Rare earth hydrides, stabilities

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. 

This is consistent with the observation that there is also a MI transition in Mg-stabilized cubic YH3 j [122] and in LaH which remains fee at room temperature for 2 < X < 3. Even Lai-zYzH alloys show the same behavior independently of their crystallographic structure and their degree of stmctural disorder [56]. This is another demonstration of the robustness ofthe MI transition in rare-earth hydrides. 

Two types of fundamental metal hydride electrodes comprising the AB, and ABj classes of alloys are currently of interest. The AB, alloys with A = rare earth or mischmetal, B = Ni and/or other transition metal are investigated. LaNi, has been well-investigated because of its utility in conventional hydrogen storage applications, but it is very expensive and corrodes rapidly. The commercial AB, electrodes use mischmetal, a low-cost combination of rare earth elements, as a substitute for La. The partial substitution of Ni by Co, Ce, Mn and Al increases the thermodynamic stability of the hydride phase, the corrosion resistance and hence the cycle life. However, the substitution reduces the hydrogen storage capacity. 

As the final set of examples of the metal-rich rare earth halides, the chemical bonding and electronic stability of the hydride halides, RXH, are presented. Their structures have been discussed in sect. 2.3. 

Although the electronic behavior of the rare earths is more consistent than that of the actinides, important variations are encountered. The enhanced stabilities of electron configurations for divalent oxidation states prior to the half-filled and filled 4f configurations is well known. Strong alkaline-earth-like properties are manifest at Eu and appear in progressively lesser extents at Yb and Sm. The increased stability of tetravalent oxidation states at Ce, Pr and Tb apparently do not influence hydride properties. 

In remarkable contrast to the behavior of the early-actinide hydrides, the physicochemical properties of the transuranium phases are suddenly more rare-earth-like. As discussed earlier, the progressive narrowing of the 5f bands favors the stabilization of the fee structure with the larger M-M distance attendant localization can then occur and the f bands become core-like and magnetic. However, in contrast to the rare earths, the 5f electrons remain near the Fermi level and one can expect that effects such as scattering will then influence resistivity, neutron scattering and other properties. 

AB5 alloys are now mainly used for nickel/metal hydride batteries. Mischmetals, which are unrefined mixtures of rare earths, are mostly substituted instead of lanthanum, mainly because they are less expensive than pure La. But such substitutions also are required to stabilize the alloy and prevent its premature degradation. 

Jones, P.M.S., J. Southall and K. Goodhead, 1964, The Thermal Stability of Metal Hydrides. Part 1 Rare Earth and Yttrium Hydrides and Deuterides, United Kingdom Atomic Energy Authority, AWRE Report No. 0-22/64. 

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