93Nb NMR study of disorder in KTa1-xNbxO3 -: art. no. 144110

The Nb-93 nuclear magnetic resonance spectra of a mixed KTa1-xNbxO3 (KTN) single crystal with x=15% have been measured. The observation of an unresolved quadrupole-induced first-order satellite background below the sharp central 1/2 <->-1/2 transition already in the cubic phase demonstrates that the Nb ions are dynamically disordered between off-center positions rather than being located at the high symmetry central perovskite site. The angular dependence of the second moment of the satellite background further shows that these distortions and the biasing of the Nb hopping are of rhombohedral symmetry, i.e., the Nb ions are effectively displaced along the [111] body diagonals. The two-order-of-magnitude difference between the spin-lattice (T-1) and spin-spin (T-2) relaxation times indicates a two-time scale behavior: the faster time scale can be tentatively assigned to biased hopping of the Nb ion and the slower one to flipping of rhombohedral nanodomains which percolate at the ferroelectric transition. This also agrees with the observed huge increase in the second moment M-2 at the transition to the ferroelectric phase at T-C=135 K.