Off-center impurities

Off-centering of impurities is an interesting phenomenon that was discovered in 1965 in KCl crystals doped with Li. The off-centering manifests in the displacement of small atoms, which substitute for larger ones, from regular sites in a lattice. The displacement can be sufficiently large, typically 0.5-1 A. The reason of instability of off-center impurity is the inability of the reduced repulsive forces to oppose the action of the polarization force (attractive interaction between the charged ion and the polarization induced by it in a lattice). Since the discovery of the off-centering of lithium in potassium chloride, the off-center behaviour was found for many other small impurities in different crystals (see the list of known off-center impurities in crystals).

Investigation of narrow-gap IV-VI semiconductors (mainly lead telluride PbTe and lead selenide PbSe) doped with off-center impurities (like germanium or sulfur) attracts much attention due to impurity-induced phase transitions in these incipient ferroelectrics. The changes in the physical properties of these semiconductors due to the ferroelectric phase transition may be used in the infrared optoelectronics (tunable infrared lasers, photoelectric devices).

Systematic studies which have been carried out in our laboratory discovered a whole class of off-center impurities in IV-VI semiconductors. New relaxation and hysteresis phenomena were found in quaternary solid solutions of IV-VI semiconductors. In addition to the ferroelectric state, two disordered dipolar states (dipolar glass and quenched dipolar disorder) were realized and studied in these crystals. Influence of different impurities on the ferroelectric phase transitions was also investigated. At present we continue to study these phenomena using EXAFS (X-ray absorption) spectroscopy and synchrotron radiation.

The recent result of these studies is the discovery of a new class of off-center ions -- off-center ions of large size.

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Physics of Semiconductors division