Polar structural modulation in proximity to Peierls distortion in CoIn₂

The presence of a disordered, short-range ordered, or modulated crystal structure separating a high-symmetry, high temperature phase from a low temperature ground state can lead to a host of interesting and technologically relevant properties including large Berry curvature, ultrahigh thermoelectric efficiency, and ferroelectricity. In CoIn₂, linear chains of equidistant Co atoms in the ambient temperature structure distort via pairwise Peierls dislocation, likely at temperature T₁ = 194 +/- 1 K. Signatures in heat capacity, electrical resistivity, magnetization, and single crystal x-ray diffraction measurements, however, reveal that a polar, non-centrosymmetric, structurally modulated phase separates the undistorted and Peierls phases over a narrow range between T₁ and T₂ = 206 +/- 2 K. Here, impending disorder is resolved by the formation of a structural modulation with propagation vector q = (0,d,0), d = 0.23 +/- 0.02 oriented in a direction perpendicular to the Co-Co chains. Moreover, we observe enhancement in heat capacity and magnetization across a broad temperature range from 140 K to 210 K, which extends deep into the Peierls ground state. These results suggest that the structural modulation may be the signature of a charge density wave (CDW), which overlaps and coexists with the Peierls phase.