New stannide ScAgSn:  determination of the superstructure via two-dimensional ⁴⁵Sc solid state NMR

Abstract

The new stannide ScAgSn was synthesized by induction melting of the elements in a sealed tantalum tube and subsequent annealing. ScAgSn crystallizes with a pronounced subcell structure:  ZrNiAl type, P6̄2m, a = 708.2(2) pm, c = 433.9(1) pm, wR2 = 0.1264, 321 F₂ values, and 14 variables. The Guinier powder pattern reveals weak superstructure reflections pointing to a TiFeSi-type structural arrangement:  I2cm, a = 708.1(1) pm, b = 1225.2(2) pm, c = 869.9(1) pm, wR2 = 0.0787, 5556 F2 values, and 49 variables. So far the growth of high-quality single crystals failed. Determination of the superstructure was partly based on merohedral triplet X-ray data augmented by ¹¹⁹Sn Mössbauer spectroscopy and ¹¹⁹Sn and ⁴⁵Sc solid-state NMR data. In particular, the observation of three crystallographically inequivalent sites in ⁴⁵Sc NMR triple quantum magic-angle spinning (TQ-MAS) NMR spectra provided unambiguous proof of the superstructure proposed. The ScAgSn structure consists of a three-dimensional [AgSn] network (with Ag–Sn distances between 273 and 280 pm) in which the scandium atoms are located in distorted hexagonal channels, each having five tin and two silver nearest neighbors. Both crystallographically independent tin sites have a tricapped trigonal prismatic coordination, that is, [Sn₁Sc₆Ag₃] and [Sn₂Ag₆Sc₃] environments, which are well distinguished in the ¹¹⁹Sn NMR and Mössbauer spectra because of their different site symmetries.