Hydrides semiconductor-metal transition

Early studies on polycrystalline samples of lanthanum and cerium hydrides by Stalinski (1957b, 1959) indicated that these materials become semiconductors as the trihydride composition is approached. More recent studies on single crystals of cerium hydride by Libowitz and Pack (1969b) showed a sharp increase in resistivity as the hydride composition approached CeH2 illustrated in fig. 26.10. As mentioned in section 5.1, samples in which H/Ce<2.7 showed metallic behavior. However, samples with H/Ce > 2.8 exhibited typical semiconductor behavior with a linear increase in resistivity with reciprocal temperature. Clearly, there is a metal-to-semiconductor transition in cerium hydride at an... 

The maj or exceptions to this simplified picture are probably CuH and ZnH2, which are reported to be stoichiometric, nonmetallic solids like the main group metal hydrides, but their crystal stmctures and physical properties are not well characterized. Borderline cases are the rare earth hydrides, which show a hydrogen concentration-dependent semiconductor-metal transition. E11H2 and YbH2 behave like alkaline earth dihydrides because of the special electron configuration of Eu + (4/ ) and Yb + (4/ " ). 

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