Raman Study and the Evolution of Order in NH₄ Br at the λ Transition

Abstract

NH₄Br is known to experience a λ-type phase transition from a disordered cubic (CsCl structure) to an ordered tetragonal structure at T_λ = 235°K[1]. Above T_λ the NH₄ ⁺ ions are randomly distributed between two energetically equivalent orientations. (This is often called phase II.) Below T_λ the crystal is tetragonally distorted along one of the cube axes and at the same time the NH₄ ⁺ions assume ordered orientations. (This is often called phase III.) The identically oriented NH₄ ⁺ ions form chains along the tetragonal direction with neighboring chains oppositely oriented. In this sense the NH₄Br ordered phase is analogous to an antiferromagnet and is therefore more complicated than the ordered phase of NH₄Cl, which is analogous to ferromagnetic order. As a consequence, the NE₄Br unit cell contains two formula units below T_λ.