Dielectric permittivity measurements show that (1−x)NaNbO 3 -(x)Gd 1/3 NbO 3 crystals, at x = 0.09, exhibit a first-order phase transition near 150 • C that is diffuse over a wide temperature interval. The area of the ε (T ) thermal hysteresis loop was found to depend both on annealing and cooling temperatures. Optical studies in polarized light show a decrease of the domain sizes at this phase transition. The results obtained are fitted assuming a spatial distribution of Curie temperatures.

We consider consequences of local disorder in systems experiencing first order phase transitions. Such systems can be of rather different nature. For example, manganates showing gigantic magnetoelectric effect, doped antiferroelectrics or biomembranes. Monte-Carlo computations performed have shown that the disorder increases the temperature interval where the high-and low-temperature phases coexist and this provides thermodynamics in the disordered systems distinct from the thermodynamics of classical homogeneous systems.

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