Structural Evolution of the BiFeO₃−LaFeO₃System

Abstract

The (1x)BiFeO 3 -xLaFeO 3 system has been investigated and characterized by room-temperature and high-temperature laboratory and synchrotron powder X-ray diffraction, electron diffraction, high-resolution transmission electron microscopy, differential scanning calorimetry, and magnetization measurements. At room temperature, the ferroelectric R3c phase is observed for 0.0 e x e 0.10. The PbZrO 3 -related ffiffi ffi p 2a p  2 ffiffi ffi p 2a p  4a p superstructure (where a p is the parameter of the cubic perovskite subcell) is observed for Bi 0.82 La 0.18 FeO 3 , while an incommensurately modulated phase is formed for 0.19 e x e 0.30 with the ffiffi ffi p 2a p  2a p  ffiffi ffi p 2a p basic unit cell. The GdFeO 3 -type phase with space group Pnma ( ffiffi ffi p 2a p  2a p  ffiffi ffi p 2a p ) is stable at 0.50 e x e 1. Bi 0.82 La 0.18 FeO 3 has no detectable homogeneity range (space group Pnam, a=5.6004(1) A ˚, b=11.2493(3) A ˚, c=15.6179(3) A ˚).The incommensurately modulated Bi 0.75 La 0.25 FeO 3 structure was solved from synchrotron X-ray powder diffraction data (Imma(00γ)s00 superspace group, a = 5.5956(1) A ˚, b = 7.8171(1) A ˚, c = 5.62055(8) A ˚, q=0.4855(4)c*, R P =0.023, R wP =0.033). In this structure, cooperative displacements of the Bi and O atoms occur, which order within the (AO) (where A=Bi, La) layers, resulting in an antipolar structure. Local fluctuations of the intralayer antipolar ordering are compensated by an interaction with the neighboring (AO) layers. A coupling of the antipolar displacements with the cooperative tilting distortion of the perovskite octahedral framework is proposed as the origin of the incommensurability. All the phases transform to the GdFeO 3 -type structure at high temperatures. Bi 0.82 La 0.18 FeO 3 shows an intermediate PbZrO 3 -type phase with ffiffi ffi p 2a p  2 ffiffi ffi p 2a p  2a p (space group Pbam; a = 5.6154(2) A ˚, b = 11.2710(4) A ˚, and c = 7.8248(2) A ˚at 570 K). The compounds in the compositional range of 0.18 e x e 0.95 are canted antiferromagnets.

Structural and magnetic properties of isovalently substituted multiferroic BiFeO3: Insights from Raman spectroscopy

“…Finally, the GdFeO 3 type P nma structure was reached above x = 0.5. 16 These subtle structural transitions were further corroborated by neutron diffraction experiments where the PbZrO 3 structure with P bam symmetry was observed for Bi 0.82 La 0.18 FeO 3 . Signs of an incommensurate structure with diffraction peaks very similar to P bam was, however, not observed until x = 0.25.…”

“…17 However, a more recent study, also employing a combination of synchrotron XRD and ED, reported a total of four structural phases. 16 A single phase R3c structure was reported up to x = 0.1, followed by a mixed phase region for x < 0.18. At this point, a single phase P nam structure of the PbZrO 3 type was observed.…”

“…15 A similar transition, albeit more complicated, is reported in the intensely studied Bi 1−x La x FeO 3 series. 5,[16][17][18][19] Here the similar ionic radii between Bi and La makes it possible to stabilize several intermediate structures before the final P nma structure is reached at 50% La content. 16 XRD and electron diffraction (ED) data indicated a stabilized R3c structure up to 20% La content where a transition towards a P nma structure of the GdFeO 3 type occurred.…”

“…5,[16][17][18][19] Here the similar ionic radii between Bi and La makes it possible to stabilize several intermediate structures before the final P nma structure is reached at 50% La content. 16 XRD and electron diffraction (ED) data indicated a stabilized R3c structure up to 20% La content where a transition towards a P nma structure of the GdFeO 3 type occurred. 17 However, a more recent study, also employing a combination of synchrotron XRD and ED, reported a total of four structural phases.…”

“…19 The P nam and P bam structures appear very similar from x-ray and neutron diffraction and the P nam structure has only been identified by spatially local electron diffraction measurements. 16 Even though PbZrO 3 crystallizes in the P bam structure, 20 we will use PbZrO 3 type to denote either P nam or P bam; this point will be discussed further in Sec. V C.…”