Formation of negative-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>U</mml:mi></mml:math>centers in ionic crystals

It is argued that a natural tendency exists for multivalent impurities in an ionic host crystal to act as negative-$U$ centers. The tendency is due to an increase in the impurity-host lattice attractive interaction when the impurity is in the donor state which is greater than the decrease in interaction in the acceptor state. Differences in lattice distortions are responsible for this difference. Results of a model calculation for Group III impurities in PbTe support the above contention.

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