Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. We report on extensive experimental studies on thin film, single crystal, and ceramics of multiferroic bismuth ferrite BiFeO 3 using differential thermal analysis, high-temperature polarized light microscopy, hightemperature and polarized Raman spectroscopy, high-temperature x-ray diffraction, dc conductivity, optical absorption and reflectivity, and domain imaging, and show that epitaxial ͑001͒ thin films of BiFeO 3 are clearly monoclinic at room temperature, in agreement with recent synchrotron studies but in disagreement with all other earlier reported results. We report an orthorhombic order-disorder ␤ phase between 820 and 925 ͑Ϯ5͒°C, and establish the existence range of the cubic ␥ phase between 925 ͑Ϯ5͒ and 933 ͑Ϯ5͒°C, contrary to all recent reports. We also report the refined Bi 2 O 3 -Fe 2 O 3 phase diagram. The phase transition sequence rhombohedral-orthorhombic-cubic in bulk ͓monoclinic-orthorhombic-cubic in ͑001͒BiFeO 3 thin film͔ differs distinctly from that of BaTiO 3 . The transition to the cubic ␥ phase causes an abrupt collapse of the band gap toward zero ͑insulator-metal transition͒ at the orthorhombic-cubic ␤-␥ transition around 930°C. Our band structure models, high-temperature dc resistivity, and light absorption and reflectivity measurements are consistent with this metal-insulator transition.

IntroductionThe recent recognition of the importance of metallomesogens (i.e., liquid crystals containing metal ions) for the development of advanced materials 1 with new electronic, optical, and magnetic properties 2 has encouraged coordination chemists to design new lipophilic rodlike or disklike complexes exhibiting mesomorphic behavior. 3 Liquid crystalline (or mesomorphic) properties result from phases in which the molecular order is intermediate between that of an ordered solid crystal and a disordered liquid. A thermotropic mesophase is formed by the influence of temperature, and the resulting thermotropic liquid crystals are divided in two main categories depending on their molecular axial anisotropies: calamitic (rodlike) and discotic (disklike). 3 A large variety of bidentate chelating units has been successfully used for the design of metallomesogens, 3 but it has been shown that metal complexes with thermotropic calamitic 5 and discotic 6 mesogenic ligands containing linear bidentate 2,2′-bipyridine units 4 display only poor 7 or no 8 mesogenic behavior. The origin of these failures has been tentatively attributed (i) to the large increase of the electric dipole moment which occurs upon complexation 8 and (ii) to the limited anisometry of the rigid core, although this has been recently overcome in extended systems. 9 The closely related tridentate receptor 2,2′:6′,2′′-terpyridine is essentially ignored in this field, 3 as are other tridentate binding units. 10 To the best of our knowledge, a single . Particular attention has been focused on structure-properties relationships, which can be modulated by the size of the lanthanide metal ions.

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