Structural and dynamic properties of the polyanionic hydrides SrAlGeH and BaAlGeH

摘要

The quaternary aluminium hydrides SrAlGeH and BaAlGeH were synthesized from either hydrogenating the intermetallic AlB_2-type precursors SrAlGe and BaAlGe or reacting SrH_2 with a mixture of Al and Ge in the presence of pressurized hydrogen. Their structures were characterized by X-ray and neutron powder diffraction of the corresponding deuterides. The compounds crystallize with the trigonal SrAlSiH structure type (space group P3m1, Z= 1, a = 4.2435(2) and 4.3450(2) A, c = 4.9710(3) and 5.2130(4) A for SrAlGeH and BaAlGeH, respectively) and feature a two-dimensional polyanion [AlGeH]~(2-) which represents a corrugated hexagon layer built from three-bonded Al and Ge atoms. H is terminally attached to Al. Polyanions [AlGeH]2" are electron precise and, according to electronic structure calculations, the quaternary hydrides display band gaps with sizes between 0.7 and 0.8 eV. Infrared and inelastic neutron scattering spectroscopy show Al-H stretching and bending mode frequencies at around 1250 and 870 cm ~(-1), respectively. SrAlGeH and BaAlGeH are thermally stable up to at least 500 °C When exposed to air the hydrides decompose rapidly to amorphous, orange colored materials.