Intermediate valence behavior

A recent addition is CeRhAs (Yoshii et al. 1996), 3iidiich is closely related to CeRhSb. All these materials contain 4f elements that ate known candidates for intermediate valence behavior and at high temperatures one finds evidence for such an electronic state in the majority of cases. Published [iSR data exist mainly for the first two cases and also, to a more limited extend, for (10). The latter is, together with (3) and (6), a prime example of a typical Kondo insulator (Kasuya 1996), while (1) and (2) have always been classified as Kondo semiconductors instead. As better samples have become available, it has been realized that they are now better described as Kondo semimetals (Takabatake et al. 1996, 1998a). More details are provided below. 

Within the fee RSe series TmSe exhibits quite unique properties, since it shows intermediate-valence behavior (Bucher et al. 1975, Campagna et al. 1974, Launois et al. 1980) in combination with low-temperature magnetic order... 

YbAlB has the YCrB4-type, Pbam, a = 5.927(2), b = 11.47(1), c = 3.492(1) and golden brown color. With a similar preparation technique (Al flux method to prepare YbB4) Pisk et al. (1981) obtained the compound YbAlB4 with intermediate valence behavior, a = 5.921, b = 11.424, c = 3.507 (structure refinement, YCrB4-type was proposed). Electrical resistivity, magnetic susceptibility and specific heat data were reported. 

The theory of intermediate valence (IV) forms the starting point for the treatment of HF materials. IV represents the limiting situation of a very strong coupling parameter g in strongly correlated electron materials, as can be seen from fig. 105. Kondo insulators in particular possess IV behavior at high temperatures as an essential ingredient. They form a borderline case. We have discussed them earlier as a separate class of materials. 

A Liii XANES spectrum in the ytterbium region of YbCuGa exhibits two distinct peaks separated by 7eV (Adroja et al. 1990), indicating an intermediate-valence state of the ytterbium atoms. This is in agreement with the magnetic behavior. 

The very same charge deformability of the mixed-valence Sm ion due to 4f" - 4f"5d excitations used for the description of the Raman intensities in fig. 37 has been used to describe the phonon anomalies (Bilz et al. 1979). Therefore we can conclude that the dominant F scattering intensities of Sm 25S near 250 cm" and 85 cm , respectively, arise mainly from the LO and LA phonon anomalies in the [111] direction, emphasizing scattering from L-point phonons. The available data on the LO(L) phonon frequencies of RS are depicted in fig. 38 as a function of the lattice parameter. The LO(L) phonons of intermediate-valence metallic SmS and Sm jY 25S lie between the divalent reference line given by YbS and EuS, and the trivalent reference line spanned by YS, GdS, PrS and LaS, thus exhibiting the behavior of an alloy of divalent and trivalent Sm ions. Figure 39 shows the bulk modulus of several RS compounds at room... 

About 25 years ago it was rare that any scientist would speak about 4f electrons being involved in bonding, since everyone knew that the 4f electrons were localized in the ion core and were well shielded by the 5p and 5s electrons, especially in the case of the lanthanide metals. However, a few scientists led by Matthias and Gschneidner believed that some of the properties of the lanthanide elements were sufficiently anomalous with respect to the rest of the periodic table that just the (5d6s) valence electrons could not account for these behaviors. Slowly the evidence built up, such that today virtually everyone believes that there is some hybridization of the 4f electrons with the outer 5d and 6s electrons, especially in the light lanthanides (see, e.g., Freeman et al. 1987). The work on intermediate valence cerium and more recently the heavy-fermion studies (see section 4.4.4) convinced even the most conservative scientists that 4f electrons must be involved. Reference to the early papers on this subject can be found in the article by Gschneidner (1971). 

YbAl2 has an anomalously large electronic specific heat coefficient 17mJ/molK. This may indicate an intermediate valence state in YbAla at low temperatures. Furthermore all dialuminides show bottleneck behavior in the Gd and Eu ESR experiments and hence allow to determine the relaxation rate 5 . Levin et al. (1982) investigated YbA (R = Nd, Eu, Gd, Dy, Er) and reported the observation of bottleneck situations for Eu and Gd as ESR probes. They broke the bottleneck by the addition of nonmagnetic thorium ions or by diminution of the Gd and Eu concentration and reached the isothermal limits Ago=+0.09 0.01 and dA/f/dT=25 5G/K for Gd and Ago = +0.06 0.01 and... 

CeRu2 is a mixed-valence system with a valency close to 4. The ESR experiments on this system are reviewed in sect. 12, which deals with local moment spin resonance in conventional superconductors. SmBs is an intermediate valence system, but belongs to a new class of small gap semiconductors. These materials exhibit Kondo-lattice behavior at elevated temperatures but evolve into semiconducting materials with small gaps of the order of a few kelvins or a few tens of kelvins as the temperature is lowered (Aeppli and... 

Electron-spin resonance (ESR) is an extremely useful tool for probing the localized magnetic moment in a variety of materials. The chapter (162) by Elschner and Loidl focuses on metallic lanthanide systems, reviewing both the classical ESR behaviors and the role of ESR in newly developed branches of solid-state physics. The former includes the determination of the site symmetry of the ESR probe and measurement of the crystal field (CF) and how it (CF) differs in metallic systems from that in insulators. The latter deals with Van Vleck systems, spin glasses, Kondo systems, heavy-fermion and intermediate-valence compounds, and high-temperature superconductors. 

The metallic Ce compounds exhibit a variety of properties which are related to their electronic states, including Kondo resistivity minima, intermediate valency, superconductivity, various types of magnetic ordering, and heavy fermion behavior (along with some of the properties previously mentioned). Some of these Ce compounds exhibit properties like those of Ce, including y-a phase transitions (related to variations in composition if not to variations in temperature or pressure). Others resemble either y-Ce or o Ce. 

In sect. 5 we discuss a large number of paramagnetic systems that exhibit no sharp crystal-field excitations. For the most part the materials are of intermediate valence (IV). Most of the experiments have been performed on polycrystalline materials. As discussed above, for the most part the interactions that lead to intermediate behavior are stochastic in nature so that little dispersion is anticipated - see also below. A good test for this is whether there is agreement between the neutron magnetic intensities (local susceptibility) and the bulk susceptibility. There is now considerable evidence that the magnetic response of these systems is inelastic at low temperatures. The energy of the peak position is... 

Here Martin and Allen (1979) have shown that for an even count of f and d electrons, and with no other electrons in the conduction band, the appUcation of the Luttinger theorem (Luttinger 1960) leads to being in the gap of an intermediate-valent system. For an odd electron count the Fermi level lies in one of the density of states peaks resulting in intermediate-valent and heavy-fermion behavior (Martin 1982). This theory has been greatly stimulated by the establishment of an insulating behavior of SmBg at low temperatures, which has taken 12 years of research (Nickerson et al. 1971) and 3 conferences on intermediate valence (1977, 1981, 1982). 

The chapter by Wachter (132), which is one of the most extensive and comprehensive ones in the entire Handbook series, reviews intermediate valence and heavy fermions in a wide variety of lanthanide and actinide compounds, ranging from metallic to insulating materials. The behaviors of these materials are discussed from the basic idea that a gap exists between two narrow f-like subbands, i.e. the hybridization gap model, which can account for the observed physical and electronic properties. As pointed out by Wachter, heavy fermions are intermediate valence materials by virtue of the fact that they have nonintegral f occupation values. The main difference between normal intermediate valence materials is that their Fermi energies are in the hybridization gap, while for the heavy fermion materials the Fermi energy is not in the gap. 

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