Thermal expansion at the incommensurate phase transition in [N(CH3)4]2ZnCl4-xBrx crystals

The temperature dependence of the thermal expansion in the vicinity of the incommensurate phase transition in [N(CH3)4]2ZnCl4-xBrx mixed crystals is found to deviate from that predicted within the Landau theory of phase transitions. It is shown that the dominant contribution to this deviation in the immediate vicinity of Ti is due to the presence of defects. The character of the deviation is found to be in qualitative agreement with the theoretically predicted behaviour in the presence of random field type defects. In some compounds (x = 0, 2.4, 2.9), the temperature dependence of the thermal expansion coefficient at T ≥ Ti + 3 K can be described by a τα-dependence with α = 0.5. These values are in reasonable agreement with α = 0.5, corresponding to the fluctuation correction to the Landau theory. In solid solutions with a rather smeared paraelectric-incommensurate phase transition (x = 0.25, 0.45), the value of this exponent is found to be between 0.8 and 0.9. This suggests that the contribution to the thermal expansion due to the defects in these crystals is, even quite far from the phase transition, comparable to the contribution due to order parameter fluctuations.