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OFF-CENTER IMPURITY IONS IN CRYSTALS

Off-center ions in crystals are substitutional impurity ions whose equilibrium position is shifted away from the regular lattice site. This shift is typically smaller than the lattice constant. Ion displacement can occur in crystals where the ionic radius of the impurity is significantly smaller (by 10% or more) than that of the host crystal. The displacement arises because the repulsive forces between impurity ion and its nearest neighbors stabilizing the ion at the regular site are strongly weakened due to small size of the impurity ion. This phenomenon is more likely to occur for the ions with high polarizability. Off-center position of substitutional ions was first discovered in lithium-doped KCl  by two groups of American  physicists in 1965 /1, 2 /. 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 tunnel movement of atomic particles in solid state, cooperative properties of the system of local centers with internal degrees of freedom, and segnetoelectricity  (see review papers /3 - 6/).

REFERENCES [1] Sack H.S.,  Moriarty  M. C.  Solid State Commun.,  3,  93  (1965). [2] Lombardo G.,  Pohl  R. O.  Phys. Rev. Lett., 15, 291 (1965). [3] Smoluchowski R.  Magnetic resonance and radiofrequency spectroscopy  (Proc. XV Colloque AMPERE). Amsterdam-London, 1969,  p. 120. [4] Narayanamurti V.,  Pohl  R. O.  Rev. Mod. Phys., 42, 201  (1970).[5] 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. [6] 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.