Intermediate valence-type materials

Wachter ( ) have presented a very clear example of this behaviour in their studies of the moment formation in TmSe Te under pressure where both mixed valency, or intermediate valence (IV), and semiconductor to metal transition are found. The particular interest of this case is not only that these materials have been extensively characterized, but also because they show, from comparison of valences determined by two different experimental methods, that a unique picture which considers only one type of... 

These materials have been of interest for many years because of their unusual properties. In the last decade reasonably large crystals have become available by using the mineralization technique (Spirlet and Vogt 1984), so that many different neutron inelastic experiments have been performed. Although TmSe orders with a moment of about 2(Ub (Bjerrum-Moller et al. 1977), we shall discuss the Tm compounds with the NaCl-type structure in sect. 5 in coimcction with effects of intermediate valence (IV). 

Intermediate between these two extremes are minerals classified as Class II compounds in which the two sites are similar but distinguishable that is, both are octahedral sites, but with slightly different metal-oxygen distances, ligand orientation or bond-type. Examples include the amphibole Ml, M2 and M3 sites (figs 4.14 and 5.18), the mica trans-Ml and c -M2 sites (fig. 5.21) and babingtonite (Bums and Dyar, 1991). Such materials still exhibit properties of cations with discrete valences, but they have low energy IVCT bands and may be semiconductors. 

By contacting the material with two high band gap semiconductors, it can be shown that, under certain restrictions regarding the properties of the states of the intermediate band, the output voltage is still given by the total energy gap between the valence and the conduction bands [112]. The theoretical efficiency for this type of cell is close to the limiting efficiency of a triple junction cell, e.g., 63 % [112]. 

On metal electrodes, the transfer coefficients typically approach 0.5. Generally, the transfer coefficients for redox reactions on moderately doped diamond electrodes are smaller than 0.5 their sum a +p, less than 1. We recall that an ideal semiconductor electrode must demonstrate a rectification effect in particular, on p-type semiconductors, reactions proceeding via the valence band have the transfer coefficients a = 0, P = 1, and thus, a +p = 1 [7]. Actually, the ideal behavior is rarely the case even with single crystal semiconductor materials manufactured by use of advanced technologies ( like germanium, silicon, gallium arsenide, etc.). The departure from the ideal semiconductor behavior is likely to be caused by the fact that the interfacial potential drop appears essentially localized, even in part, in the Helmholtz layer, due, e.g., to a high density of surface states, or the surface states directly participate in the electrochemical reactions. As a result, the transfer coefficients a and p have intermediate values, between those characteristic of semiconductors (O or 1) and metals (-0.5). Semiconductor diamond falls in with this peculiarity. However, for heavily doped electrodes, the redox reactions often proceed as reversible, and the transfer coefficients approach 0.5 ( metaMike behavior). 

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