TEMPERATURE-DEPENDENCE OF THE ORDER PARAMETER OF THE 2 DIMENSIONAL ANTI-FERROELECTRIC SQUARIC ACID AS DETERMINED FROM BIREFRINGENCE MEASUREMENTS

The temperature dependence of the partial birefringence Δn is measured on single crystals of squaric acid. In contrast to previous results a phase transition of the first order is detected. The temperature dependence of Δn close to Tc supports the validity of a classical Landau expansion which also has been suggested recently in order to account for the behaviour of the specific heat. The results are discussed in relation to those of previous works. © 1979.