First order β, γ phase transition in NH4Br

The course of the order parameter around the β, γ phase transition (235 K) in the NH₄Br has been closely observed by means of linear optical birefringence. The experimental data were fitted in the critical region T > 0.9T₀ by the Landau theory and a simple power law. Both relations reveal a first order transition, with the jump height in the birefringence amounting to Δn(T₀)/?Δn(120 K) = 0.145.