Ferroelastoelectric ordering in (NH4)2CuBr4·2H2O single crystal

Abstract

The diammonium tetrabromocuprate dihydrate single crystal, (NH₄)₂CuBr₄·2H₂O, was studied by six different techniques: single-crystal X-ray and neutron diffraction, differential scanning calorimetry, dielectric permittivity, conoscopic figures and piezoelectric resonances. The compound undergoes a weak second order phase transition at 158 K changing the symmetry from P4₂/mnm to P2₁m. The piezoelectric effect observed below 158 K confirmed the non-centrosymmetric low-temperature crystal phase. The X-ray diffraction studies performed at 296 and 120 K and the single neutron diffraction studies at 293 and 20 K determined structures of both phases with high precision. Only neutron results allowed to describe the mechanism of the phase transition, which is entirely connected with the orientational ordering of hydrogen atoms from ammonium groups. The phase transition is classified as an order–disorder continuous one. The low-temperature phase has a ferroelastoelectric higher-order ferroic ordering and the piezoelectric tensor is the macroscopic order parameter.