High Temperature Phase Transitions in BiFeO 3
- PDF / 520,358 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 54 Downloads / 232 Views
1199-F11-08
High Temperature Phase Transitions in BiFeO3 Donna C. Arnold1, Christopher M. Kavanagh1, Philip Lightfoot1 and Finlay D. Morrison1 1 School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9ST, UK. ABSTRACT The high temperature phases of BiFeO3 have courted much controversy with many conflicting structural models reported, in particular for the paraelectric β-phase. High temperature powder neutron diffraction (PND) experiments indicate that the ferroelectric (R3c) α-phase transforms to the paraelectric β-phase at approximately 820 °C via a first order phase transition. We demonstrate that this phase is unambiguously orthorhombic, adopting the GdFeO3 structure-type with a space group Pbnm. On further heating BiFeO3 undergoes another first order phase transition (β-γ) at approximately 930 °C which is marked by a discontinuous decrease in cell volume consistent with an insulator-metal transition. Close inspection of the PND data show no evidence of any symmetry change, with the postulated γ-phase remaining orthorhombic Pbnm. In addition we present PND and impedance spectroscopy data for BiFeO3 which suggest that the so-called ‘Połomska’ transition observed by some authors at approximately 185 °C is not intrinsic. INTRODUCTION In recent years multiferroic BiFeO3 has attracted a lot of attention, due primarily to its high temperature magnetic and electric transition temperatures: ferroelectric TC ~ 810 – 830 °C [1,2] and antiferromagnetic TN ~ 350 – 370 °C [3]. Despite extensive studies on bulk BiFeO3 there are still many contradictions in the literature ([4] and references therein). In particular the high temperature phase diagram has recently been studied extensively, though the exact natures of these phase transitions are still not fully understood. It is generally accepted that BiFeO3 undergoes two temperature driven phase transitions above room temperature. The first coincides with the ferroelectric – paraelectric transition at TC (~820 °C) with the second occurring at a much higher temperature (~930 °C) and thought to coincide with the postulated metal-insulator transition [5]. In addition anomalies are observed at TN and at ~ 185 °C [6]. However, the nature of this latter ‘Polomska’ transition has never been fully understood, with some authors reporting evidence for this transition in structural studies whilst others make no reference to it. Whilst the structure of the α-phase is widely accepted to exhibit a rhombohedral (R3c) symmetry, as determined by both powder x-ray diffraction (PXRD) and powder neutron diffraction (PND) studies [7-9], much controversy has surrounded the structures of both the β- and γ-phases. In fact recently the β-phase has been reported to exhibit tetragonal (space group I4/mcm) [10], monoclinic (P21/m) [11], orthorhombic (Pbnm) [12] and rhombohedral (R-3c) [13] symmetries by first principle and PXRD techniques, whilst the γ-phase has been reported as cubic [5]. Most of these inconsistencies arise as a result of difficulties in making single phase Bi
Data Loading...