RESUMO
The vibrational properties of the recently discovered aluminum hydrides SrAl2H2 and SrAlSiH have been investigated by means of inelastic neutron scattering (INS) and first-principles calculations. Both compounds contain Al-H units being part of a two-dimensional polyanionic layer, [(AlH)(AlH)]2- and [Si(AlH)]2-, respectively. The INS spectrum of SrAlSiH is characterized by very weakly dispersed Al-H modes with well-resolved overtones, while SrAl2H2 yields a solid-state dispersed phonon spectrum. The frequency of the stretching mode of the Al-H unit in SrAlSiH is the hitherto lowest observed for a terminal Al-H bond. At the same time, SrAlSiH displays the highest decomposition temperature known for an aluminum hydride compound. It is proposed that the stability of solid-state aluminum hydrides correlates inversely with the strength of Al-H bonding.
RESUMO
The hydrogenation behavior of the polar intermetallic systems AeE2 (Ae = Ca, Sr, Ba; E = Al, Ga, In) has been investigated systematically and afforded the new hydrides SrGa2H2 and BaGa2H2. The structure of these hydrides was characterized by X-ray powder diffraction and neutron diffraction of the corresponding deuterides. Both compounds are isostructural to previously discovered SrAl2H2 (space group P3m1, Z = 1, SrGa2H2/D2: a = 4.4010(4)/4.3932(8) A, c = 4.7109(4)/4.699(1) A; BaGa2H2/D2: a = 4.5334(6)/4.5286(5) A, c = 4.9069(9)/4.8991(9) A). The three hydrides SrAl2H2, SrGa2H2, and BaGa2H2 decompose at around 300 degrees C at atmospheric pressure. First-principles electronic structure calculations reveal that H is unambiguously part of a two-dimensional polyanion [E2H2]2- in which each E atom is tetrahedrally coordinated by three additional E atoms and H. The compounds AeE2H2 are classified as polyanionic hydrides. The peculiar feature of polyanionic hydrides is the incorporation of H in a polymeric anion where it acts as a terminating ligand. Polyanionic hydrides provide unprecedented arrangements with both E-E and E-H bonds. The hydrogenation of AeE2 to AeE2H2 takes place at low reaction temperatures (around 200 degrees C), which suggests that the polyanion of the polar intermetallics ([E2]2-) is employed as precursor.
