Nature of the phase transition inK1−xNaxTaO3

We present dielectric, elastic, and NMR measurements that point at the special character of the phase transition in ${\mathrm{K}}_{1-x}{\mathrm{Na}}_x{\mathrm{TaO}}_3$ mixed crystals. At the onset of birefringence, a critical behavior is observed in the dielectric constant and in the elastic compliance, whereas the NMR measurements do not reflect any criticality. It is suggested that, related to the transition, an off-center displacement of the sodium ions of less than 0.1 Å occurs.