Stoichiometry and Structure of LiH upto 6.0 GPa and 820 K

Abstract

Under suitable conditions, elemental lithium reacts with hydrogen to form a 1:1 stoichiometric hydride possessing the rock-salt structure. We have conducted experimental investigations a t pressures between 2.5 and 6 . 0 GPa in the temperature interval 500 to 820 K to determine (i) if LiH reacts further with hydrogen to form a quenchable, higher hydride, LiH , where n > 1; and (ii) the pressure at n which LiH i s transformed into the denser cesium-chloride structure. In addition to these experiments, we have investigated the effects of time on LiH stoichiometry under these conditions. Experiments below 4 . 0 GPa were performed in a piston-cylinder apparatus simto that of Boyd and England (1). The high pressure die and sample assembly ilar have been previously described by Stephens and co-workers (2, 3). Experiments conducted above 5.0 GPa were carried out in a girdle-anvil device previously described by Stromberg and Stephens ( 4 , 5). All temperature readings are accurate to 220 K pressures a r e accurate to kO.1 GPa. Run products were analyzed by conventional Debye-Scharrer X-ray powder techniques. Complete details of the sample preparation and containment, experimental runs, and X-ray results a r e given elsewhere (6) The results and interpretation of the experimental data are complicated by the nature and multiplicity of the contaminating phases. The presence of intermetallic compounds from the encapsulating materials hinders the interpretation of results for pressures lower than 4 . 0 GPa. However, in this pressure interval, the basic LiH rock-salt structure was still observed. The most clearly defined results were obtained in an assembly consisting of a Ag-Pd inner capsule and a tantalum outer capsule at 5.0 GPa and 773 K . These conditions produced no change in the stoichiometry of normal LiH having the rock-salt structure. A s a result of our investigation, we conclude that up to 6 . 0 GPa and 820 K no change can be produced in LiH 1) Work performed under the auspices of the U . S. Energy Research & Development Admini s t rat ion.