Mixed valence compounds electrical conduction

All these data verify that in real systems, the rate of electron transfer between components of a conductive chain is high. There are states of a mixed valence. Enhanced electric conductivity and other unusual physical properties are widespread among those inorganic or coordination compounds that contain metals in intermediate -valence states. In cases of organic metals, nonstoi-chiometric donor/acceptor ratios provide even better results. For example, the salt of (TTF)i (Br)oj composition displays an electric conductivity of 2 X 10 cm while (TTF)i(Br)i salt does not... 

Magnetite, Fe30>4, is a unique material. It is a mixed-valency compound (Fe " and Fe " on crystallographic sites of the same symmetry) with a low electrical conductivity below 120K, and a nearly metallic conductivity above this temperature. Between 120 and 770 K, magnetite is an inverse spinel, (Fe )[Fe Fe ]04, and all the transition phenomena occur in the octahedral sites. For T > 770 K, the redistribution of Fe on tetrahedral sites becomes non-negligible (Wu Mason, 1981). The reversible, sharp discontinuity in conductivity was first observed by Okamura (1931). Ferroelectric features have also been observed at low temperatures (Rado Ferrari, 1975 Kato et al, 1983). 

Reaction with nucleophiles yields 2,10-disubstituted products.284 Partial oxidation of these macrocyclic complexes with iodine yields mixed valence non-stoichiometric compounds of high electrical conductivity.285,286... 

These are one of the most studied classes of mixed valence complexes and have been reviewed recently by Keller and by Clark.134,135 They all possess a common structural unit X,MII X"M1V"X,MII, X. Table 5 gives a selection of examples for which electrical conduction studies are available with particular emphasis on results obtained on single crystals. The compounds are arranged depending on the charge of the complex. 

Mixed Valence Complexes. A number of compounds containing both Cu1 and Cu11 are well established. The sulfides, for example, K3Cu8S6made by heating K2C03, Cu, and S, may have metal-like electrical conductivities and magnetic properties. 

A determined search for superconductivity in metallic oxides was initiated in mid-summer of 1983 at the IBM, Zurich Research Laboratories in Riischliken, Switzerland. This research effort was an extension of previous work (145) on oxides, namely, Sr1.xCaxTiOs, which exhibited some unusual structural and ferro-electric transitions (see Section 2.2a). During the summer of 1983, the superconductivity research was focussed on copper-oxide compounds. Muller had projected the need for mixed Cu2+/Cu3+ valence states, Jahn-Teller interactions (associated with Cu2+ ions), and the presence of room temperature metallic conductivity to generate good superconductor candidates. These researchers then became aware of the publication by Michel, Er-Rakho, and Raveau (146) entitled ... 

---