Intermediate temperature structural modulation and properties of CoIn 2

At temperature $T=293\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the crystal structure of ${\mathrm{CoIn}}₂$ is characterized by linear chains of equidistant Co atoms, but these chains undergo pairwise dislocation via Peierls distortion at some $T>90\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ [Z. Kristallogr. 237, 239 (2022)]. Recently published differential scanning calorimetry measurements revealed at least two intervening thermodynamic effects near ${T}₁=194(1)\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}₂=206(2)\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ but did not clarify how these features were related to the structural distortion or distortions. In the present paper, we report a suite of measurements on high-quality single crystals of ${\mathrm{CoIn}}₂$. We observe clear signatures of both transitions in $T$-dependent measurements of heat capacity, electrical resistivity, and dc magnetization. Moreover, we find a new crystal structure with polar orthorhombic space group $Fdd2$ to reside between the two transitions at $T=198(2)\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Like the high-$T$ structure, the lattice of this intermediate-$T$ structure remains characterized by a single intrachain Co-Co distance ${d}_{\mathrm{Co}\ensuremath{-}\mathrm{Co}}=2.69324(10)\phantom{\rule{0.16em}{0ex}}\AA{}$, but sparse $\mathcal{O}(1%)$ structural disorder is resolved by the formation of a structural modulation with propagation vector $\mathbf{q}=(0,\ensuremath{\delta},0), \ensuremath{\delta}=0.23\ifmmode\pm\else\textpm\fi{}0.02$ oriented in a direction perpendicular to the Co-Co chains. We discuss the results in light of the potential onset of a charge density wave state above the Peierls distortion.