Birefringence studies of the phase transition in RbH2PO4

Abstract
The authors have measured the temperature and field dependence of the birefringence Delta n6of RbH2PO4in the vicinity of its ferroelectric phase transition. Their results are consistently fitted by assuming that this birefringence is proportional to the order parameter and that its variations are given by the Landau theory where the coefficient of the quantic term in the free-energy expansion is allowed to vary linearly with the temperature. The transition is found to be second order but close to a tricritical one.