Covalent Si–H Bonds in the Zintl Phase Hydride CaSiH1+x (x ≤ 1/3)？

The crystal structure of the Zintl phase hydride CaSiH ≈4/3 was discussed controversially, especially with respect to the nature of the silicon-hydrogen interaction. We have applied X-ray and neutron powder diffraction as well as total neutron scattering on a deuterated sample, CaSiD 1.1. Rietveld refinement (CaSiD 1.1, Pnma, a = 14.579(4) ？, b = 3.8119(4) ？, c = 11.209(2) ？) and an analysis of the neutron pair distribution function show a silicon-deuterium bond length of 1.53 ？. The Si–H bond may thus be categorized as covalent and the main structural features described by a limiting ionic formula Ca 2+H ？(Si ？) 2/3(SiH ？) 1/3. Hydrogen atoms decorating the ribbon-like silicon polyanion made of three connected zigzag chains are under-occupied, resulting in a composition CaSiH 1.1. Hydrogen-poor Zintl phase hydrides CaSiH <1 with hydride ions in Ca 4 tetrahedra only were found in an in situ neutron diffraction experiment at elevated temperature. Hydrogen (deuterium) uptake and release in CaSiD x (0.05 ≤ x ≤ 0.17) is a very fast process and takes less than 1 min to complete, which is of importance for possible hydrogen storage applications. View Full-Tex