  Dielectric permittivity measurements show that (1-x)NaNbO3-(x)Gd1/3NbO3
crystals, at $x=0.09$, exhibit a first order phase transition near 150 C which
is diffuse over a wide temperature interval. The area of the \epsilon(T)
thermal hysteresis loop was found to depend both on the annealing and cooling
temperatures. Optical studies in polarized light show the decrease of the
domains sizes at this phase transition. The results obtained are fitted
assuming the space distribution of Curie temperatures.
