Linear birefringence studies on mixed crystals of Rb1-x(NH4)xH2AsO4

Abstract

The linear birefringences, δn_ac and δn_ab, of Rb_1−x(NH₄)_x H₂AsO₄ have been measured on ferroelectric (x=0.13), structural glass (x=0.20 and 0.37), and antiferroelectric mixtures (x=0.49). Domain observations reveal one and two orthorhombic low-T phases for x=0.13 (T_c=85.5 K: second-order transition with random interaction Ising model critical exponent β=0.35) and for x=0.49, respectively (T_c1=85 K: second-order transition, T_c2∼60 K: smeared first-order transition). In the glassy systems, x=0.20 and 0.37, Δ_ac reveals 3 ²≫O for T≤150 K, whereas Δn_ab=0 at any T.