Structure Study of Paraelectric-Ferroelectric Phase Transition in Monoclinic K_<b>2</b>ZnBr_<b>4</b>

Abstract

The phase transition from the paraelectric phase (α phase) to the ferroelectric one of monoclinic K 2 ZnBr 4 at about 159 K is investigated by means of X-ray diffraction. The space groups of the para- and ferroelectric phases are confirmed to be P 2 1 / m and P 2 1 , respectively. The crystal structures of both phases are also determined at 296 and 123 K, respectively. In the paraelectric phase, the displacive and the disordered models are examined: The significance test shows that ZnBr 4 tetrahedron is in disorder. In the ferroelectric phase, ZnBr 4 tetrahedron is ordered and two independent potassium ions are translated in a same direction along the b -axis.