Abstract
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) A, b = 12.6213(30) A and c = 5.0321(10) A in the space group Pbcm (No. 57) and Z = 4. The Al–D distances (1.77–1.81 A) 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−.
Highlights
Al originated from AlD3 decomposition or the mechanochemical milled SrD2 + 2AlD3 since the presence of a complex anion, [AlD4 ], in the mechanochemical milled sample was identified by Raman spectroscopy
The remaining Bragg peaks were indexed by an orthorhombic unit cell with a ≈ 4.66 Å, b ≈ 12.71 Å and c ≈ 5.03 Å, and these values are close to the theoretically and experimentally reported unit cell parameters of SrAlH5 [7,11]
SrAlD5 is the main phase present in the synthesised sample
Summary
Aluminium-based complex hydrides (alanates) composed of metal cation(s) (typically alkali or alkaline-earth metals) and a complex anion, [AlH4 ]− or [AlH6 ]3− with covalent Al–H bonds, have attracted tremendous attention as hydrogen storage materials since Bogdanović and Schwickardi reported the reversible hydrogen desorption and absorption reactions on Ti-enhanced NaAlH4(Equation (1)) [1,2,3,4,5].NaAlH4 ↔ 1/3Na3 AlH6 + 2/3Al + H2 (g) ↔ NaH + Al + 1/2H2 (g) (1)NaAlH4 is composed of Na+ and [AlH4 ]− , and Na3 AlH6 is composed of Na+ and [AlH6 ]3− ; hereafter, these as referred to as Na tetra-alanates and Na hexa-alanates, respectively.In addition to studies on NaAlH4 with Ti-based additives as hydrogen storage materials, exploratory studies on new alanates with different metal cations have been conducted.Interestingly, alkaline-earth metal tetra-alanates M(AlH4 ) composed of an alkaline-earth metal (M)and [AlH4 ]− decompose into MAlH5 containing corner-sharing AlH6 octahedron chains [3,6,7,8,9]Crystals 2018, 8, 89; doi:10.3390/cryst8020089 www.mdpi.com/journal/crystalsCrystals 2018, 8, 89 after releasing hydrogen from M(AlH4 ) (Equation (2)). Aluminium-based complex hydrides (alanates) composed of metal cation(s) (typically alkali or alkaline-earth metals) and a complex anion, [AlH4 ]− or [AlH6 ]3− with covalent Al–H bonds, have attracted tremendous attention as hydrogen storage materials since Bogdanović and Schwickardi reported the reversible hydrogen desorption and absorption reactions on Ti-enhanced NaAlH4. NaAlH4 is composed of Na+ and [AlH4 ]− , and Na3 AlH6 is composed of Na+ and [AlH6 ]3− ; hereafter, these as referred to as Na tetra-alanates and Na hexa-alanates, respectively. In addition to studies on NaAlH4 with Ti-based additives as hydrogen storage materials, exploratory studies on new alanates with different metal cations have been conducted. Alkaline-earth metal tetra-alanates M(AlH4 ) composed of an alkaline-earth metal (M). Crystals 2018, 8, 89 after releasing hydrogen from M(AlH4 ) (Equation (2)). We refer to MAlH5 as an alkaline-earth metal penta-alanate.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
