Phase diagram of BiFeO3/LaFeO3 superlattices: antiferroelectric-like state stability arising from strain effects and symmetry mismatch at heterointerfaces

摘要

(BiFeO3)(1-x)Λ/(LaFeO3)xΛ superlattices (SLs) have been grown using pulsed laser deposition and studied by x-ray diffraction, transmission electron microscopy (TEM) and Raman spectroscopy. The composition was varied, 0.30 ≤x≤ 0.85, while the modulation period Λ was kept constant at about 10nm. Unit cell doubling signatures typical of orthorhombic Pnma symmetry for x= 0.80 and 0.85 SLs and ¼{011} antiferroelectric PbZrO3 like reflections in SLs with 0.30 ≤x≤ 0.7 are detected by TEM showing a complex structural mixture at the nanoscale level. The Raman spectra confirm these observations and show a change in the SLs from a Pnma LaFeO3 like spectra for LaFeO3-rich period to a PbZrO3 like spectra for BiFeO3-rich period. Electron-phonon interactions and resonant-like excitations were also observed in the SLs. A temperature dependent x-ray diffraction investigation shows a large shift of the paraelectric-antiferroelectric phase transition scaling with the BiFeO3 thickness. This shift is correlated with the strain state and can be explained by a strong interplay between octahedral rotation/tilt and anti-polar Bi displacement. Thickness-temperature phase diagram is constructed and differs from previous report showing the extreme sensitivity of the BiFeO3 phase stability to strain effects and rotation/tilt degrees of freedom.