  Both IrV and RhV crystallize in the alpha-IrV structure, with a transition to
the higher symmetry L1_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
alpha-IrV structure. The transition has long been thought to be second order,
with a simple doubling of the L1_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_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_0 but still above alpha-IrV. 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 alpha-IrV phase. In addition, we find that all of the relaxed
structures, stable and unstable, have a density comparable to the L1_0 phase
(and less than the alpha-IrV 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_0 to alpha-IrV is
probably weakly first order.
