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The dynamical properties of solids have recently attracted renewed
interest in connection with the increasing understanding of phase
transitions and re lated phenomena. In particular, soft modes or,
more generally, phonon 'anom alies' seem to play an important role
in structural and electronic phase tran sitions, such as
ferroelectric or superconducting transitions. The understanding of
the mechanisms responsible for the occurrence of unusually low
frequencies in phonon spectra requires a detailed analysis of the
microscopic forces governing the lattice vibrations. Of particular
importance is the influence of the electron lattice interaction in
the adiabatic approximation which in many cases is the origin of
peculiarities in the phonon self-energy. In this work the
vibrational spectra of pure non-metals and of those con taining
point defects are investigated. ' In these materials the
interrelation be tween the pseudo-harmonic forces (determining the
phonon dispersion re lations) and the non-linear anharmonic and
electron-phonon forces (as they act in infrared and Raman spectra)
is most obvious and can be quantitatively analysed in terms of
appropriate models. The main task is to arrive at a physically
correct treatment of electronic degrees of freedom, as for example
in an electronic 'shell' model, which leads to the description of
phonon spectra in terms of long-range polarizabilities and
short-range deformabilities. The pur pose of our review is to
stimulate further investigations which, we hope, will result in
explicit relations between the parameters of the semi-microscopic
models and the matrix elements from the electronic band structure."
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