Preparation, Crystal Structure at 298 and 90 K and Phase Transition in (C2H5NH3)2[SbBr5] Studied by the Single Crystal X-Ray Diffraction Method

The reaction of antimony(III) oxide with ethylamine, in molar ratios from 1:1 to 1:10, in concentrated hydrobromic acid leads to the formation of one product – bis(ethylammonium) pentabromoantimonate(III). The structure of (C2H5NH3)2[SbBr5] was determined at 298 and 90 K, below and above the phase transition that occurs at about 158.5 K. The orthorhombic system was found in both phases, space groups Cmca and Pbca at 298 and 90 K, respectively. At both temperatures the structure consists of [SbBr6]3− octahedra connected via cis bromine atoms forming one-dimensional zig-zag [{SbBr5}2−]n chains. The ethylammonium cations fill the space between polyanionic chains. The organic and inorganic substructures are held together by a system of N(–H)· · ·Br interactions. Their influence on the deformation of [SbBr6]3− octahedra is well reflected in differences in the corresponding Sb–Br bond lengths and Br–Sb–Br angles in both phases. The phase transition is of the first order and the order-disorder type. It is related to changes in the molecular dynamics of the ethylammonium cations. In the low-temperature phase the organic cations are ordered, while at 298 K both crystallographically independent cations are disordered. The type of disorder is realized by the presence of two positions of the methyl carbon atoms.