Solid hydrogen in hydrogenated amorphous silicon

The thermal properties of hydrogenated amorphous Si at temperatures between 0.1 and 5 K are shown by calorimetry to be dominated by the presence of molecular hydrogen H2 in microvoids. A spontaneous release of heat due to conversion of o-H2 to p-H2 is observed from which the concentration (0.5 at.%) and bimolecular conversion rate (0.025 h-1) are obtained. The specific heat is time dependent, varies roughly as T2 between 0.1 and 3 K, and is attributed to a highly broadened orientational ordering transition of H2 in restricted geometry.