FIRST-PRINCIPLES THEORY OF SHORT-RANGE ORDER, ELECTRONIC EXCITATIONS AND SPIN POLARIZATION IN NI-V AND PD-V ALLOYS

The short-range order (SRO) and long-range order (LRO) of Ni-V and Pd- V alloys are studied theoretically by a combination of first-principle s calculations of Ising-like interaction energies (J(f)) with a Monte Carlo solution of the Ising Hamiltonian. We find the following: (i) Th ere are several compositions in these alloys for which the dominant wa ve vectors of LRO and those of SRO do not coincide, indicating that th e low-temperature (T) LRO may not always be inferred from the high-T S RO. (ii) In Ni3V and Pd3V, the density of states at the Fermi level, n (epsilon(F)), is much larger in L1(2) than in the stable D0(22) struct ure. This has two consequences: (a) thermal electron-hole excitations across epsilon(F) are energetically more favorable in the L1(2) struct ure and lead to a T dependence of J(f), and (b) magnetic stabilization is larger in L1(2), so spin polarization affects structural stability . As a result, (iii) calculations using T-dependent J(f)'s are needed to obtain quantitative agreement with experimental measurements of LRO , SRO, and transition temperatures in Ni0.75V0.25, Ni0.67V0.33, and Pd 0.75V0.25. (iv) We provide predictions of the SRO patterns where there is currently no experimental evidence for Pd0.67V0.33, Ni0.6V0.4, Pd0 .6V0.4, and Pd0.5V0.5. (v) For Ni3V and Pd3V, discrepancies are found between the total-energy differences of the L1(2) and D0(22) structure s as determined by T=0 first-principles calculations and those inferre d from diffuse neutron scattering measurements at high T. By performin g temperature-dependent self-consistent local-density-approximation ca lculations, we find that electronic excitations are responsible for re ducing the discrepancy by similar to 25% and the combination of spin p olarization and electronic excitations reduce the discrepancy by simil ar to 30-50%. Thus, electronic excitations and spin polarization are n ot fully responsible for the T dependence of J(f) used in the SRO calc ulations.