RESUMO
The investigation focuses on the crystal structure, microstructure, local ferroelectric and magnetic properties of the Bi0.9Sr0.1Fe1-x Ti(x)O(3-δ) (x = 0.05, 0.1, 0.15; δ = (0.1 - x)/2) multiferroics prepared by a solid-state reaction method. All the samples have been found to be isostructural with the pure BiFeO3 (the material crystallizes in a polar rhombohedral structure belonging to the space group R3c). It has been shown that the pattern of changes in the lattice parameters of the Bi0.9Sr0.1Fe(1-x)Ti(x)O(3-δ) samples can be interpreted as consistent with the doping-driven elimination of anion vacancies at x ⩽ 0.1 and the formation of cation vacancies at x > 0.1. The readjustment of the defect structure associated with the mechanism of charge compensation in the aliovalent-substituted BiFeO3 is accompanied by correlated changes in the morphology, ferroelectric/ferroelastic domain structure and magnetic properties of the materials. In particular, it has been found that the deviation from the ideal (δ = 0) cation-anion stoichiometry in the Bi0.9Sr0.1Fe(1-x)Ti (x)O(3-δ) system leads to a significant decrease in the average size of crystal grain and ferroelectric domains and gives rise to an antiferromagnetic-weak ferromagnetic transformation. Results of this study have been compared with those obtained for equally substituted samples of the Bi0.9Ca0.1Fe(1-x)Ti(x)O(3-δ) series (Khomchenko and Paixão 2015 J. Phys.: Condens. Matter 27 436002) to demonstrate how the variation in the chemical pressure introduced by the partial replacement of Bi(3+) with bigger (Sr(2+)) and smaller (Ca(2+)) ions can affect the multiferroic behavior of Ti-doped bismuth ferrites.
RESUMO
Recognition of the factors that may significantly affect the multiferroic properties of BiFeO3-based perovskites remains one of the most challenging tasks in condensed matter physics. To reveal the reasons behind the doping-driven instability of the cycloidal antiferromagnetic order in the polar phase of Bi(1-x)Ca(x)FeO(3-x/2), synthesis and investigation of the crystal structure, microstructure, local ferroelectric and magnetic properties of the ceramic samples of Bi0.9Ca0.1Fe(1-x)Ti(x)O(3-δ) (x = 0.05, 0.1, 0.15) have been carried out. The compounds possess a rhombohedral structure (space group R3c). The compositional dependence of unit cell volume in this series can be interpreted as suggesting the doping-induced elimination of anion vacancies at x ⩽ 0.1 and the formation of cation vacancies at x > 0.1. The filling of oxygen vacancies suppresses a weak ferromagnetic contribution characteristic of the parent Bi0.9Ca0.1FeO2.95. The appearance of cation vacancies restores the weak ferromagnetic phase. The key role of lattice defects in the magnetic behavior of Ca-doped BiFeO3 has been confirmed by the observation of a correlation between the magnetic properties and the morphology/ferroelectric domain structure of the Bi0.9Ca0.1Fe(1-x)Ti(x)O(3-δ) ceramics.
RESUMO
Neutron powder diffraction and magnetization measurements of the Bi(1-x)Ca(x)FeO3 (0.05 ≤ x ≤ 0.14) compounds were carried out to follow the effect of the heterovalent A-site doping on the long-range structure and magnetic properties of the BiFeO3 multiferroic. Ca substitution induces the appearance of weak ferromagnetism in the initial ferroelectric R3c phase, but modifies the picture of polar displacements, so the average PbZrO3-like antiferroelectric structure is stabilized at x = 0.11. Further increase of the Ca content leads to transformation of the antipolar ionic shifts to give rise to the Pbam â Imma transition near x = 0.14. A structural study performed for the x = 0.05 compound at high temperature revealed the R3c â Pnma phase transition at 950 K. For x = 0.1 samples, an intermediate heating-induced structure separating the R3c and Pnma phases was found.