Off-center impurities in alkali halides: reorientation, electric   polarization and binding to F center. V. Temperature-dependent electrostatic   polarizabilities
  We derive and discuss expressions for the temperature-dependent electrostatic polarizabilities of off-center ions holding good under various experimental conditions. At low temperatures and electric-field strengths, all of them reasonably reduce to values characteristic of phonon-coupled two-level systems. Prospects for further studies of the dispersive coupling are also considered. 
Mladen Georgiev