Density functional study of the L10–αIrV transition in IrV and RhV

Both IrV and RhV crystallize in the αIrV structure, with a transition to the higher symmetry L10 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 αIrV structure. The transition has long been thought to be second order, with a simple doubling of the L10 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 L10 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 L10 but still above αIrV. 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 αIrV phase. In addition, we find that all of the relaxed structures, stable and unstable, have a density comparable to the L10 phase (and less than the αIrV 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 L10 to αIrV is probably weakly first order. We also examine the behavior of similar compounds, and show that the αIrV structures of both IrTi and RhTi are lower in energy than the experimentally observed high-temperature L10 structure.

Research highlights▶ The computational determination of the ground state of a material can be a difficult task, particularly if the ground state is uncommon and so not found in usual databases. In this paper we consider the alpha-IrV structure, a low temperature structure found only in two compounds, IrV and RhV. In both cases this structure can be considered as a distorted tetragonal structure, and the tetragonal “L10” structure is the high temperature structure for both compounds. We show, however, that the logical path for the transition from the L10 to the alpha-IrV structure is energetically forbidden, and find a series of unstable and metastable structures which have a lower energy than the L10 phase, but are higher in energy than the alpha-IrV phase. We also consider the possibility of the alpha-IrV structure appearing in neighboring compounds. We find that both IrTi and RhTi are candidates.