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Density functional study of the L1 sub(0)-aIrV transition in IrV and RhV
Ist Teil von
Journal of alloys and compounds, 2011-01, Vol.509 (3), p.560-567
Erscheinungsjahr
2011
Link zum Volltext
Quelle
Elsevier ScienceDirect Journals Complete
Beschreibungen/Notizen
Both IrV and RhV crystallize in the aIrV structure, with a transition to the higher symmetry L1 sub(0) structure at high temperature, or with the addition of excess Ir or Rh. Here we present evidence that this transition is driven by the lowering of the electronic density of states at the Fermi level of the aIrV structure. The transition has long been thought to be second order, with a simple doubling of the L1 sub(0) unit cell due to an unstable phonon at the R point (0 1/2 1/2). We use first-principles calculations to show that all phonons at the R point are, in fact, stable, but do find a region of reciprocal space where the L1 sub(0) structure has unstable (imaginary frequency) phonons. We use the frozen phonon method to examine two of these modes, relaxing the structures associated with the unstable phonon modes to obtain new structures which are lower in energy than L1 sub(0) but still above aIrV. We examine the phonon spectra of these structures as well, looking for instabilities, and find further instabilities, and more relaxed structures, all of which have energies above the aIrV phase. In addition, we find that all of the relaxed structures, stable and unstable, have a density comparable to the L1 sub(0) phase (and less than the aIrV phase), so that any transition from one of these structures to the ground state will have a volume change as well as an energy discontinuity. We conclude that the transition from L1 sub(0) to aIrV is probably weakly first order. We also examine the behavior of similar compounds, and show that the aIrV structures of both IrTi and RhTi are lower in energy than the experimentally observed high-temperature L1 sub(0) structure.