Article pubs.acs.org/cm
Intrinsic Compositional Inhomogeneities in Bulk Ti-Doped BiFeO3: Microstructure Development and Multiferroic Properties M. S. Bernardo,*,† T. Jardiel,† M. Peiteado,‡ F. J. Mompean,§ M. Garcia-Hernandez,§ M. A. Garcia,† M. Villegas,† and A. C. Caballero† †
Departmento de Electrocerámica, Instituto de Cerámica y Vidrio (CSIC), Kelsen, 5, 28049, Madrid, Spain Departamento de Física Aplicada, E.T.S.I. Telecomunicación (UPM), Avda. Complutense 30, Ciudad Universitaria, 28040 Madrid, Spain § Departamento de Materiales para las Tecnologías de la Información, Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, 28049 Madrid, Spain ‡
S Supporting Information *
ABSTRACT: Ti-doped BiFeO3 ceramics prepared by a mixed-oxide route were structurally characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), and highresolution transmission electron microscopy (HRTEM), giving evidence of the formation of an inner structure at the nanometric scale. The observed nanograins are separated by Ti-rich areas that originate due to the tendency of the titanium dopant to segregate from the perovskite lattice. Such a peculiar nanostructure is responsible for the changes produced in both the electrical and the magnetic properties of BiFeO3 upon titanium doping: the Ti-rich interfaces act as resistive layers that increase the direct-current (dc) resistivity of the material, while the existence of structural domains in the scale of tens of nanometers causes a ferrimagnetic-like behavior with a huge coercive field (on the order of 20 kOe), even at room temperature. KEYWORDS: BiFeO3, nanostructural domains, multiferroics
I. INTRODUCTION Multiferroic materials have received increasing interest in the past few years, because the coexistence of electric and magnetic ordering provides extra degrees of freedom in the storage process, which may either simplify the operation of present device structures or offer new architectures.1,2 BiFeO3 is one of the most promising multiferroic materials, since it can exhibit both ferroelectricity and antiferromagnetism at room temperature. It presents considerably high phase-transition temperatures, which are 645 K for the antiferromagnetic-to-paramagnetic transition (Néel temperature, TNéel) and 1103 K for the ferroelectric-to-paraelectric transition (Curie temperature, TCurie).3 At atmospheric pressure and room temperature, BiFeO3 presents a rhombohedrically distorted perovskite structure belonging to the R3c group.3 However, BiFeO3 materials usually present a high electrical conductivity that hampers their practical applications. The origin of this high leakage current is attributed to the possible coexistence of Fe3+/ Fe2+ as well as to the presence of oxygen vacancies.3,4 In order to stabilize the perovskite structure and reduce the electrical losses, the use of different synthesis methods,5−7 the addition of dopants,8−10 or the formation of solid solutions with other perovskites,11,12 are currently under study. Among the different possible strategies to avoid the mentioned problems, titanium doping has been proposed as a © 2013 American Chemical Society
promising method to enhance the properties of the BiFeO3 materials.13−20 However, the properties of Ti-doped BiFeO3 materials are extremely sensitive to the process conditions and they are often not consistent among different works. For example, titanium doping has been reported both to increase and to decrease the electrical conductivity.13,20−22 Magnetic properties of Bi(Fe,Ti)O3 materials also diverge: although most works report an increased magnetization due to the structural distortion and/or disturbance of antiparallel magnetic ordering,13,16,20,23 some authors affirm that Ti-substitution into the Fe positions causes paramagnetic behavior, as a consequence of the nonmagnetic nature of Ti4+ ions.21 It is certainly clear that both the magnetic and the electric responses are strongly affected by the structural defects originated, as a consequence of the Ti-substitution: as an aliovalent dopant, Ti4+ substitution must imply the formation of some structural defects in order to compensate the charge. However, in the literature it is still discussed whether the main charge compensation mechanism is the formation of Fe2+ or the annihilation of pre-existent oxygen vacancies,16,18,19,24,25 although the existence of Fe2+ is usually discarded when the leakage current decreases in the doped Received: November 20, 2012 Revised: April 4, 2013 Published: April 4, 2013 1533
dx.doi.org/10.1021/cm303743h | Chem. Mater. 2013, 25, 1533−1541
Chemistry of Materials
Article
Figure 1. X-ray diffractograms and structural refinement (observed and calculated patterns) for the (a) BFO and (b) BFTO sintered materials. The insets evidence the presence of (▼) Bi25FeO39 sillenite and (■) Bi2Fe4O9 mullite secondary phases; all other peaks correspond to perovskite BiFeO3.
material.20 The fact is that, in view of the published papers, tailoring the bismuth ferrite properties through titanium doping needs a good understanding of the mechanisms implied in Tisubstitution which is still lacking. The aim of this contribution is to report a thorough study of the relationship between titanium incorporation, microstructure evolution and changes in the electrical and magnetic properties of Ti-doped BiFeO3 polycrystalline materials.
1073 K for 2 h, while BFTO materials were obtained by annealing at 1073 K for 2 h and sintering at 1098 K for 2 h. After sintering, densities of 95% and 97% of the BiFeO3 theoretical density were obtained for the BFO and BFTO ceramics, respectively. Sintered ceramics were structurally characterized through X-ray diffraction (XRD) on a Bruker D8 Advance diffractometer using Cu Kα radiation. Step-scanned diffraction patterns of the powdered ceramics (ground in a tungsten mortar) were collected between 15° and 90°, in steps of 0.02° and with a counting time of 1.5 s per step. The XRD experimental data were refined using the FullProf software and its graphical interface WinPLOTR in the profile matching mode (Le Bail fitting method).27,28 Microstructural characterization was carried out on polished and chemically etched surfaces via fieldemission scanning electron microscopy (FESEM), using a Cold FESEM Hitachi S-4700 microscope supplied with an energy-dispersive spectroscopy (EDS) microanalysis probe. Grain-size measurements were evaluated from FESEM micrographs by an image processing and
II. EXPERIMENTAL PROCEDURE Ceramic samples of BiFeO3 and BiFe0.95Ti0.05O3 nominal composition (referenced hereafter as BFO and BFTO, respectively) were prepared using a mixed-oxide procedure. In order to obtain the highest density and the lowest amount of secondary phases, annealing and sintering conditions were optimized as described elsewhere.26 In particular, undoped BFO samples were annealed at 1023 K for 2 h and sintered at 1534
dx.doi.org/10.1021/cm303743h | Chem. Mater. 2013, 25, 1533−1541
Chemistry of Materials
Article
Figure 2. FESEM micrographs of polished surfaces of BFO (a and b) and BFTO (c and d) materials. Images on the right correspond to chemically etched surfaces. analysis program (Leica). High-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL 2100F transmission electron microscope (TEM/STEM) operating at 200 kV and equipped with a field-emission electron gun, providing a point resolution of 0.19 nm. Specimens for TEM were prepared by grinding and ion milling of sintered ceramics. In this microscope, EDS microanalyses were conducted in the STEM mode, with a spatial resolution of 1 nm. In order to avoid thermal drifting and recrystallization phenomena under the electron beam, exposition times no longer that 50 s were used for the EDS-STEM measurements. For each region, grain cores, grain boundaries and triple junctions, up to 10 different analyses were averaged, yielding uncertainty values of
