond order, which have recently been derived within tight-binding Huckel theory, are shown to satisfy two different sum rules. The first relates to the moments of the density of states about a given atom. The second provides a constraint on the terminator of any approximate bond order potential. The bond order potentials are found to provide well converged results for simple s-valent lattices if terms up to the fourth moment are retained in the many-atom expansion and the terminator is chosen to satisfy the second sum rule.

Magnetic properties of pseudo-binary compounds Y(Fe,Al)2 and Y(Co,Al)2

Abstract The electronic structures of binary compounds YM 2 (M = Fe and Co) and ordered ternary compounds Y(M 0.75 Al 0.25 ) 2 and Y(Fe 0.25 Al 0.75 ) 2 with the cubic Laves phase structure are calculated by the self-consistent APW method. The anomalous magnetic properties observed in Y(Fe,Al) 2 and Y(Co,Al) 2 are discussed using the calculated results of the density-of-states curve.