Diffuse first-order phase transition in NaNbO$_3$:Gd (a report)
  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. 
I. P. Raevski, S. I. Raevskaya, S. A. Prosandeev, V. A. Shuvaeva, A.   M. Glazer and M. S. Prosandeeva