Mechanism of the Appearance of an Energy Gap in Mixed-Valent Rare-Earth Compounds

Mitsuo Kasaya

doi:10.1007/978-1-4615-2868-5_3

Mitsuo Kasaya

https://doi.org/10.1007/978-1-4615-2868-5_3

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Publication Date: Jan 1, 1993

Citations: 2

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The origin of an energy gap at the Fermi level in mixed-valent SmB6, gold-phase SmS, YbB12, CeNiSn and Ce3Pt3Bi4 has been a subject of controversy.1–6 In case of SmB6, for instance, valence of Sm ions is about +2.6, and then the number of conduction electrons per formula unit has been believed to be 0.6, because RB6 compounds with R=Ca2+ and Eu2+ are semiconductors and La3+B6 is a monovalent metal. How to construct an energy gap and how to obtain a semiconducting property in such a metallic state have been an attractive theme for theorist. Models of hybridization gap,7 Wigner-crystal formation8 and coherence pseudogap9 have been proposed so far. From an experimental point of view, it has been required to distinguish the gap due to the effect of f-electrons from the one in the non-f electronic states. Furthermore, to find a requisite condition for the gap opening, it is also necessary to find other exemplifications, especially, in isomorphous compounds in which rare-earth ions exist as a well-defined valence state in one compound and as different valence state in another one, both of which show semiconducting properties. Recently, based on our experimental finding of the appearance of semiconducting properties in both Ce3Au3Sb4 with well-defined Ce3+ ions and the mixed-valent compound Ce3Pt3Sb4, we have pointed out that the valence of Ce ions in Ce3Pt3X4(X=Bi,Sb) is formally 4+ and the origin of the energy gap is not so much different from those of Ce3Au3Sb4 and La3Au3Sb4.10 Furthermore, we pointed out that there are common features in the band structures of their reference systems without f electrons.10,11 For Sm- and Yb-based mixed-valent compounds, for instance, the band calculations of CaB6 (ref.12) and YB12 (ref.13) predicted that non-f R2+B6 and non-f R2+B12 can be semiconductors, i.e., band structures of compounds with rare-earth sites replaced by non-f 2+ ions have a gap at the Fermi level.

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