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Berlin 2005 – scientific programme

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M: Metallphysik

M 21: Intermetallische Phasen II

M 21.3: Talk

Saturday, March 5, 2005, 11:30–11:45, TU H2038

First-principles prediction of novel ground state structures for the binary systems made of Nb, Ta, Mo, W — •Volker Blum1 and Alex Zunger21Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin — 2National Renewable Energy Laboratory, Golden, Colorado 80401, USA

The refractory metals Nb, Ta, Mo, and W are the most prominent body-centered cubic transition metals in the periodic system of elements. While the measured mixing enthalpies are negative, implying the existence of some long-range ordered binary crystal structures, the phase diagrams, restricted so far to high-temperature measurements, have revealed only random alloys at high T. It was previously guessed that the ordered structures are ordinary B2 (CsCl type) or D03 (BiF3 type), but in principle could be any of the infinite number of other configurations. Combining DFT calculated energies of O(50) configurations for each binary with a “Mixed Basis Cluster Expansion” whose interaction types are chosen by a genetic algorithm search, we derive the energy for any bcc configuration. This (Ising-like) functional is then searched to find T=0 ground state structures, and to compute (via Monte Carlo) finite T thermodynamics. The ground states reflect directly the complexity of the underlying interatomic interactions, with the observed absence of high-temperature ordered phases as a consequence. We also predict atomic short-range order in the solid solutions at finite T.
Work performed at NREL under DOE-BES support.

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