A theory of hydrogen-like impurity centres in ionic crystals in the presence of an external magnetic or electric field is developed. The coupling t o longitudinal optical phonons is taken into account, and the results are valid for arbitrary interaction strength. Feynmans path-integral representation for the partition function of the phonon-impurity system and a variational principle connected with a quadratic trial Lagrangian are used. A simple

b) -136, K129 (1986) Subject classification: 71.38 V. I. Lenin State University, Kishinev F r e e C a r r i e r s in Ionic Semiconductors with a Degenerate Valence Band Edge

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