Dielectric relaxation spectroscopy and the ground state of K _1-xLi_xTaO₃

Dielectric susceptibility data are presented for frequencies between 10 and 109 Hz and for several Li concentrations. A low-frequency relaxation peak is attributed to an impurity mode slowing down to below 1 Hz at a concentration-dependent temperature. For Li concentrations x<or=0.04, temperature-dependent relaxation rates and the broadening of relaxation peaks are accounted for by analytical and numerical predictions from spin-glass models. A high-frequency relaxation peak is attributed to the ordered motion of the lattice. Its relaxation rate increases with Li concentration. The lattice does not order ferroelectrically for any sample reported. The impurity modes for x>or=0.06 exhibit disorder features that cannot be attributed to glass models nor to ferroelectricity. Non-random occupation of lattice sites by impurities could be at the origin of these features.