Donald M. Nicholson

Ab Initio Theory of the Ground State Properties of Ordered and Disordered Alloys and the Theory of Ordering Processes in Alloys

We review some of the advances in the calculation of the electronic structure and energetics of ordered and disordered alloys that hold out the possibility of obtaining, in thenot-too-distant future, an ab initio theory of ordering and phase stability in alloys. In particular, we focus on the calculation of the ground state properties of Ni 3 Al and discuss the competition between the Li 2 and D0 22 ordered structures. We review the ab initio concentration functional theory of ordering developed by Gyorffy and Stocks and its application to the short-range-ordered solid-solution state in Cu c Pd 1-c alloys. Finally, we review the generalized perturbation method approach to calculation of multisite interchange potentials in Ni 3 Al, Pd 3 V, and Al 3 Ti and again discuss L1 2 /D0 22 competition as well as antiphase boundary energies in Ni 3 Al.

Reinventing atomic magnetic simulations with spin-orbit coupling

We propose a powerful extension to the combined molecular and spin dynamics method that fully captures the coupling between the atomic and spin subsystems via spin-orbit interactions. Moreover, the foundation of this method lies in the inclusion of the local magnetic anisotropies that arise as a consequence of the lattice symmetry breaking due to phonons or crystallographic defects. By using canonical simulations of bcc iron with the system coupled to a phonon heat bath, we show that our extension enables the previously unachievable angular momentum exchange between the atomic and spin degrees of freedom.

The Effect of Scandium on the Phase Stability of Al3Nb AND Al3Zr

A series of Al-25 (Nb,Sc), at.%, and Al-25 (Zr,Sc), at.%, alloy buttons were arccast in argon and homogenized for 2 h in vacuum at temperatures ranging from 1473 to 1573K. X-ray powder-diffraction indicated that almost all the Nb in Al-25 Nb (DO 22 ) had to be replaced by Sc in order to obtain the Ll 2 structure. In the case of Al 3 Zr much less Sc was required - the single phase Ll 2 composition is approximately Al 16 Zr 8 Sc. Recent calculations [1,2] show that once the tetragonality is properly included the phase stability of the trialuminide transition-metal binary alloys can be understood in terms of their electronic density of states. The dominating feature is a deep minimum in the density of states just below the major transition metal d-band peak. The exact position of the minimum changes with structure type (i.e., Ll 2 , DO 22 , or DO 23 , and with c/a). The alignment of the Fermi energy with the minimum appears to determine the equilibrium structure. The results of linearized-muffin-tin-orbital [3] (LMTO) electronic structure calculations are compared to the rigid band model and checked against the experimentally determined phase boundaries.

A first-principles study of short range order in Cu-Zn

Recently, measurements of short range order (SRO) neutron diffuse scattering intensity have been performed on quenched Cu-Zn alloys with 22.4 to 31.1 at.% Zn and pair interactions were obtained by Inverse Monte Carlo simulation. These results are compared to SRO intensities and effective pair interactions obtained from first- principles electronic structure calculations. The theoretical SRO intensities were calculated with the Cluster Variation Method in the tetrahedron-octahedron approximation with first-principles pair interactions as input. More generally, phase stability in the Cu-Zn alloy system are discussed, using ab-initio energetic properties.

Magnitude of the Wang-Landau error

The Wang-Landau algorithm is an entropic sampling method that incoroporates an update factor ln fi , which introduces a self-avoidance tendency into the random walk. Continued sampling at constant ln fi leads to a steady state estimate of the density of states ln gi (E). We find numerically that the difference between ln gi (E) and the true density of states ln g (E) is proportional to the update factor.

Magnetic Materials at finite Temperatures: thermodynamics and combined spin and molecular dynamics derived from first principles calculations

We present a unified approach to describe the combined behavior of the atomic and magnetic degrees of freedom in magnetic materials. Using Monte Carlo simulations directly combined with first principles the Curie temperature can be obtained ab initio in good agreement with experimental values. The large scale constrained first principles calculations have been used to construct effective potentials for both the atomic and magnetic degrees of freedom that allow the unified study of influence of phonon-magnon coupling on the thermodynamics and dynamics of magnetic systems. The MC calculations predict the specific heat of iron in near perfect agreement with experimental results from 300K to above Tc and allow the identification of the importance of the magnon-phonon interaction at the phase-transition. Further Molecular Dynamics and Spin Dynamics calculations elucidate the dynamics of this coupling and open the potential for quantitative and predictive descriptions of dynamic structure factors in magnetic materials using first principles derived simulations.

Ordering energy of B2 alloys calculated in the frozen potential and Harris Approximations

It has been established that the coherent potential approximation can successfully describe the energy of random alloys. It has also served as the basis of generalized perturbation method and concentration wave calculations of the energy of short range ordered alloys. The multisublattice coherent potential, (MCPA) is the natural extension of the CPA with which to address long rang order (LRO). Using the recently developed multisublattice coherent potential approximation Korringa Kohn Rostoker, (MCPA-KKR) code the eigenvalue sum can be calculated as a function of LRO. This allows the evaluation of the ordering energy by either or two approximations. The frozen potential approximation (FPA) assumes that the muffintin single site potentials do not change as the long range order is varied; the Harris Approximation, (HA) as applied in this work, assumes that the single site charge densities do not change as the long range order is changed. These two methods of calculating the ordering energy will be compared with each other and to experiment for several systems including CuZn, NiAl, and NiAl with zinc additions. 14 refs., 4 figs., 2 tabs.