Add time:09/04/2019         Source:sciencedirect.com

Ho5Si3Hx hydrides (2 < x < 4) exhibit an α ⇌ β order-disorder transformation with desorption of 1H per Ho5Si3 in contravention of the phase rule, suggesting two forms of hydrogen exist in the hydride. Reaction of Ho5Si3H2.76 with D2 indicated rapid exchange of 1.81H followed by a slower exchange process. Cycling through the α ⇌ β transformation did not accelerate mixing of H and D in the hydride and supported the existence of two discrete forms of hydrogen. Gd5Si3H7 rapidly exchanged 5.2H as expected from its hydriding behaviour. The Ho5Si3Hx behaviour is attributed to ready filling of octahedral metal columns of the D88 structure by one hydrogen and then by a second hydrogen to give H—H distances near 0.21 nm. This increases repulsion, causes reversible absorption of the second hydrogen and displaces the metal atoms slightly to cause super cell formation. Rapid diffusion in these columns contrasts with slow diffusion elsewhere in the D88 structure. D88 silicides with smaller unit cells do not behave similarly because H-H repulsion prevents absorption of a second hydrogen and for D88 silicides with larger cells repulsion is insufficient to displace the metal atoms and the larger interstices enhance diffusion and allow greater hydrogen absorption.

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