Trimorphic TaCrP – A diffraction and 31P solid state NMR spectroscopic study

The metal-rich phosphide TaCrP forms from the elements by step-wise solid state reaction in an alumina crucible (maximum annealing temperature 1180 K). TaCrP is trimorphic. The structural data of the hexagonal ZrNiAl high-temperature phase (space group P 6 ‾ 2 m $P\overline{6}2m$ ) was deduced from a Rietveld refinement. At room temperature TaCrP crystallizes with the TiNiSi type (Pnma, a = 623.86(5), b = 349.12(3), c = 736.78(6) pm, wR = 0.0419, 401 F 2 values, 20 variables) and shows a Peierls type transition below ca. 280 K to the monoclinic low-temperature modification (P121/c1, a = 630.09(3), b = 740.3(4), c = 928.94(4) pm, β = 132.589(5)°, wR = 0.0580, 1378 F 2 values, 57 variables). The latter phase transition is driven by pairwise Cr–Cr bond formation out of an equidistant chain in o-TaCrP. The phase transition was monitored via different analytical tools: differential scanning calorimetry, powder synchrotron X-ray diffraction, magnetic susceptibility measurements and 31P solid state NMR spectroscopy.

Peierls distortion of the cobalt chain in the low-temperature structure of CoIn2

CoIn2 (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 (CuMg2 type, Fddd, a = 529.95(10), b = 940.49(13), c = 1785.8(3) pm, wR2 = 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 C2/c and exhibits a new structure type (a = 933.7(7), b = 526.91(10), c = 1000.8(2) pm, β = 117.81(5)°, wR2 = 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-CoIn2 (270.1 pm, 175.2° Co–Co–Co) show pairwise dislocation in LT-CoIn2 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-CoIn2 along with a shift to lower energy, manifesting the stabilization by pair formation through Peierls distortion.