Hexagonal tungsten bronze-type FeIII fluoride: (H2O)0.33FeF3; crystal structure,magnetic properties,dehydration to a new form of iron trifluoride

(H₂O)_0.33FeF₃, grown by hydrothermal synthesis, crystallizes in the orthorhombic system with cell dimensions a = 7.423(3) Å, b = 12.730(4) Å, c = 7.526(3)Å, and space group Cmcm, Z = 12. The structure, derived from single crystal X-ray diffraction data (605 independent reflections) is refined to R = 0.019 (R_ω = 0.021). The framework of the Fe^IIIF₆ octahedra is related to that of hexagonal tungsten bronze (HTB) Rb_0.29WO₃. At 122°C, zeolithic water is evolved from hexagonal tunnels without any noticeable change of the fluorine skeleton. The related anhydrous compound represents a new form of iron trifluoride which is denoted HTBFeF₃; at 525°C, it transforms into the cubic form of ReO₃-type. (H₂O)_0.33FeF₃ and HTBFeF₃ are antiferromagnetic, with Néel temperatures of T_N = 128°7 ± 0.5 K and T_N = 97 ± 2 K, respectively.