Competing structural modulations above first order phase transition in CoIn₂

CoIn₂ undergoes a Peierls distortion upon cooling through temperature T₁ = 194(1) K.  Measurements of the T-dependences of magnetic susceptibility, electrical resistivity, and heat capacity reveal that the Peierls phase is separated from the high temperature, high symmetry Fddd structure by at least two structural phase transitions, one at T₁ and a second at T₂ = 206(2) K.  We previously reported a polar, modulated structure at T = 198(2) K with propagation vector q = (0, δ, 0), δ = 0.23(2) oriented normal to the Co-Co 1D chains.  Subsequent scattering measurements performed with synchrotron x-rays reveal, however, that the superstructure/CDW reflections undergo a pairwise splitting at T = 196.0(5) K and persist some 8 K into the Peierls phase with two competing modulated structures thus spanning beyond the regime T₁ < T < T₂.  Additionally, improved single crystals obtained via a liquid transport technique have permitted us to clarify that the transition at T₁ is likely first-order in nature, thereby resolving the mechanism by which inversion symmetry is apparently restored in the Peierls ground state.