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.

The ternary system Sc–Co–In at 870 K: the isothermal section and the crystal structures of the compounds

The isothermal section of the Sc–Co–In system at T = 870 K has been constructed using X-ray powder diffraction and SEM/EDX data. At the studied temperature, nine ternary compounds are formed: Sc50Co12.5In3.5 (Ag7+x Mg24−x -type structure, space group Fm 3 ‾ $\overline{3}$ , a = 17.7411 Å), Sc14Co3.10In2.59 (Lu14Co3In3 type, P42/nmc, a = 8.8913, c = 21.387 Å), Sc6Co2.18In0.82 (Ho6Co2Ga type, Immm, a = 8.867, b = 8.780, c = 9.321 Å), Sc2CoIn (Pt2ZnCd type, P4/mmm, a = 3.2887, c = 7.1642 Å), Sc11Co4In9 (Nd11Pd4In9 type, Cmmm, a = 13.836, b = 20.758, c = 3.351 Å), Sc5Co2In4 (Lu5Ni2In4 type, Pbam, a = 17.3400, b = 7.5940, c = 3.3128 Å), Sc3Co1.64In4 (Lu3Co2−x In4 type, P 6 ‾ $\overline{6}$ , a = 7.6702, c = 3.3595 Å), Sc10Co9In20 (Ho10Ni9In20 type, P4/nmm, a = 12.8331, c = 9.0226 Å), Sc0.5In0.5Co2 (ternary non-centrosymmetric derivative of the Laves phase MgNi2, P63 mc, a = 4.8910, c = 16.0166 Å, with a homogeneity range from 12.3 to 16.7 at% of indium). The cubic phase ScCo4In (MgCu4Sn type, F 4 ‾ $\overline{4}$ 3m, a = 6.9230(8) Å) is evidently present in the cast samples and disappears after the annealing procedure. A Sc/In substitution was observed for the solid solution Sc1−x In x Co2 (x = 0–0.26) with the structure of the cubic Laves phase (MgCu2 type) and also for the hexagonal Sc0.5In0.5Co2-type phase.