Structure and Characterization of KSc(BH₄)₄

A new potassium scandium borohydride, KSc(BH₄)₄, is presented and characterized by a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. The title compound, KSc(BH₄)₄, forms at ambient conditions in ball milled mixtures of potassium borohydride and ScCl₃ together with a new ternary chloride K₃ScCl₆, which is also structurally characterized. This indicates that the formation of KSc(BH₄)₄ differs from a simple metathesis reaction, and the highest scandium borohydride yield (∼31 mol %) can be obtained with a reactant ratio KBH₄:ScCl₃ of 2:1. KSc(BH₄)₄ crystallizes in the orthorhombic crystal system, a = 11.856(5), b = 7.800(3), c = 10.126(6) Å, V = 936.4(8) ų at RT, with the space group symmetry Pnma. KSc(BH₄)₄ has a BaSO₄ type structure where the BH₄ tetrahedra take the oxygen positions. Regarding the packing of cations, K⁺, and complex anions, [Sc(BH₄)₄]⁻, the structure of KSc(BH₄)₄ can be seen as a distorted variant of orthorhombic neptunium, Np, metal. Thermal expansion of KSc(BH₄)₄ in the temperature range RT to 405 K is anisotropic, and the lattice parameter b shows strong nonlinearity upon approaching the melting temperature. The vibrational and NMR spectra are consistent with the structural model, and previous investigations of the related compounds ASc(BH₄)₄ with A = Li, Na. KSc(BH₄)₄ is stable from RT up to ∼405 K, where the compound melts and then releases hydrogen in two rapid steps approximately at 460−500 K and 510−590 K. The hydrogen release involves the formation of KBH₄, which reacts with K₃ScCl₆ and forms a solid solution, K(BH₄)_1−xCl_x. The ternary potassium scandium chloride K₃ScCl₆ observed in all samples has a monoclinic structure at room temperature, P2₁/a, a = 12.729(3), b = 7.367(2), c = 12.825(3) Å, β = 109.22(2)°, V = 1135.6(4) ų, which is isostructural to K₃MoCl₆. The monoclinic polymorph transforms to cubic at 635 K, a = 10.694 Å (based on diffraction data measured at 769 K), which is isostructural to the high temperature phase of K₃YCl₆.