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The d~electric suscept~b~lity of substltutional defect4 m alkali halides shows a surprising dependence on concentration c I11 the dilute case c < 100 ppm, the susceptibility is proportional to the number of defects It reaches a maxlmum at a concentration where the average dipole-d~pole interaction 1s equal to the bare tunnelling amplitude of a single defect and for still higher concentrations c > 1000 ppm it strongly decreases The data for various defect systems are in quantitative agreement with a paranleter-free master curve der~vcd from a continued fraction expansion for the relevant correlation function With increasing defect concentration a universal crowover from coherent tunnelling to incoherent relaxation is driven by a novel relaxation mechan~sm, based on the &polar ~nteraction

We report measurements of the dielectric loss of lithium defects in potassium chloride (KCl:Li). Experiments were performed at audio frequencies and in the temperature range between 8 mK and 30 K on single crystals doped with different compositions of 6 Li and 7 Li. Our results clearly show that the mean relaxation rate exhibits a rather surprising isotope dependence, because the tunneling states associated with the heavier isotope 7 Li relax much faster than the ones due to 6 Li defects. Such an unusual behavior can be understood in the framework of a novel relaxation mechanism, based on the collective motion of the Li 1 defects.[S0031-9007(96)02094-7]

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