Chapter 96 The electron-phonon interaction in intermetallic compounds

Abstract

This chapter focuses on the electron–phonon coupling in metallic rare-earth (R) compounds. The electron-phonon coupling in R compounds can lead to low-temperature effects in many physical quantities connected with elastic or vibrational degrees of freedom, for example, in the elastic constants, thermal expansion, and magnetostriction and phonon frequencies. The physical nature of these effects depends crucially on the type of electronic ground state and low-lying excited states in these compounds. In insulators, the spectrum of localized, partly filled 4f n states of R ions needs to be considered. The lowest states are the crystalline electric field (CEF) levels of the relevant Russell–Saunders LSJ ground-state multiplet. Electron–phonon coupling to CEF states is commonly known as “magnetoelastic coupling,” and it arises from the strain modulation of the energies of CEF states. In insulators, for example, the R vanadates, arsenates, and phosphates, this coupling can lead to cooperative Jahn–Teller phase transitions in which the degeneracy of the lowest CEF multiplet is lifted and accompanied by a lattice distortion.