Structural instability of epitaxial (001) BiFeO₃ thin films under tensile strain.

Zhen Fan,David J Singh,John Wang,Alfred Huan,Khuong P Ong,Michael B Sullivan

doi:10.1038/srep04631

Abstract

We explore BiFeO3 under tensile strain using first-principles calculations. We find that the actual structures are more complex than what had been previously thought, and that there is a strong shear deformation type structural instability which modifies the properties. Specifically, we find that normal tensile strain leads to structural instabilities with a large induced shear deformation in (001) BiFeO3 thin films. These induced shear deformations in (001) BiFeO3 thin films under tension stabilize the (001) BiFeO3 thin films and lead to Cc and Ima2 phases that are more stable than the Pmc21 phase at high tensile strain. The induced shear deformation shifts the Cc to Ima2 phase transition towards lower tensile strain region (~1% less), prevents monoclinic tilt and oxygen octahedral tilts, and increases the ferroelectric polarization. The induced shear deformation also strongly affects the electronic structure. The results are discussed in relation to growth of BiFeO3 thin films on cubic and tetragonal substrates involving high levels of tensile strain.

Highlights

We explore BiFeO3 under tensile strain using first-principles calculations
Theoretical and experimental studies show that compressive strain induces successive Rhombohedral (R) - Monoclinic (MA) - Monoclinic (MC) Tetragonal (T) phase transitions in BFO (001) thin films accompanied by changes in the magnitude and orientation of ferroelectric polarization[22,23]
Our initial motivation was that while both compressive strain and tensile strain have been investigated for epitaxial BFO thin films, the influence of shear deformation on BFO was not studied even though the coupling of strain to physical properties suggests that new phenomena may be observed

Summary

Introduction

We explore BiFeO3 under tensile strain using first-principles calculations. We find that the actual structures are more complex than what had been previously thought, and that there is a strong shear deformation type structural instability which modifies the properties. We find that normal tensile strain leads to structural instabilities with a large induced shear deformation in (001) BiFeO3 thin films. Ferroelectricity may be regarded as a polar lattice instability that arises because of poorly satisfied bonding, while magnetism and magnetic interactions depend strongly on bond lengths and bond angles, e.g. through hopping integrals that control superexchange For this reason, strain engineering of oxide ferroelectrics and magnetoelectrics through epitaxial growth has been a effective approach for realizing new properties[1,2,3,4,5,6,7,8]. BiFeO3 (BFO) is a interesting material from this point of view It can be epitaxially grown on a variety of oxide substrates with an exceptionally large range of achievable strain, and this leads to a wide range of properties and structures. We revisited the stability of phases Cc, Ima[2] and Pmc[21] (without influence of deformation) focusing on the stability of Pmc[21] structure and find quantitative differences from prior reports regarding the stability of the orthorhombic phase; in particular, it is less stable than previously reported even without deformation

Methods

Results

Conclusion