Angular Dependence of 93Nb NMR in KTa1-xNbxO3.

The ⁹³Nb NMR spectra of a mixed KTa_1-x Nb_x O₃ (KTN) single crystal with x = 15% have been measured. The observation of an unresolved quadrupole-induced first-order satellite background below the sharp central ½ ↔ -½ transition already in the cubic phase demonstrates that the Nb ions occupy off-center positions rather than the high symmetry central perovskite site. The angular dependence of the second moments of the satellite background further shows that these distortions are of rhombohedral symmetry, i.e. the Nb ions are displaced along the [111] body diagonals. [ABSTRACT FROM AUTHOR]