Order and disorder in pure and dopedKTaO3: ATa181NMR study

Measurements of quadrupolar effects on the NMR spectrum and of nuclear spin-lattice relaxation $T_1$ are reported for ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ in pure and weakly Nb-, Li-, and Na-doped ${\mathrm{KTaO}}_3$ crystals. A new effect related to the ${\mathrm{KTaO}}_3$ host lattice is observed in the line-shape measurements indicating that below 40 K the Ta ions are in a local noncubic symmetry on a time scale longer than ${10}^{\mathrm{-}7}$ sec. The off-center displacement of the Ta ion is estimated to 0.04 Å. In ${\mathrm{KTa}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Nb}}_{\mathrm{x}}$ ${\mathrm{O}}_3$ the ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ and ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ $T_1^{\mathrm{-}1}$ data near the transition temperature $T_c$ confirm the existence of a collective ordering of the Nb ions. The Nb displacement below $T_c$ is estimated to be less than 0.15 Å. Moreover, the ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ line shape is consistent with a nonuniform distortion of the host lattice around each Nb ion below $T_c$. The NMR results in ${\mathrm{K}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Li}}_{\mathrm{x}}$ ${\mathrm{TaO}}_3$ are in agreement with the usual picture of an order-disorder transition dominated by the Li-ion hopping.