Trimorphic TaCrP. A diffraction and ${}^{31}$P 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$\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 (P12${}_1$/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 ${}^{31}$P solid state NMR spectroscopy.