Atomic Structure of Defects in Anion-Deficient Perovskite-Based Ferrites with a Crystallographic Shear Structure

Abstract

Crystallographic shear (CS) planes provide a new structure-generation mechanism in the anion-deficient perovskites containing lone-pair cations. Pb2Sr2Bi2Fe6O16, a new n = 6 representative of the A(n)B(n)O(3n-2) homologous series of the perovskite-based ferrites with the CS structure, has been synthesized using the solid-state technique. The structure is built of perovskite blocks with a thickness of four FeO6 octahedra spaced by double columns of FeO5 edge-sharing distorted tetragonal pyramids, forming 1/2[110](101)p CS planes (space group Pnma, a = 5.6690(2) Å, b = 3.9108(1) Å, c = 32.643(1) Å). Pb2Sr2Bi2Fe6O16 features a wealth of microstructural phenomena caused by the flexibility of the CS planes due to the variable ratio and length of the constituting fragments with {101}p and {001}p orientation. This leads to the formation of "waves", "hairpins", "Γ-shaped" defects, and inclusions of the hitherto unknown layered anion-deficient perovskites Bi2(Sr,Pb)Fe3O8.5 and Bi3(Sr,Pb)Fe4O11.5. Using a combination of diffraction, imaging, and spectroscopic transmission electron microscopy techniques this complex microstructure was fully characterized, including direct determination of positions, chemical composition, and coordination number of individual atomic species. The complex defect structure makes these perovskites particularly similar to the CS structures in ReO3-type oxides. The flexibility of the CS planes appears to be a specific feature of the Sr-based system, related to the geometric match between the SrO perovskite layers and the {100}p segments of the CS planes.

A Novel, Layered Phase in Ti‐Rich SrTiO3 Epitaxial Thin Films

“…This typically causes the material to possess complex rotations of the CSS giving rise to what some have called waves, hairpins, and Γ-shapes as in the case of the A n B n O 3 n −2 homologous series of perovskite-based CSS. [ 42 ] In the Sr 2 Ti 7 O 14 phase, however, we have observed only one variant of the potential CSS that runs parallel to the growth plane of the fi lm in a highly ordered, periodic fashion across the sample.…”

“…CSS can be considered as an example of so-called non-conservative defect structures -or defect structures that change the composition of the parent material -and have been found in a range of materials. [38][39][40][41][42] In transition metal oxides, …”

“…[ 38,43 ] While early work on CSS focused on binary oxide materials, more recent work has explored CSS in ternary, anion-defi cient perovskites [ 42,[44][45][46][47] and others have gone on to extend the idea to cation-excess systems such as the RP homologous series. [48][49][50] In a similar spirit, it is possible to discuss Sr 2 Ti 7 O 14 within the confi nes of CSS.…”

Recent Advances in Transmission Electron Microscopy for Materials Science at the EMAT Lab of the University of Antwerp

“…Some types of samples can be difficult to study with powder diffraction, but are excellent for study using TEM, for example multiphase samples, samples with local defects and modulated materials [4,5,6,7]. While often such materials can still be analysed with powder diffraction, the data might be difficult for interpretation due to a variety of factors such as the presence of a large number of reflections, reflection overlap and anisotropic reflection broadening.…”

Structural, optical and vibrational properties of self-assembled Pbn+1(Ti1−xFex)nO3n+1−δ Ruddlesden-Popper superstructures

“…While a small deviation from the 1:1:3 stoichiometric ratio produces point defects, larger deviations can lead to extended (planar) defect formation. Non-conservative crystallographic shear planes can account for oxygen deficiency910 in A n B n O 3 n −2 , while the Ruddlesden-Popper (RP) homologous series A n +1 B n O 3 n +1 has extra AO planes11. The unit cell of the RP phase consists in the z -direction of n ABO 3 layers, a planar stacking fault [ a /2, b /2, d ] (where ), another n ABO 3 layers and a second stacking fault.…”