Perovskite and Perovskite Related Ferroelectric and Multiferroic Materials
Multiferroics represent an important technological class of materials which simultaneously exhibit magnetic and ferroelectric ordering. If magnetoelectric coupling can be achieved between these two phenomena the ability to switch the (ferro)electric state with a magnetic field and vice versa is expected to lead to a whole host of new multifunctional devices such as transducers, actuators, and sensors as well as ultimate memory devices. One multiferrioic material of interest is BiFeO3 (BFO) which exhibits both room temperature antiferromagnetic ordering (TN ~ 350-370 oC) and ferroelectric ordering (TC ~ 810-830 oC). Whilst many authors have investigated the crystal structure of BFO the structural phase transitions are still not well known. Two high temperature phase transitions have been reported, namely -BFO (ferroelectric) – β-BFO (paraelectric) at approximately 820 oC and a β-BFO to γ-BFO phase transition at approximately 930 oC. Whilst it is widely accepted that the -phase crystallises in the rhombohedral R3c space group various symmetries including orthorhombic, rhombohedral and monoclinic symmetries have been reported for the -phase. The γ-phase has been investigated to a lesser extent primarily due to the instability of BFO at high temperatures although some authors have postulated that this phase is cubic in nature. This later phase transition (i.e β-γ) is also reportedly coupled with an insulator-metal transition. In this paper we report unambiguous powder neutron diffraction (PND) data which confirms the -phase to be orthorhombic with a space group Pbnm and a full crystallographic model is proposed. We also show the evolution of the paraelectric -phase through time-resolved PND studies and demonstrate the phase transition occurs as a first order transition with co-existence of both the rhombohedral - and the orthorhombic -phases at temperatures between 820 oC and 830 oC. Moreover we demonstrate that no symmetry change is observed at the insulator-metal transition with both the β-BFO and γ-BFO phases exhibiting orthorhombic symmetry.
I will also present ongoing advances investigating novel (multi)ferroic materials with the perovskite related tetragonal tungsten bronze (TTB) structure. Preliminary data indicates that TC is highly dependent on structural distortion and can subsequently be controlled through selective doping. This raises the enticing possibility for designing new room temperature multiferroic materials with the TTB structure.
 Donna C. Arnold, Kevin S. Knight, Finlay D. Morrison, Philip Lightfoot, Phys. Rev. Lett., 2009, 102, 027602
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