Phonon anomalies in intermediate valence compounds

Abstract

Phonon anomalies in the intermediate valence compounds SmS, TmSe and YS were studied extensively using a three-body force rigid shell model. Three-body interactions were used because, due to valence fluctuations, the radii of rare-earth ions change significantly and, therefore, the amount of overlap with neighboring chalcogen ions also changes. Change in the radii of the rare-earth ions results in the deformation of the rare-earth ions, whereas change in the amount of overlap with the neighboring chalcogen ion gives rise to long-range three-body interactions between the transferred charges of the lattice. Both of these changes strongly influence the phonon dispersion curves, particularly along the zone boundary. Results obtained from the model are in agreement with the measured data for phonon dispersion as compared to the results obtained using a three-body rigid ion model. Results for Debye temperature variations and two-phonon IR/Raman spectra for these crystals have also been presented, probably for the first time, but could not be compared with experimental data due to their non-availability.

Keywords:

• phonon anomaly
• heat capacity
• optical properties
• intermediate valence compound

Acknowledgements

The authors are thankful to the Principal, K.N. Govt. P.G. College, Gyanpur, SRN Bhadohi (India) for providing necessary facilities. They are also thankful to the Computer Centre B.H.U., Varanasi (India) for providing computational assistance.