Valence fluctuation phenomena

Lanthanides with fractional valences have II, III and IV valences, as well as mixed II/III and III/IV valences. Depending on temperature and pressure, the degree of oxidation can change. This effect may result in a change in the different properties of nanoparticles, such as the stability, heat capacity, conductivity and magnetic susceptibility [218]. Valence fluctuation phenomena have been reported to occur... 

Although several concentrated alloys and compounds of Sm (notably the Sm monochalcogenides and SmB ) have been studied in connection with valence fluctuation phenomena, there is only one dilute alloy system reported to date containing Sm impurities which exhibits Kondo-like behavior. 

The term mixed valence is widely used in the literature to describe a phenomenon rather different from that considered here. Typically it refers to a metal (or a rare-earth alloy, or a compound such as SmS) which features a broad conduction band (formed by the overlap of s, p, or d orbitals), and a very narrow band such as an /-band, slightly above the Fermi level. This band becomes partially populated, hence the mixed, or fractional, valence. Fluctuations with time, and various degrees of localization in space, result from electron-phonon interaction. A useful review appeared recently.Many of the ideas used in this field parallel those used in other more chemical types of electron transfer. Recent articles on mixed-valence as a polaronic effect and on local polaronic effects exemplify this, and the dynamical properties have been discussed. Other recent reviews of this area deal with spectroscopic techniques and with mixed valency in rare-earth compounds. ... 

Closely related to the heavy fermions and spin fluctuators are the valence fluctuation/intermediate valence materials. The origin of this phenomenon starts with cerium and its a 7 transformation (see sections 3.3.4 and 3.7.2). Today it involves many cerium materials and also compounds of samarium, europium, thulium and ytterbium. Because of the breadth of the subject matter and space limitations in this chapter we refer the reader to the following reviews Jayaraman (1979), Lawrence et d. (1981), de Chatel (1982), Coqblin (1982), Nowik (1983), Brandt and Moshchalkov (1984), Varma (1985) and Stassis (1986). 

Concerted Phenomenon Coupled with Spin Equilibrium and Valence Fluctuation... 

Nonstoichiometry can be caused by oxygen deficiency (or excess) or by fractional valences of the metal components. For example, the existence of Cu " in nonstoichiometric cuprates has been widely discussed [9,10]. It is essential that in nonstoichiometric oxides the microscopic fluctuations of the composition should proceed (the so-called phase separation). The characteristic size of heterogeneities induced can exceed atomic dimensions by an order of magnitude. This phenomenon is attributable to the fact that the electron-nonuniform state of such chemically singlephase materials appears to be energetically more advantageous. 

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