Berlin 2005 – wissenschaftliches Programm
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M: Metallphysik
M 21: Intermetallische Phasen II
M 21.3: Vortrag
Samstag, 5. März 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 Zunger2 — 1Fritz-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.