Intermediate valent

Also YbPdln shows the typical behavior of an intermediate-valent compoimd. The magnetic susceptibility was measured up to about 1000 K (Cirafici et al. 1985). The intermediate-valent behavior is also evident from the temperature dependence of the lattice parameters. A significant anomaly is detected only for the c parameter, while a increases linearly. 

The gallide YbCuGa (Adroja et al. 1990, Malik et al. 1987a,b) shows a susceptibility behavior typical for an intermediate-valent compound (fig. 13). This behavior can be understood in terms of the ionic-configuration-fluctuation (ICF) model (Sales and Wohlleben 1975). A fit according to this model results in a temperature-independent... 

Intermediate-valent YbCuAl shows the well-known nonlinear temperature dependence (above 50 K) with a strong saturation. The temperature dependence of the Hall coefficient shows a discontinuity at aroimd 40 K. This peculiar behavior is attributed to an extraordinary contribution to the Hall coefficient (Cattaneo 1985, 1986). High-pressure resistivity measurements were also performed on this material (Mignot and Wittig 1981, 1982). 

Intermediate-valent YbCuGa shows a TEP of —4 xV/K at room temperature as is observed in most ytterbium-based mixed valent systems. This suggests that the majority earners in YbCuGa are electrons (Adroja et al. 1990). 

High-resolution photoemission studies were performed for YbPdAl, YbPtAl, and YbAuAl (Reinert et al. 1999). For these intermediate-valent and Kondo compounds a clear correlation of the linewidths, the binding energies and the spectral-weight distributions of the 4f spectra with the characteristic temperature Tmax of the maximum in the magnetic susceptibility is observed. 

The most outstanding YbZX compounds are ferromagnetic YbNiSn, intermediate-valent YbCuAl, and the bismuthide YbPtBi with an extremely large linear coefficient of the specific heat y=8 J/molK. More than 85 publications have been devoted to these three peculiar materials in recent years. 

The supposition that the spin-orbit coupling is relatively insensitive to the local ionic environment (see sect. 2.1.2) is amply confirmed by neutron measurements on metallic systems. The observed spin-orbit splittings are all very close in energy to those previously measured in optically transparent compounds. The only exceptions to this rule so far are in the intermediate valent compounds EuPd2Si2 (Holland-Moritz et al. 1987) and Smp 25S (Holland-Moritz et al. 1988), in which the transition energies are nearly 20% lower than in the free ion (Martin et al. 1978). 

Investigations of Coulomb transitions in intermediate valent thulium (and samarium) compounds are an obvious development in this field. 

The electrical resistivity of the intermediate-valent ytterbium compound YbAu2ln has been investigated under high-pressure conditions (Alami-Yadri et al., 1998) up to 8 GPa. The Kondo temperature decreases with increasing pressure, while the residual electrical resistiv-... 

Structural details and phase equilibria of numerous intermediate valent halides were reviewed in this handbook (Haschke 1979). Intermediate valent compounds remain an area of exceedingly high activity. Lattice parameters of numerous phases which contain either a lanthanide or an actinide in an intermediate oxidation state, or an alkaline earth substituted for one of these elements, are listed in table 2. 

Similar observations can be obtained for the thulium chalcogenides. TmTe is obviously divalent. With increasing internal pressure (decreasing lattice constant), the tendency towards trivalency increases. This means that TmS is purely trivalent and TmSe is mixed valent as is evident from the magnitude of their lattice constants. One of the first intermediate valent system studied by photoemission was the... 

Fig. 18. The valence of Sm in Smo.j Yq,2S between 77 and 300 K at ambient pressure. 20 at % Y stabilizes the strongly intermediate valent state of Sm (1 2.6) at high temperatures. At T< 150 K the system transforms into the semiconducting (black phase). The semiconducting phase turns out to be mixed valent also (adapted from Boyce et al. 1981).

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