Paraelectric impurities

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. 

This kind of microstructure also influences other kinds of conductors, especially those with positive (PTC) or negative (NTC) temperature coefficients of resistivity. For instance, PTC materials (Kulwicki 1981) have to be impurity-doped polycrystalline ferroelectrics, usually barium titanate (single crystals do not work) and depend on a ferroelectric-to-paraelectric transition in the dopant-rich grain boundaries, which lead to enormous increases in resistivity. Such a ceramic can be used to prevent temperature excursions (surges) in electronic devices. 

Now let us discuss the behavior recently observed in doped ferroelectric crystals in the paraelectric phase (see Section IV.C). Because process B was observed only in crystal 1 (see Fig. 18), it must be attributed to the presence of the Cu impurities embedded at random in the KTN crystal. The Arrhenius nature of the process at elevated temperatures above 354 K indicates normal relaxation of the independent Cu+ ions [179]. These ions are significantly smaller than the K+ sites in which they reside (the radii of the Cu+ and the K+ ions are 0.77 A and 1.52 A, respectively [250]), leading to off-center displacements. The Cu+ ions can therefore hop between the eight symmetrical minima of their potential wells. Indeed, the energy of activation of EBa = 0.37 eV corresponds to the activation energies for the hopping of transition metal ion impurities in KTa03 [251]. 

Thus, one may summarize the physical picture of the relaxation dynamics in KTN crystal-doped with Cu+ ions in the following way In the paraelectric phase, as the ferroelectric phase transition is approached, the Nb5+ ions form dipolar clusters around the randomly distributed Cu+ impurity ions. The interaction between these clusters gives rise to a cooperative behavior according to the AG theory of glass-forming liquids. At the ferroelectric phase transition the cooperative relaxation of the Cu+ ions is effectively frozen. ... 

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