Polar correlations and defect-induced ferroelectricity in cryogenic KTaO3

KTaO3 is an incipient ferroelectric material with an extrapolated transition temperature below 0 K. It contains a small number of "unavoidable defects" which are randomly distributed. Some of these defects are polar and their interaction leads to macroscopic coherent polar structures at low temperatures. In this article it is shown that freezing of local defect dipoles coincides with elastic stiffening and damping of ultrasonic waves in KTaO3. The elastic freezing anomalies are accompanied by stepwise increases of piezoelectricity, forming a thermal polar staircase below ca. 120 K and a gigantic enhancement below 50 K. A small spontaneous polarization also emerges below this temperature, gradually increasing to a value of 0.045 mu C cm(-2) at 5 K with increasing coherency of defect dipoles. The orientation of this spontaneous polarization depends on a weak strain-induced anisotropy of the macroscopic sample. Defect-induced ferroelectricity, as demonstrated for KTaO3, may be a possible way forward to develop functional device materials based on the switching of coherently interacting defects.