Thermodynamics of Materials with Valence Fluctuations
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概要
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Valence instabilities are found in solids with narrow bandgap, and cations which may achieve two different valence states. The change in valence results here not by transfer of electrons from the cation to the anion, but by promotion of the electrons of the cation from a localized (4f or 5f) state (characteristic of the divalent cation) to a delocalized 5d-Band (characteristic of the trivalent cation). Application of pressure increases the crystal field splitting of the 5d-Band and makes the energy gap of e.g. semiconducting, black, divalent SmS smaller. At 6.7 Kbr a semiconductor-metal transition takes place to the trivalent, golden, metallic SmS. XPS measurements show that both Sm^<2+> and Sm^<3+> are present in the collapsed state. It is not, however, clear if a new mixed valent state appears. Unique for this phase transition however is an approximately 10% decrease in volume (due to the Me^<2+> → Me^<3+> transformation) without change of the NaCl-structure. TmSe contains already at normal pressure mixed valence. Up to now, samples of TmSe showed irreproducible average valence and therefore large scattering of their lattice constant (Bucher et al.). In this lecture, it is shown that a) The lattice constant of TmSe samples (and therefore the valence of Tm) becomes reproducible when the nonstoichiometry is controlled. b) The valence of Tm can be varied between Tm^<3+> and Tm^<2.71+> by proper adjustment of stoichiometry. Variations almost in the whole range between Tm^<3+> and Tm^<2+> can be achieved in the systems TmSe-TmTe and TmSe-EuSe. c) Measurements of the density show that these variations of valence are followed by the formation of high concentration of vacancies. d) Investigations of the Tm-Se phase diagram up to the m.p. of TmSe (2030℃) show several phase transition which may be the result of valence segregation (separation of Tm^<3+>Se and Tm^<2+>Se). e) The phase diagram shows that the m.p. of the stoichiometric compound is lower. This is an indication of instability for a composition at which the valence fluctuation is expected to appear.
- 日本結晶成長学会の論文
- 1978-12-25
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関連論文
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- Thermodynamics of Materials with Valence Fluctuations