Local properties, long-range order, and quantum ferroelectricity in KTa1−xNgxO3

Abstract

We present the first data ${(}^{39}$K, ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$, and ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ NMR) on local properties at the phase transition in slightly Nb-doped ${\mathrm{KTaO}}_3$, where quantum effects have been thought to be important. The Nb ions go off center at low temperatures, but not all Ta do the same: The distortion of the host lattice has no long-range order. Each off-center Nb ion polarizes a cloud of some 100 ${\mathrm{KTaO}}_3$ unit cells. We suggest that coupling with the soft mode creates orientational order among these clouds, but with varying magnitude of the dipole moment per ${\mathrm{KTaO}}_3$ unit cell.