Structural transitions and complex domain structures across a ferroelectric-to-antiferroelectric phase boundary in epitaxial Sm-dopedBiFeO3thin films

Abstract

We have investigated structural phase transitions across a ferroelectric-to-antiferroelectric phase boundary in epitaxial ͑001͒ oriented Bi ͑1−x͒ Sm x FeO 3 thin films. For the Sm 3+ concentration of 0.1Յ x Յ 0.14, we observe short-range antiparallel cation displacements, verified by the appearance of localized 1 4 ͕011͖ weak reflections in the selected area electron diffraction patterns. At the critical composition of x = 0.14, the system adopts a complex nanoscale domain mixture with appearance of 1 4 ͕011͖, 1 2 ͕011͖, 1 2 ͕010͖, and 1 2 ͕111͖ reflections and an incommensurate phase bridging the rhombohedral and orthorhombic phases. For compositions 0.14Ͻ x Ͻ 0.2, orientational twin domains coupled with antiphase oxygen octahedral tilts, identified by 1 2 ͕hkl͖ weak superstructure are observed. The above systematic changes in the microstructure as a function of Sm 3+ doping are linked to the macroscopic functional properties.

Phase coexistence near a morphotropic phase boundary in Sm-doped BiFeO3 films

“…6 At this critical doping, the film has features of a morphotropic phase boundary ͑MPB͒ solid solution, consisting of a cell-doubled orthorhombic Pnma phase beyond the MPB and a PbZrO 3 ͑PZO͒-like structural phase in local regions below the MPB. [6][7][8] Electron and laboratory x-ray diffraction studies in Ref. 7 show a superlattice peak at 1/2-order along b in pseudocubic notation that appears to be uniquely associated with the orthorhombic phase.…”

“…[6][7][8] Electron and laboratory x-ray diffraction studies in Ref. 7 show a superlattice peak at 1/2-order along b in pseudocubic notation that appears to be uniquely associated with the orthorhombic phase. In this same work, high resolution transmission electron microscopy ͑HR-TEM͒ imaging has revealed nanoscale twin domains with a size of 10-20 nm.…”

“…We also observe antiphase domain boundaries in the orthorhombic phase that may correspond to nanodomains identified in HR-TEM. 7 Finally, we note that relatively thicker films ͑200 nm͒ exhibit a substantial volume collapse associated with the structural transition that is suppressed in thin films ͑20 nm͒.…”

“…6 Wavelength dispersive x-ray spectroscopy maps over a 100ϫ 100 m 2 area showed homogenous composition within Ϯ1%, and extensive TEM studies using HR-TEM and high angle annular dark field scanning TEM have not revealed macroscale composition inhomogeneity. 7,9 This composition allows us to straddle the MPB as depicted in the temperature-composition phase diagram 10 in Fig. 1͑a͒.…”

“…Previous work did not reveal the weak peak at this position in the low temperature phase and were not of sufficient resolution to distinguish the splitting. 7 The differences in film thickness manifest themselves in two ways: in the magnitude of the splitting of the satellite peaks and in the ratio of intensities of the peaks associated with each phase. Splitting about the h = 0 value, coupled to the lack of such splitting in the main Bragg peaks, is not consistent with orthorhombic twin domains but more likely associated with an antiphase domain boundary of FeO 6 octahedra rotations.…”