Abstract
The structure and phase variation of Ca 5Si 3 upon hydrogenation were systematically investigated using combined neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), and first-principles calculations. The hydrogen absorption equilibrium was first attained with formation of Ca 5Si 3H(D) 0.53 ( I4/ mcm) with H exclusively located in Ca 4-tetrahedral sites. More hydrogen absorbed into the system under higher pressure leads to dissociations into CaH 2 (an amorphous hydride at higher pressures) and CaSi. The hydrogen-induced formation of an amorphous phase under higher pressures is very unusual in Cr 5B 3-type compounds and the observed formation of CaH 2 upon hydrogen absorption confirmed the proposed composition equilibrium between A 5Tt 3 (A = Ca, Sr; Tt = Si, Ge, Sn) and AH 2.
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