Thermodynamic potential and phase diagram for multiferroic bismuth ferrite (BiFeO3)

Dmitry V Karpinsky,Anna N Morozovska,Alexandra Franz,Long-Qing Chen,Sergey A Gavrilov,Eugene A Eliseev,Venkatraman Gopalan,Maya D Glinchuk,Fei Xue,Igor O Troyanchuk,Maxim V Silibin

doi:10.1038/s41524-017-0021-3

Abstract

We construct a Landau–Ginzburg thermodynamic potential, and the corresponding phase diagram for pristine and slightly doped bismuth ferrite, a ferroelectric antiferromagnet at room temperature. The potential is developed based on new X-ray and neutron diffraction experiments complementing available data. We demonstrate that a strong biquadratic antiferrodistortive-type coupling is the key to a quantitative description of Bi1−xLaxFeO3 multiferroic phase diagram including the temperature stability of the antiferromagnetic, ferroelectric, and antiferrodistortive phases, as well as for the prediction of novel intermediate structural phases. Furthermore, we show that “rotomagnetic” antiferrodistortive–antiferromagnetic coupling is very important to describe the ferroelectric polarization and antiferrodistortive tilt behavior in the R3c phase of BiFeO3. The Landau–Ginzburg thermodynamic potential is able to describe the sequence of serial and trigger-type phase transitions, the temperature-dependent behavior of the order parameters, and the corresponding susceptibilities to external stimuli. It can also be employed to predict the corresponding ferroelectric and antiferrodistortive properties of Bi1−xLaxFeO3 thin films and nanoparticles by incorporating the gradient and surface energy terms that are strongly dependent on the shape, size, and preparation method.

Highlights

The very high AFD transition temperature of multiferroic BiFeO3 offers the unique possibility to study the influence of AFD order on the FE and AFM phase transitions
Structural data obtained for the BiFeO3 compounds during the Xray diffraction (XRD) and neutron powder diffraction (NPD) measurements confirmed their single-phase rhombohedral structhe calculated one for the room temperature structure
The mentioned structural rearrangement is most probably associated with the oxygen octahedra tilts and rotations, which become more pronounced across the phase transition into the orthorhombic phase as discussed below

Summary

Introduction

State-of-the art Multiferroics, defined as ferroics with more than one long-range order, are ideal systems for fundamental studies of couplings among the ferroelectric (FE) polarization, structural antiferrodistortion, and antiferromagnetic (AFM) order parameters.[1,2,3,4,5] BiFeO3 (BFO) is the one of the rare multiferroics with a strong FE polarization, antiferromagnetism at room temperature as well as conduction and magnetotransport at domain walls.[6,7,8]Multiferroic properties have been extensively demonstrated BiFeO3 thin films and heterostructures.[9,10,11,12] In particular, the studies of thin epitaxial BiFeO3 films revealed a universal field induced phase transition, modulated phases and microstructure changes as a function of rare-earth (RE) elements (e.g., Dy, Sm, Ho) doping concentration.[13,14,15,16,17] Further RE doping effect was studied systematically for thin BiFeO3 films, and it was shown experimentally and theoretically[16,17,18] that the doping of BiFeO3 by Sm can lead to the enhancement of dielectric properties and tunability, as well as to the stabilization of the polar phase over a wide range of temperatures for the thin (Bi,Sm)FeO3 films and the short period superlattices BiFeO3–(Bi,Sm)FeO3.Bulk BiFeO3 exhibits antiferrodistortive (AFD) order at temperatures below 1200 K; it is FE with a large spontaneous polarization below 1100 K and is AFM below Neel temperature TN ≈ 650 K.19, 20 The very high AFD transition temperature of multiferroic BiFeO3 offers the unique possibility to study the influence of AFD order on the FE and AFM phase transitions. Expansion coefficients of the LG potential for pure BiFeO3 and BiFeO3 doped with 5% of La extracted from the fitting of measured the AFD and FE order parameters in rhombohedral R3c, orthorhombic Pbnm and tetragonal I4/mcm phases are listed in the Table 2.

Results

Conclusion