${}^7\mathrm{Li}$ NMR Investigation of Li-Li Pair Ordering in the Paraelectric Phase of Weakly Substitutionally Disordered ${\mathbf{K}}_{1-x}{\mathrm{Li}}_x{\mathrm{TaO}}_3$

Breaking of the average cubic symmetry in Li-doped potassium tantalate was observed with quadrupole-perturbed ${}^7\mathrm{Li}$ NMR at temperatures (150–400 K) far above the nominal glass transition temperature ($\approx 50\mathrm{K}$ for Li concentration $x=0.03$). The observed spectrum consists of contributions from both isolated Li ions (i.e., with no nearest-neighbor Li) and from Li-Li pairs. The isolated Li ions move among six equivalent off-center sites in a potential having cubic symmetry. These have zero average electric field gradient and, hence, exhibit no quadrupole splitting. In addition, very low intensity, but well resolved, quadrupole satellites having a temperature-dependent splitting were observed. This splitting indicates that the various Li-Li pair configurations are not all equally probable. These are the first direct observations of biased Li ion ordering that persists in the paraelectric phase at temperatures high above the glass phase.

Figure 1

Configuration of “[100],” “[010],” and “[001]” Li-Li pairs. The instantaneous Li position is shown in a solid color, whereas the other five possible states are shown in a translucent color.

Figure 2

(a) Room temperature ${}^7\mathrm{Li}$ NMR spectrum in ${\mathrm{K}}_{0.958}{\mathrm{Li}}_{0.042}{\mathrm{TaO}}_3$ at a general orientation at which three satellite doublets can be resolved, belonging to the three respective orientations of Li-Li pairs (labels above the peaks). (b) Angular dependence ($\Theta \angle [010]$, ${\mathbf{B}}_0$) of the positions of the satellite transitions obtained by rotating the crystal about $[100]\perp {\mathbf{B}}_0$. Solid lines represent best fits to Eq. (2). (c) Concentration dependence of the relative intensity $I_s/I_c$ of ${}^7\mathrm{Li}$ NMR spectral components in KTL, determined from the spectra like that of Fig. 2a.

Figure 3

(a) Temperature dependence of the ${}^7\mathrm{Li}$ quadrupole frequency in ${\mathrm{K}}_{0.9895}{\mathrm{Li}}_{0.0105}{\mathrm{TaO}}_3$ (circles) and ${\mathrm{K}}_{0.958}{\mathrm{Li}}_{0.042}{\mathrm{TaO}}_3$ (squares). Solid line is a theoretical fit according to Eq. (3) with ${\nu }_{Qi}$ values listed in panel b. (b) Possible configurations of the two Li ions belonging to a “[100]” Li-Li pair. Only the basic configuration of each multiplet state is shown, together with the number of multiplet members ($N_i$) and the average multiplet quadrupole frequency ${\nu }_{Qi}$.