Off-center ions

Off-center ions in crystals are substitutional impurity ions whose equilibrium position is shifted away from the regular lattice site. The magnitude of the shift typically ranges from 0.2 to 1.0 Å. There are two possible mechanisms that can cause impurity ion displacement. If the impurity ion is smaller than the regular ion (by 10% or more), the displacement arises because the repulsive forces between the impurity ion and its nearest neighbors stabilizing the ion at the regular site are strongly weakened.[1][2][3][4] If the impurity ion is bigger than the regular ion, the displacement arises because of different covalency of the chemical bonds with the nearest neighbors for the impurity and regular ions.[5][6]

Off-center position of substitutional ions was first discovered in lithium-doped KCl by two groups of American physicists in 1965.[7][8] Since these pioneer works crystals with off-center impurity ions have attracted continuous attention. The cause of such interest is that these crystals can be used as good model objects for the investigation of such key phenomena in solid state physics as quantum tunnelling of atomic particles in solid state, cooperative properties of the system of local centers with internal degrees of freedom, and ferroelectricity.

References
  1. Smoluchowski R. Magnetic resonance and radiofrequency spectroscopy (Proc. XV Colloque AMPERE). Amsterdam-London, 1969, p.120.
  2. Narayanamurti, V.; Pohl, R. O. (1970-04-01). "Tunneling States of Defects in Solids". Reviews of Modern Physics. American Physical Society (APS). 42 (2): 201–236. Bibcode:1970RvMP...42..201N. doi:10.1103/revmodphys.42.201. ISSN 0034-6861.
  3. Glinchuk M.D. In “Modern problems in condensed matter sciences”, v.7 – The dynamical Jahn-Teller effect in localized systems, ed. by Yu. Perlin, M.Wagner. North-Holland, Amsterdam-Tokyo, 1984, p.819.
  4. L.S. Sochava, V.S. Vikhnin, V.E. Bursian. Proc. of the XII Internat. Conf. on Defects in Insulating Materials, Nordkirchen, Germany, August 16–22, 1992, ed. by O.Kanert and J.-M.Spaeth, World Scientific Publ. Co, 1993, p. 390.
  5. Lebedev, A. I.; Sluchinskaya, I. A. (1993). "Unusual phase transitions in Pb1−xSnxTe1−ySey and Pb1−xSnxTe1−ySy, crystals induced by sn off-center ions". Ferroelectrics. Informa UK Limited. 143 (1): 91–98. Bibcode:1993Fer...143...91L. doi:10.1080/00150199308008317. ISSN 0015-0193.
  6. Lebedev, A. I.; Sluchinskaya, I. A.; Erko, A.; Veligzhanin, A. A.; Chernyshov, A. A. (2009). "XAFS studies of the local environment of Pb impurity atoms in barium, strontium, and calcium titanates". Physics of the Solid State. Pleiades Publishing Ltd. 51 (5): 991–997. Bibcode:2009PhSS...51..991L. doi:10.1134/s1063783409050175. ISSN 1063-7834. S2CID 97771194.
  7. Sack, H.S.; Moriarty, M.C. (1965). "Low-temperature dielectric phenomena in doped ionic crystals". Solid State Communications. Elsevier BV. 3 (5): 93–95. Bibcode:1965SSCom...3...93S. doi:10.1016/0038-1098(65)90229-2. ISSN 0038-1098.
  8. Lombardo, G.; Pohl, R. O. (1965-08-16). "Electrocaloric Effect and a New Type of Impurity Mode". Physical Review Letters. American Physical Society (APS). 15 (7): 291–293. Bibcode:1965PhRvL..15..291L. doi:10.1103/physrevlett.15.291. ISSN 0031-9007.
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