Valence instability

Koshihara S, Tokura Y, Mitani T, Saito G, Koda T (1990) Photoinduced valence instability in the organic molecular compound tetrathiafulvalene-p-chloranil (TTF-CA). Phys Rev B42 6853-6856... 

Johansson, B. in Valence Instabilities and Related Narrow Band Phenomena, (ed. Parks, R. D.), Plenum Press, New York, p. 435 (1977)... 

Figure 2.21 UVPS (Hell spectra) in the valence-band region showing final-state effects (a) Sm (b) Tb. The valence band of Sm shows the coexistence of both divalent and trivalent species due to valence instability (mixed valence). (After Rao Sarma, 1982.)...

Core-level spectra are useful in studying mixed valence (valence instability or interconfigurational fluctuation) in rare-earth systems (e.g. SmS, Ce) which arises when Eexc = n (E -i + FJ 0 where( — , )istheenergydifferencebetweenthe4/ and 4T states and is the energy of the promoted electron. The time scale involved... 

Valence Instabilities held at Rochester, New York, in November 1976, shortly to be released by Pergamon Press. A further illustrative appendix to the present text appears in those proceedings. 

The interesting aspects of the physical properties of systems with valence instabilities from the point of view of critical phenomena have also been discussed (44). A mean-field behavior has been found to hold for the systems CeTh and Snij Gd S. Further investigations along these lines will improve our understanding of the role of electronic and elastic effects in alloys (43). 

Further XPS studies with various substituents and valences, and especially studies of temperature dependent effects (41), should give microscopic information on the relative importance of electronic and elastic effects on valence instabilities. 

Symposia on Valence Instabilities, Kondo Effect, and Mixed Valence and Kondo Compounds, J. Appl Phys., 55, 1904, 1930, 1963 (1984). 

A considerable number of lanthanides show valence instabilities. Among them Sm, Eu, Tm and Yb are able to fluctuate between the trivalent and divalent electronic configurations, depending on the environmental conditions. On the tetravalent side, although Pr and Tb are suspected to have some hybridization of localized-conduction states, only Ce clearly shows such an effect. Some of its compounds are unique for the study of the electronic fluctuation between the trivalent and tetravalent configurations. The 4f electron hybridization is the origin of the Ce demagnetization, volume reduction, anomalous temperature dependence of the transport properties, and a number of other abnormal behaviours. 

Due to the 11% reduction of its molar volume, CeN was the first compound to be recognized more than fifty years ago by landelli and Botti (1937), as having valence instabilities. A similar volume reduction was also found in several Ce(TM)2 Laves compounds (Olcese 1966a). In particular, the Ce ionic radius in CeCo2 was evaluated to be 0.89 A, which is smaller than the value reported for ionic Ce" (0.94 A) (landelli and Palenzola 1979). Furthermore, Ce is the only lanthanide to form a LnRhj compound with the AuCuj structure, for which an extremely small atomic volume of the Ln element is required. 

Mignot, J.M., and J. Wittig, 1982, in Proc. Int. Symp. on Valence Instabilities (North-Holland, Amsterdam) p. 203. 

Balster, 1982a, Phys. Rev. Lett. 49, 65. Rdhler, J., D. Wohlleben and G. Kaindl, 1982b, in Valence Instabilities, eds P. Wach-ter and H, Boppart (North-Holland, Amsterdam) p. 341. 

The ease with which an electron may be excited out of the 4f shell is of great interest because it determines the extent to which the 4f electrons may be regarded as local or itinerant. Models of itinerant 4f electrons have been proposed to explain the superconducting property of La, the lattice and valence instabilities of Ce and a number of rare earth compounds. 

Lawrence et al. (1975) have studied the electronic properties of the Ce, Th system in some detail. While there are definite Kondo anomalies in this system, the main thrust of this work is concerned with the thermodynamic behavior near valence instabilities, particularly near the critical poinL A detailed discussion of the work is beyond the scope of this article and we refer the reader to the following references Parks and Lawrence (1976) and Varma (1976 and references therein). 

---