Trihydrides, rare earth

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. 

Thermodynamic properties of cubic rare earth trihydrides. 

Enthalpies and entropies of formation of hydrogen-deficient hexagonal rare earth trihydrides from hydrogen-rich dihydrides. 

In all these cases, hydride formation corresponds to partial occupation of the available holes, reminiscent of multihole polyhedra behavior. Occupation of all available holes would require a limiting stoichiometry MH3, and would correspond to occupation of the unique hole in isolated polyhedra. This situation is known for some rare earth hydrides (see Table III). Significantly, transformation of the metallic dihydride to the trihydride occurs with a decrease in apparent metal-metal distances and with a large increase in resistivity. These observations indicate a salt-like character and the disappearance of metal-metal bonds (27). 

Chemical loading of rare-earth films by immersion in an aqueous KOH solution containing NaBH4 has been demonstrated by Van der Sluis [94] for Gd. The redox reaction forms BO2 and GdHs. The attractive feature of this technique is that, due to the high redox potential difference, it leads to almost stoichiometric trihydrides. The reaction is reversed by immersion in a 0.3 % H2O2 aqueous solution. 

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. 

The heat of formation of magnesium hydride (—33 kJ/mol H) is similar to that of the transition between the rare earth dihydride and trihydride. This was essential to keep the hydrogen uptake reversible. 

In addition, most of the rare earths will also form trihydrides. Both the dihydrides and trihydrides are normally nonstoichiometric, usually exhibiting very wide existence ranges. The rate at which reaction (26.1) proceeds depends upon temperature, hydrogen pressure, and the condition of the metal surface. In some cases, the reaction will occur at room temperature and low pressures (< 1 torr) if the surface is clean (Beavis et al., 1974 Curzon and Singh, 1975). In all cases, however, the reaction will proceed readily at moderate temperatures (100-500°C) and pressures (slatm of H2). A more detailed discussion of the reaction mechanisms is given in section 6. 

A schematic generalized phase diagram for rare earth-hydrogen systems is shown in fig. 26.2. The two-phase cubic + hexagonal region does not occur in systems where the rare earth dihydrides form continuous solid solutions with their trihydrides in those cases the cubic hydride phase field extends to H/M = 3. The actual positions of the phase boundaries depend on the particular metal-bydrogen system involved, as well as the temperature. Room temperature existence ranges of the rare earth hydrides are summarized in table 26.1. 

Recently, Mintz et al. (1974) reported that neodymium trihydride, NdHs-, samples prepared at or below 350 C have a hexagonal structure (oo = 3.84 A, Co = 6.80 A) similar to the hexagonal trihydrides of the heavier rare earths. If this phase is heated above 300°C it transforms to the f.c.c. neodymium hydride, and it retains the cubic structure when cooled down again. 

Lattice parameters for all the rare earth di- and trihydrides are presented in table 26.2. Lattice parameters of the corresponding deuterides are usually a few tenths of a percent smaller. 

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