Phase transitions and twinned low-temperature structures of tetraethylammonium tetrachloridoferrate(III)

The title compound, [(C2H5)4N][FeCl 4], has at room temperature a disordered structure in the high-hexagonal space group P63mc. At 230K, the structure is merohedrally twinned in the low-hexagonal space group P63. The volume has increased by a factor of 9 with respect to the room-temperature structure. At 170 and 110K, the structure is identical in the orthorhombic space group Pca21 and twinned by reticular pseudomerohedry. The volume has doubled with respect to the room-temperature structure. All three space groups, viz. P63mc, P63 and Pca21, are polar and the direction of the polar axis is not affected by the twinning. In the P6 3 and Pca21 structures, all cations and anions are well ordered