Hydrides orthorhombic dihydrides

Eu—H system.—The lighter rare earth trihydrides have a face centered cubic structure whilst those of samarium and the heavier rare earths are hexagonal close packed. Europium and ytterbium, however, form orthorhombic dihydrides [236]. Although it has been possible to prepare [237] a hydride of ytterbium with empirical formula YKH2.55 all attempts to prepare the europium trihydride failed. 

The third subgroup consists of Eu and Yb dihydrides, which have an orthorhombic structure and resemble the alkaline-earth hydrides, CaHj, SrHj and BaHj (see 1.8.3.3). 

Hydrides are made from 1 1 intermetallics of groups V and VIII transition metals. For example, TiFe, TiCo and TiNi have the cubic CsCl structure, but on hydriding the first two form monohydrides and dihydrides that are orthorhombic and monoclinic, respectively, although they may be viewed as distorted CsCl structures. No hydride phase is formed from TiNi at RT (or above), but a solid solution of maximum composition TiNiH, 7 forms . The ZrCo and HfCo intermetallics also have the CsCl structure but transform to the orthorhombic CrB structure on formation of a monohydride... 

The rare earths absorb hydrogen readily and form solid solutions and/or hydrides exothermally at temperatures of several hundred C. Their phase diagrams consist, in general, of three basic parts (a) the metallic solid solution, or a-phase, with the H atoms inserted in the tetrahedral interstices of the host-metal lattice (b) the equally metallic dihydride 3-phase, where the two H atoms occupy ideally the two available tetrahedral sites this phase crystallizes in the fee fluorite system (c) the insulating trihydride, or y-phase, which possesses an hep unit cell with both tetrahedral sites and the one octahedral site filled up. A schematic view is given in fig. 1. Exceptions are the divalent lanthanides Eu and Yb, whose dihydrides are already insulators and exhibit an orthorhombic structure, and Sc whose very small unit cell does not normally accept more than two H atoms. 

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