Cluster size and long-range order in K1−xNaxTaO3

We present second harmonic generation results which, for the first time, allow us to assess the correlation length of the order parameter exhibited by the mixed crystal K1−xNaxTaO3. Our data, including linear birefringence results, evidence a transition to a polar state characterized, for a 15% Na doped sample, by clusters of size 0.12 to over 1 μm and a spontaneous polarization of 503 mC/m2. Even if these findings can be consistent with a ferroelectric transition characterized by long-range order, precursor effects and the behaviour of hysteresis are more indicative of a glass-type transition.