Mixed-valence materials

Iron mixed-valence materials such as green rust and fougerite are sensitive to air exposure and soil pollution by nitrates. A MIMOS instrument autonomously lowered or raised within a plexiglas tube, which was put down a bore hole, allowed... 

A miniaturized MB spectrometer MIMOS II was developed for the robotic exploration of Mars, where it provided fundamental information about mineralogical composition and alteration processes, helped to classify rocks and soils, aided geologic mapping, was instrumental in assessing habitability of past and present environments, and identified potential construction resources for future human explorers. The applicability of the instrument as a process monitor for oxygen production and prospecting tool for lunar ISRU has been demonstrated. The characterization of air pollution sources and the study of mixed-valence materials as a function of depth in soil are examples of terrestrial in situ applications. MIMOS lla with additional XRF capability will open up new applications. 

After reduction with dithionite, this dendrimer was cast into a film the electrical properties of which were isotropic. (This means that on the molecular and macroscopic levels there is a three-dimensional electron delocalization.) The conductivity was humidity dependent (water may take part in long-distance electron transfer). At 95% humidity, a mixed-valence film (0.55 electron per diimide) showed conduction at room temperature around 11 fl 1-cm 1 (Miller Mann 1996). As shown later, partially reduced mixed-valence materials are required for organic metals of high electrical conductivity. 

It is noteworthy that the structures of 2 and 3 are distinct from those of previously reported extended materials of the Mo/0/H(1P041 n family [104—106]. While these Mo(V) and Mo(V)-Mo(VI) mixed valence materials exhibit a variety of embedded molybdate clusters, the pentanuclear ring of 1-3 is not among these. 

When the f-states are pushed even more closely towards EF, condition (2.10) is no longer fulfilled. The interaction with the conduction-band states further increases. The f-states become broader due to lifetime broadening as the frequency of the transitions of the f electrons to the conduction-band states increases and the mean occupation of the f states decreases (from 4f1 state in Ce-compounds). This f-state instability is typical, e.g., for a number of Ce compounds. For this reason they are called mixed-valence materials. 

Unlike the Kondo systems, a qualitatively different behaviour was observed in the Ce and Yb mixed-valence materials. A stronger coupling between the 4f moments and the conduction-band states is manifest in the much faster relaxation rates and the values of residual IT are about one order of magnitude higher. The striking contrast is underlined by the nearly temperature independent values of T. 

In summary then, the behaviour of the low-energy transitions of the mixed-valence [Os20u-Cl)3(PMe2Ph) ] show i) no solvent dependence and ii) intense, structured bands. Thus we suggest that the diosmium species belongs to class III mixed-valence materials with the definite possibility of metal-metal interactions occurring in the oxidised states. 

Figure 14 also contains U estimates obtained by Hiifner and Wertheim (1973) from XPS data on insulators, and it is evident that they compare as favorably with our results for the metals as do the measurements on the mixed-valence materials. This may at first seem surprising since the individual 4f binding energies for metals and insulators can differ significantly. Rewriting t/(4f") defined by eq. (1) in the form... 

In mixed valence materials macroscopic and microscopic measurements indicate the coexistence of two adjacent valence states of the lanthanide atom. There are two types of such solids. In one the two valence states occur on crystallographically distinguishable sites (inhomogeneously mixed valence). The main interest is in the other type, where the lanthanide atoms occupy only sites with identical point symmetry (homogeneously mixed valence). Such solids are certain intermetallic compounds and related sufficiently dilute lanthanide alloys and certain elements. 

The same experiments have been performed on mixed-valence materials CeSn3 (Stassis et al. 1979a, b) and CePdj (Stassls et al. 1982). The data for CeSnj from 300 K down to 40 K are shown in fig. 13, and those at 4.2 K are shown in fig. 14. For a... 

It was reviewed in section 2.4 that, at sufficient low temperatures, the neutron magnetic form factors of a number of Ce-based mixed-valence materials do exhibit an additional feature which has the spatial distribution of Ce 5d electrons. This part follows the temperature dependence of the so-called susceptibility tail, and becomes vanishingly small when the temperature is raised to 40 K, which is much below the temperature of maximum susceptibility, about 150 K for CeSnj. There has been one suggestion (Liu 1989a) that this unusual feature is the contribution of the spin fluctuation resonance band because it has the expected wave function characteristic. If this interpretation is correct, then one must find a different reason for the characteristic temperature for susceptibility and resistivity peaks. Also, it is not understood why such a feature has not been seen in heavy-fermion systems. 

To conclude, the model gives a good qualitative to semi-quantitative account of many thermodynamic and transport properties of heavy-fermion and mixed-valence materials. It elucidates the cross-over from Fermi liquid behavior at low temperatures to local moment behavior at high temperatures, although it is not capable of treating the problem fully in either extreme region. It can, however, provide insight into the... 

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