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.

Members of the group:

• Alexander I. Lebedev
  
• Irina A. Sluchinskaya

Selected publications:

• Kh.A. Abdullin, A.I. Lebedev, A.M. Gas'kov, V.N. Demin and V.P. Zlomanov. Structural phase transition in the solid solution PbTeS. Pis'ma v Zh. Teor. Eksp. Fiz. 40, 229 (1984) (in Russian). [Translation: JETP Lett. 40, 998 (1984)].

• Kh.A. Abdullin, V.N. Demin and A.I. Lebedev. Scattering of free carriers near a phase transition in the solid solution PbTeS. Fizika Tverdogo Tela 28, 1020 (1986) (in Russian) [Translation: Sov. Phys. - Solid State 28, 570 (1986)].

• V.F. Kozlovskii, A.I. Lebedev and Yu.E. Petrov. X-ray and electrical studies of the phase transition in PbGeSe. Fizika Tverdogo Tela 28, 3610 (1986) (in Russian). [Translation: Sov. Phys. - Solid State 28, 2035 (1986)].

• A.I. Lebedev and I.A. Sluchinskaya. Phase transitions in PbGeTeS solid solutions caused by noncentral Ge and S ordering. Bull. Acad. Sci. USSR Phys. ser. 51(10), 17 (1987).

• A.I. Lebedev and I.A. Sluchinskaya. Possible appearance of the dipole-glass phase in solid solutions of the semiconductors of the group IV-VI. Pis'ma v Zh. Teor. Eksp. Fiz. 46, 425 (1987) (in Russian) [Translation: JETP Lett. 46, 536 (1987)].

• V.F. Kozlovskii and A.I. Lebedev. X-ray diffraction investigations of phase transitions in narrow-gap IV-VI semiconductors. Fizika Tverdogo Tela, 30, 531 (1988) (in Russian). [Translation: Sov. Phys. - Solid State 30, 303 (1988)].

• A.I. Lebedev and I.A. Sluchinskaya. Structural disorder and phase transition in PbSnTeSe. Fizika Tverdogo Tela 32, 1780 (1990) (in Russian). [Translation: Sov. Phys. - Solid State 32, 1036 (1990)].

• A.I. Lebedev and I.A. Sluchinskaya. New arguments in support of the off-center positions of the tin atoms in PbSnTeSe and PbSnTeS. Fizika Tverdogo Tela 34, 1491 (1992) (in Russian). [Translation: Sov. Phys. - Solid State 34, 793 (1992)].

• A.I. Lebedev and I.A. Sluchinskaya. Influence of dopants on ferroelectric phase transitions in PbTeS and PbGeTe. Fizika Tverdogo Tela 35, 629 (1993) (in Russian) [Translation: Physics of the Solid State 35, 321 (1993)].

• A.I. Lebedev and I.A. Sluchinskaya. Unusual phase transitions in PbSnTeSe and PbSnTeS crystals induced by Sn off-center ions. Ferroelectrics 143, 91 (1993).

• A.I. Lebedev and I.A. Sluchinskaya. Ferroelectric phase transitions in IV-VI semiconductors associated with off-center ions. Ferroelectrics 157, 275 (1994).

• A.I. Lebedev and I.A. Sluchinskaya. Low-temperature phase transitions in some quaternary solid solutions of IV-VI semiconductors. J. Alloys and Compounds 203, 51 (1994).

• A.I. Lebedev and I.A. Sluchinskaya. Influence of random fields on the ferroelectric phase transitions in IV-VI semiconductors. Ferroelectrics 169, 293 (1995).

• A.I. Lebedev, I.A. Sluchinskaya, V.N. Demin and I.H. Munro. EXAFS studies of the influence of impurities on the phase transition in GeTe. Izv. Akad. Nauk, Ser. Fiz. 60, N 10, 46 (1996) (in Russian).

• A.I. Lebedev, I.A. Sluchinskaya, V.N. Demin and I.H. Munro. Off-centering of Pb and Sn impurities in GeTe induced by strong local stress. The 9th Int. Conf. on X-ray Absorption Fine Structure (Grenoble, August 1996), Program and Abstracts, Abstract PS3-82.
  

• A.I. Lebedev, I.A. Sluchinskaya, V.N. Demin and I.H. Munro. Off-centering of Pb and Sn impurities in GeTe induced by strong local stress. Pis'ma v Zh. Eksp. Teor. Fiz. 63, 600 (1996) (in Russian) [Translation: JETP Letters 63, 635 (1996)].

• A.I. Lebedev, I.A. Sluchinskaya, V.N. Demin and I.H. Munro. Off-centering of Pb and Sn impurities in GeTe. Phys. Rev. B 55, 14770 (1997).

• A.I. Lebedev, I.A. Sluchinskaya, V.N. Demin and I.H. Munro. Influence of Se, Pb and Mn impurities on the ferroelectric phase transition in GeTe studied by EXAFS. Phase Transitions 60, 67 (1997).

• A. Lebedev, I. Sluchinskaya, I. Munro. EXAFS study of PbS-SnS solid solution. J. Synchr. Rad. 8, 800 (2001).

• A.I. Lebedev, I.A. Sluchinskaya. Direct determination of the shape of potential well for off-center Ge atom in GeTe-SnTe solid solution by EXAFS technique. 7th Russia/CIS/Baltic/Japan Symposium on Ferroelectricity (St. Petersburg, June 2002). Abstract book, p. 29.

• A.I. Lebedev, I.A. Sluchinskaya. Parameters of the potential well of an off-center Ge atom in GeTe-SnTe solid solution. Fizika Tverdogo Tela 49, 1077 (2007) (in Russian). [Translation: Physics of the Solid State 49, 1132 (2007).]

• A.I. Lebedev, I.A. Sluchinskaya, A. Erko, A.A. Veligzhanin, A.A. Chernyshov. XAFS studies of the local environment of Pb impurity atoms in barium, strontium, and calcium titanates. Physics of the Solid State 51, 991 (2009).

• A.I. Lebedev, I.A. Sluchinskaya, A. Erko, V.F. Kozlovskii. Direct evidence for off-centering of Mn impurity in SrTiO3. JETP Letters 89, 457 (2009).

• I.A. Sluchinskaya, A.I. Lebedev, A. Erko. Local environment and oxidation state of Mn impurity in SrTiO₃ determined from XAFS data. -- Bull. Russian Acad. Sci.: Physics 74, 1235 (2010); [local copy].