Rare-earth hydrides

The subject index provides access to the text by way of methods, techniques, reaction types, apparatus, effects and other phenomena. Also, it lists compound classes such as organotin compounds or rare-earth hydrides which cannot be expressed by the empirical formulas of the compound index. 

Rare-earth hydrides, 13 627 Rare-earth industry, recycling and disposal in, 14 646—647... 

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). 

Uranium metal reacts with hydrogen at 250-300°C to form a well-defined hydride, which resembles the rare-earth hydrides in many respects. The formula of this substance has been shown to be UH3.00 The hydride undergoes decomposition with increasing temperature the dissociation pressure of UH3 is one atmosphere at 436°C. 

G.D. Sandrock, Development of low cost nickel-rare Earth hydrides for hydrogen storage, In Hydrogen Energy Systems, W. Seifritz, T.N. Vezeroglu, Editor. (1978), Pergamon Press, Oxford, U.K. pp. 1625-1656. 

The solubility of rare earth hydrides in organic solvents is increased by appropriate additives, too. For this purpose the hydrides are reacted with electron-donor ligands such as alkyl benzoates, alkyl propionates, alkyl tolu-ates, dialkylethers, cyclic ethers, alkylated amines, N,N -dimclhylacelamide, AT-methyl-2-pyrrolidone, trialkyl and triaryl phosphines, trialkyl phosphates and triaryl phosphates, trialkyl phosphates, hexamethylphosphoric triamide, dimethyl sulfoxide, etc. Prior to use as a polymerization catalyst the prereacted mixture of the rare earth hydride plus the additive is prereacted with Al-alkyl-based Lewis acids in the temperature range of 60-100 °C for 10 min to 24 h [351,352]. 

The properties of the nonstoichiometric hydrides are discussed in groups the saline hydrides, the Group IVA hydrides, the rare earth hydrides, the actinide hydrides, the Group VA hydrides, and palladium hydride. 

From the standpoint of nonstoichiometry, the rare earth hydrides are probably the most interesting. They can be divided into three classes La through Nd Sm through Lu, excluding Eu and Yb and Eu and Yb. 

Table III. Homogeneity Ranges of Some Rare Earth Hydrides ...

It had the nonstoichiometric composition YbH2i55 and a fluorite-type structure, and, therefore, appears to be analogous to the first class of rare earth hydrides. 

Recently, higher hydrides of niobium and of vanadium have been prepared by special techniques (5, 32). They both have the cubic fluorite-type structure of the rare earth hydrides. These compounds are discussed in more detail by Gibb (11),... 

The group IIIA transition metals include the lanthanides and actinides, as well as Sc and Y however, the hydrides of Sc and Y are so similar to those of the lanthanides that they are included with the rare-earth hydrides. 

The rare-earth hydrides are prepared by heating the metal (> 200 C) under to initiate the reaction after equilibrium is reached the sample is slowly cooled to RT. Alternatively, the metal is first heated in vacuum, followed by admission of gas and slow cooling to RT. Finely divided metal can react with even at RT. Initial heating removes surface oxidation and other surface impurities. 

Rare-earth hydrides are also prepared by methods other than direct reaction with Hj e.g., rare-earth dihydrides can be formed from the metal with H O vapor at 100-150°C ... 

An interesting consequence of studies of the CFI in rare earth hydrides is that it serves to provide information about the oft-disputed electronic state of hydrogen, whether it is protonic or anionic. For those systems which have been studied by this approach the unambiguous conclusion is that hydrogen is anionic. 

Studies of the rare earth hydrides have been largely concerned with the stabilities of the phases formed, and the thermodynamic properties of the solid solutions [16-18], The decompositions of hydrides of metals with pronoimced catalytic properties (Ni, Pd) are often limited by surface steps involving hydrogen atom reactions, which are sensitive to poisoning and resemble rate processes occurring at surfaces of heterogeneous catalysts. In contrast, the decomposition of hydrides of other metals (Be, Zn) apparently proceed by nucleation and growth mechanisms. 

Apart from these specific hydrogen absorption and desorption properties, it is clear that the intermetallic compounds are more often used as precursors of new and more active catalysts. The decomposition of intermetallic compounds into transition-metal particles and rare-earth hydride, oxide or hydroxide in the course of the catalytic reaction was evidenced in a large number of catalytic reactions, especially those involving CO dissociation, which produces oxidizing species. The rise of catalytic activity was... 

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