Paraelectric and molecular impurities in ionic solids

Dipolar ions like CN and OH can be incorporated into solids like NaCl and KCl. Several small dopant ions like Cu and Li ions get stabilized in off-centre positions (slightly away from the lattice positions) in host lattices like KCl, giving rise to dipoles. These dipoles, which are present in the field of the crystal potential, are both polarizable and orientable in an external field, hence the name paraelectric impurities. Molecular ions like SJ, SeJ, Nf and O J can also be incorporated into alkali halides. Their optical spectra and relaxation behaviour are of diagnostic value in studying the host lattices. These impurities are characterized by an electric dipole vector and an elastic dipole tensor. The dipole moments and the orientation direction of a variety of paraelectric impurities have been studied in recent years. The reorientation movements may be classical or involve quantum-mechanical tunnelling. 

Ions like S2, SeJ and SSe are found to align along the 110 directions in most alkali halides, while in Nal, KBr and KI, the S—bond of S2 is oriented along the 100 direction. In the case of Oj, the p orbitals are parallel to the 100 direction in sodium salts but are parallel to the n0 direction in rubidium and potassium salts. Extensive spectroscopic studies have been reported on molecular ions such as NO, NO3, Cr04 and MnO . Several reviews(Corish et al, 1977 Bridges, 1975 Grimes, 1976) are available on such impurity-doped solids. 

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