The observation of critical behaviour in the thermal expansion of PbZr_0.9Ti_0.1O₃

The lattice parameters have been determined as a function of temperature for single crystals of the ferroelectric PbZr_0.9Ti_0.1O₃. By associating the difference in rhombohedral angle with the square of the order parameter, P_s, the critical index beta has been determined for the ferroelectric to paraelectric phase transition. This transition appears to be of second order with beta approximately=0.33 for (T₀-T)/T₀<or=0.06. This difference from the mean-field value ( beta =0.5) suggests the presence of critical fluctuations in the order parameter. Outside the critical region the order parameter deviates even further from mean-field behaviour. This is explained in terms of further fluctuations associated with a transition at a lower temperature.