Crystal optical studies of spontaneous and precursor polarization inKNbO3

High-resolution measurements of the cubic refractive index $n$ ($700 \mathrm{K}<T<\mathrm{}1000\mathrm{} \mathrm{K}$) and of the tetragonal and orthorhombic birefringence $\Delta n$ ($480 \mathrm{K}<T<\mathrm{}705\mathrm{} \mathrm{K}$ and $300 \mathrm{K}<T<\mathrm{}490\mathrm{} \mathrm{K}$, respectively) are performed on ${\mathrm{KNbO}}_3$ in order to determine the temperature dependences of the fluctuating polarization, $〈P^2〉$, in the cubic phase and of the spontaneous polarization, $P_s$, in the ferroelectric phases. An Ornstein-Zernike analysis suggests that $n$ is sensitive to precursor short-range order up to above 1000 K and that the fluctuating polarization, mainly due to collective order-disorder processes, reaches 58% of $P_s=0.33\mathrm{}$ ${\mathrm{Cm}}^{-2\mathrm{}}$ at $T_{c1\mathrm{}}=705\mathrm{}$ K. The latter value as determined from $\Delta n$ is 22% larger than hitherto reported. Better agreement between optically and conventionally determined $P_s$ values is found for the orthorhombic phase.