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

The title compound, [(C2H5)(4)N][FeCl4], has at room temperature a disordered structure in the high-hexagonal space group P6(3)mc. At 230 K, the structure is merohedrally twinned in the low-hexagonal space group P6(3). The volume has increased by a factor of 9 with respect to the room-temperature structure. At 170 and 110 K, the structure is identical in the orthorhombic space group Pca2(1) and twinned by reticular pseudomerohedry. The volume has doubled with respect to the room-temperature structure. All three space groups, viz. P6(3)mc, P6(3) and Pca2(1), are polar and the direction of the polar axis is not affected by the twinning. In the P6(3) and Pca2(1) structures, all cations and anions are well ordered.