Centrosymmetric silicide ScNiSi3 – Structure, chemical bonding, and 45Sc solid state NMR

The SmNiGe3 type silicide ScNiSi3, previously reported to crystallize in the non-centrosymmetric space group Amm2, was synthesized from the elements by arc-melting. Two different single crystals were investigated on the basis of X-ray diffractometer data: a = 382.0(1), b = 2058.2(1), c = 381.0(1) pm, wR2 = 0.0531, 947 F2 values for ScNi0.913(2)Si3 and a = 382.5(1), b = 2058.7(1), c = 380.8(1) pm, wR2 = 0.0494, 554 F2 values for ScNi0.929(4)Si3 with 23 variable parameters per refinement. The single crystal data unambiguously confirm the centrosymmetric space group Cmmm. The structure consists of a three-dimensional [NiSi3] polyanion in which the scandium atoms fill distorted hexagonal channels. The polyanion contains three crystallographically independent silicon sites which are all involved in Si–Si bonding with Si–Si distances ranging from 232 to 235 pm. Each nickel atom has a square-pyramidal silicon coordination with rather short Ni–Si distances (217–222 pm). The nickel sites of both crystals show significant underfilling. The 45Sc magic-angle spinning NMR spectrum reveals the presence of a single scandium site, thereby confirming the centrosymmetric structure. In addition, a second, weaker resonance is attributed to scandium nuclei in the vicinity of a nickel vacancy. The fractional area of this resonance is quantitatively consistent with the nickel site occupancy of 92% determined from single crystal X-ray diffraction. Electronic structure calculations reveal metallic behaviour with strong Ni–Si and Si–Si bonds as expected, but additionally significant Sc–Ni bonding in contrast to YNiSi3, where Y–Ni bonding is reported to be absent.

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