Intermediate phonon anomalies

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... 

Anomalies in the phonon spectra of UTe have also been found (Buyers and Holden 1985) and attempts were made by the authors to use the same theories described above, which essentially relate to an intermediate-valence picture. Unfortunately, as Buyers and Holden describe (see their p. 304) this does not lead to a convincing conclusion. There is, of course, no direct electronic evidence (e.g., from photoemission) that materials such as UTe exhibit valence fluctuations. We must conclude that the electron-phonon interaction is different in detail between the 4f intermediate-valence matmals and tiie actinide compounds. In both cases, however, the result is a negative Poisson ratio (which is directly related to the elastic constant C12). 

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