Linear birefringence and phase transitions in triammonium hydrogen disuiphate, (NH₄)₃H(SO₄)₂

Abstract

Abstract Phase transitions have been determined for triammonium hydrogen disulphate (NH4)3H(SO4)2 (NHS) over the temperature range 30–300 K using linear birefringence Δn. We have measured the critical exponent α for the II → III and III → IV phase transitions. We find that the II → III transition shows a classical mean-field behaviour with α = 0.49 ± 0.01, whereas the III → IV transition shows fluctuation-dominated Ising-like behaviour with α = 0.63 ± 0.01.