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March 28, 2022
Abstract
CoIn 2 ( Z. Metallkd . 1970, 61 , 342–343) forms by reaction of the elements at 1470 K followed by annealing at 770 K for five days. The room temperature structure is orthorhombic (CuMg 2 type, Fddd , a = 529.95(10), b = 940.49(13), c = 1785.8(3) pm, w R 2 = 0.0563, 444 F 2 values, 17 variables) and shows a phase transition at 195(1) K (DSC data). The low-temperature modification crystallizes in the translationengleiche monoclinic subgroup C 2/ c and exhibits a new structure type ( a = 933.7(7), b = 526.91(10), c = 1000.8(2) pm, β = 117.81(5)°, w R 2 = 0.0374, 843 F 2 values, 30 variables). The structural phase transition is a consequence of a Peierls type distortion. The equidistant cobalt chains in HT-CoIn 2 (270.1 pm, 175.2° Co–Co–Co) show pairwise dislocation in LT-CoIn 2 with shorter (252.4 pm) and longer (284.1 pm) Co–Co distances. Each cobalt atom has coordination number 10 in the form of slightly distorted square antiprisms of indium, capped by cobalt on the rectangular faces. Density-of-states calculations reveal metallic behavior for both modifications. Integrated crystal orbital overlap populations featuring the bonding characteristics indicate a slightly higher intensity area for LT-CoIn 2 along with a shift to lower energy, manifesting the stabilization by pair formation through Peierls distortion.
