Critical behavior near the ferroelectric-ferroelastic phase transition of tanane

The nitroxyde 2,2,6,6-tetramethyl piperidine-1-oxyl (tanane) is an organic free radical which undergoes a ferroelectric-ferroelastic phase transition at $T_c=13.4\mathrm{}$°C. Static optical measurements have been performed on several tanane single crystals in both the high- and low-temperature phases. We present here a study of the spontaneous birefringence $\Delta n_6$ which is considered to be the order parameter of the transition; and a study of the anomaly of the longitudinal electro-optical coefficient $f_{63}^{\sigma }$ which is considered to be the susceptibility of the material near the transition point. Analysis of the data shows that they can be interpreted as a critical behavior of short-range interactions, except for a small range of temperature near $T_c$ which is dominated by long-range dipolar interactions. This view is supported by an evaluation of the dipolar energy compared to $k{T}_c$ which allows us to present a new classification of ferroelectric phase transitions.