Crystal structural determination of SrAID₅ with corner-sharing AID₆ octahedron chains by x-ray and neutron diffraction

Aluminium-based complex hydrides (alanates) composed of metal cation(s) and complex anion(s), [AlH₄ ]⁻ or [AlH₆]³⁻ with covalent Al–H bonds, have attracted tremendous attention as hydrogen storage materials since the discovery of the reversible hydrogen desorption and absorption reactions on Ti-enhanced NaAlH₄. In cases wherein alkaline-earth metals (M) are used as a metal cation, MAlH₅ with corner-sharing AlH₆ octahedron chains are known to form. The crystal structure of SrAlH₅ has remained unsolved although two different results have been theoretically and experimentally proposed. Focusing on the corner-sharing AlH₆ octahedron chains as a unique feature of the alkaline-earth metal, we here report the crystal structure of SrAlD₅ investigated by synchrotron radiation powder X-ray and neutron diffraction. SrAlD₅ was elucidated to adopt an orthorhombic unit cell with a = 4.6226(10) Å, b = 12.6213(30) Å and c = 5.0321(10) Å in the space group Pbcm (No. 57) and Z = 4. The Al–D distances (1.77–1.81 Å) in the corner-sharing AlD₆ octahedra matched with those in the isolated [AlD₆ ]³⁻ although the D–Al–D angles in the penta-alanates are significantly more distorted than the isolated [AlD₆]³⁻.