Meghdad Palizdar

Crystallographic and magnetic identification of secondary phase in orientated Bi5Fe0.5Co0.5Ti3O15 ceramics

Oxide materials which exhibit both ferroelectricity and ferromagnetism are of great interest for sensors and memory applications. Layered bismuth titanates with an Aurivillius structure, (BiFeO<inf>3</inf>)nBi<inf>4</inf>Ti<inf>3</inf>O<inf>12</inf>, can possess ferroelectric and ferromagnetic order parameters simultaneously. It has recently been demonstrated that one such example, Bi<inf>5</inf>Fe<inf>0.5</inf>Co<inf>0.5</inf>Ti<inf>3</inf>O<inf>15</inf>, where n = 1 with half the Fe³⁺ sites substituted by Co³⁺ ions, exhibits both ferroelectric and ferromagnetic properties at room temperature. Here we report the fabrication of highly-oriented polycrystalline ceramics of this material, prepared via molten salt synthesis and uniaxial pressing of high aspect ratio platelets. Electron backscatter images showed that there is a secondary phase within the ceramic matrix which is rich in cobalt and iron, hence this secondary phase could contribute in the main phase ferromagnetic property. The concentration of the secondary phase obtained from secondary electron microscopy is estimated at less than 2.5 %, below the detection limit of XRD. TEM was used to identify the crystallographic structure of the secondary phase, which was shown to be cobalt ferrite, CoFe<inf>2</inf>O<inf>4</inf>. It is inferred from the data that the resultant ferromagnetic response identified using VSM measurements was due to the presence of the minor secondary phase. The Remanent magnetization at room temperature was M<inf>r</inf> ≈ 76 memu/g which dropped down to almost zero (M<inf>r</inf> ≈ 0.8 memu/g) at 460 °C, far lower than the anticipated for CoFe<inf>2</inf>O<inf>4</inf>.

Reactive template grain growth of BiFeO 3 -PbTiO 3 by using Bi 4 Ti 3 O 12 , PbBi 4 Ti 4 O 15 and SrTiO 3 as templates

Bismuth ferrite exhibits multiferroic properties at room temperature. It is a perovskite with a rhombohedrally distorted lattice and has a ferroelectric Curie point of 836 °C. Lead titanate is a tetragonal perovskite with a ferroelectric Curie point of 490°C. The solid solution between bismuth ferrite and lead titanate xBiFeO3-(1−x)PbTiO3, or BFPT, possesses a morphotropic phase boundary (MPB) between the rhombohedral and tetragonal forms at x=0.7. It is of interest to investigate the influence of field-driven rhombohedral-tetragonal phase transitions across the MPB, to determine whether correctly oriented BFPT can provide both giant piezoelectric properties and significant magnetoelectric coupling. Here, we used the reactive template grain growth (RTGG) technique to prepare crystallographically textured BFPT ceramics. Template grain growth (TGG) utilizes an oriented template as a seed to improve and increase preferred directional growth within a polycrystalline matrix. In this example, Bi4Ti3O12, PbBi4Ti4O15 and SrTiO3 have been used separately as templates during the process of BFPT preparation. The samples were characterized using X-ray diffraction, using both conventional and synchrotron sources and scanning electron microscopy. The results show that PbBi4Ti4O15 is the most effective template providing a level of texture ITEX = 1.80, with the tetragonal (001) reflection orientated along the cast direction. The limited penetration depth of laboratory XRD renders it insufficient to characterise the texture of these materials.

Effect of different templates on reactive templated grain growth of BiFeO 3 -PbTiO 3

The rhombohedral perovskite BiFeO<inf>3</inf>, which displays both antiferromagnetic and ferroelectric properties at room temperature, has a ferroelectric Curie point of 836 °C. The solid solution between bismuth ferrite and lead titanate, which is perovskite with either tetragonally or rhombohedrally distorted lattice and a ferroelectric Curie point around 490°C, (xBiFeO<inf>3</inf>-(1−x)PbTiO<inf>3</inf> or BFPT) possesses a morphotropic phase boundary (MPB) between the rhombohedral and tetragonal forms at x=0.7, with a spontaneous strain of 18% on the tetragonal side of the boundary. Coupling between the ferroelectric and magnetic orders near the MPB would be facilitated by the availability of crystallographically textured ceramics. The approach taken here is to investigate templated grain growth (TGG) as a route to produce oriented BFPT. TGG utilizes an oriented template as a seed to regulate preferred directional growth within a polycrystalline matrix. In this example we have employed different types of templates such as Bi<inf>4</inf>Ti<inf>3</inf>O<inf>12</inf>, BaTiO<inf>3</inf> and SrTiO<inf>3</inf>. X-ray diffraction and scanning electron microscopy have been used to examine the influence of the different templates. We have shown that by using Bi<inf>4</inf>Ti<inf>3</inf>O<inf>12</inf> the texture in BFPT ceramic has been improved.

Electric-field-induced phase switching in textured Ba-doped bismuth ferrite lead titanate

The template grain growth technique was used to synthesis textured 60BiFeO3-PbTiO3 (60:40BFPT) by using platelets of BaTiO3 as template. Synchrotron measurement clearly showed textured 60:40BFPT. Moreover, in situ high energy synchrotron radiation was employed to investigate the influence of an external electric filed on crystallographic structure of mixed phase 60:40BFPT. Application of an electric field ≥ 1 kV/mm resulted in phase transformation from mixed rhombohedral/tetragonal phases (≈ 73.5% tetragonal / 26.5% rhombohedral) to predominately tetragonal phase (≈ 95%) at applied field of 6 kV/mm.

Synchrotron texture analysis of thick BiFeO 3 -PbTiO 3 layers synthesised by tape casting using Aurivillius and non-Aurivillius templates

The solid solution between bismuth ferrite and lead titanate (xBiFeO₃-(1-x)PbTiO₃ or BFPT) possesses a morphotropic phase boundary (MPB) between the rhombohedral and tetragonal forms at x=0.7. It is of interest to investigate the influence of field-driven rhombohedral-tetragonal phase transitions across the MPB, to determine whether correctly oriented BFPT can provide both giant piezoelectric properties and significant magnetoelectric coupling. Here, we used the reactive templated grain growth (RTGG) technique to prepare crystallographically textured 0.6BiFeO₃-0.4PbTiO₃ (60:40 BFPT) ceramics. Both Aurivillius structure templates (Bi₄Ti₃O₁₂ and PbBi₄Ti₄O₁₅) and perovskite templates (BaTiO₃ and SrTiO₃) were used to prepare textured 60:40BFPT. Synchrotron radiation experiments were used to determine the degree of texture. A current data suggests that only BaTiO₃ templates survive the sintering process, and other candidate template materials reacted with the 60:40BFPT matrix at high temperature. In the case of SrTiO₃, this reaction results in a low Curie temperature (T_c = 350°C) due to the substitution of Sr²⁺. Aurivillius templates resulted in high Curie temperatures (610°C) and may be chemically suitable if they could be stabilized during the sintering process. However, the resulting ceramics show low remanent polarization (P_r = 3 μc/cm²), while SrTiO₃ and BaTiO₃ templated ceramics show higher remanent polarizations of 36 and 30 μc/cm², respectively. Because of their high chemical stability in this system, BaTiO₃ templates appear to be the best candidate for fabricating textured BFPT by the TGG method, displaying a high degree of crystallographic texture.

Synthesis of platelets Bi 5 Fe 0.5 Co 0.5 Ti 3 O 15 via the molten salt method

Oxide materials which exhibit both ferroelectricity and ferromagnetism are of great interest. Layered bismuth titanates with a Aurivillius structure, (BiFeO3)n Bi4Ti3O12, can potentially posses ferroelectric and ferromagnetic order paramaters simultaneously. It has recently been demonstrated that one such example, Bi5Fe0.5Co0.5Ti3O15 where n = 1 with half the Fe3+ sites substituted by Co3+ ions exhibits both ferroelectric and ferromagnetic properties at room temperature. Here we report on the fabrication of well oriented polycrystalline ceramics of this material, via molten salt synthesis and uniaxial pressing of high aspect ratio platelets. Electron backscatter images showed that there is an extra secondary phase within the obtained ceramic which is rich in cobalt and iron. The concentration of the secondary phase obtained from secondary electron microscopy is estimated at less than 2.5 %, below the detection limit of XRD. Further, the sintering temperature was varied and excess addition of bismuth oxide was employed in an attempt to reduce the secondary phase with limited success. The samples have been characterized by X-ray diffraction, polarization-field measurements and SQUID magnetometery as a function of sample orientation. It is inferred from the data that the resultant ferromagnetic response identified using SQUID measurements is due to the presence of the secondary phase.

Texture analysis of thick bismuth ferrite lead titanate layers

The template grain growth technique was used to synthesis textured 60BiFeO3-PbTiO3(60:40BFPT) by using platelets of BaTiO3 as template. Synchrotron measurement clearly showed textured 60:40BFPT. Moreover, in situ high energy synchrotron radiation was employed to investigate the influence of an external electric filed on crystallographic structure of mixed phase 60:40BFPT. Application of an electric field ≥ 1 kV/mm resulted in phase transformation from mixed rhombohedral/tetragonal phases (≈ 73.5% tetragonal / 26.5% rhombohedral) to predominately tetragonal phase (≈ 95%) at applied field of 6 kV/mm. A crystallographic texture refinement was done by using software package materials analysis using diffraction (MAUD) with a 4th order spherical harmonic orientation distribution function (ODF). This refinement was completed using a P4mm+Cm structure model. Texture coefficients were constrained such that the equivalent texture coefficients of each phase are the same. The resulting texture refinement determined that sample has a 1.3 multiples of random distribution (MRD) {100} crystallographic texture.

Synthesis of oriented BiFeO 3 -PbTiO 3 by molten salt method

The solid solution between bismuth ferrite and lead titanate, is a perovskite structure either tetragonally or rhombohedrally distorted lattice. It exhibits antiferromagnetic and ferroelectric properties at room temperature. xBiFeO3-(1−x)PbTiO3 or BFPT possesses a morphotropic phase boundary (MPB) between the rhombohedral and tetragonal forms around x=0.7, accompany with a spontaneous strain of 18% on the tetragonal side of the boundary. BFPT crystallographically textured ceramic could have coupling between its ferroelectric and magnetic orders near the MPB. The approach taken here was to make textured BFPT by employing the molten salt method. PbBi4Ti4O15 have been used as precursor. Samples have been characterized by X-ray diffraction, scanning electron microscopy and polarization/strain-field measurements. The materials obtained are ferromagnetic, with a Mr = 1.01 emu/g. However some iron rich secondary phases were obtained which potentially contribute in the obtained ferromagnetic results. Due to the high electric coercive field, it was not possible to obtain saturated PE loops, or strain field butterfly loops. However, an electrostrictive strain of 0.017% was achieved.

Crystallographic and magnetic identification of secondary phase in orientated Bi 5 Fe 0.5 Co 0.5 Ti 3 O 15 ceramics

Oxide materials which exhibit both ferroelectricity and ferromagnetism are of great interest for sensors and memory applications. Layered bismuth titanates with an Aurivillius structure, (BiFeO 3 )nBi 4 Ti 3 O 12 , can possess ferroelectric and ferromagnetic order parameters simultaneously. It has recently been demonstrated that one such example, Bi 5 Fe 0.5 Co 0.5 Ti 3 O 15 , where n = 1 with half the Fe3+ sites substituted by Co3+ ions, exhibits both ferroelectric and ferromagnetic properties at room temperature. Here we report the fabrication of highly-oriented polycrystalline ceramics of this material, prepared via molten salt synthesis and uniaxial pressing of high aspect ratio platelets. Electron backscatter images showed that there is a secondary phase within the ceramic matrix which is rich in cobalt and iron, hence this secondary phase could contribute in the main phase ferromagnetic property. The concentration of the secondary phase obtained from secondary electron microscopy is estimated at less than 2.5 %, below the detection limit of XRD. TEM was used to identify the crystallographic structure of the secondary phase, which was shown to be cobalt ferrite, CoFe 2 O 4 . It is inferred from the data that the resultant ferromagnetic response identified using VSM measurements was due to the presence of the minor secondary phase. The Remanent magnetization at room temperature was M r ≈ 76 memu/g which dropped down to almost zero (M r ≈ 0.8 memu/g) at 460 °C, far lower than the anticipated for CoFe 2 O 4 .

Flux growth and characterisation of rhombohedral BiFeO 3 -PbTiO 3 single crystals

Single crystals of bismuth ferrite-lead titanate (BFPT) were grown using a flux method. Starting from bismuth ferrite, we added lead titanate, to produce a range of compositions within the BiFeO3-PbTiO3 solid solution, through to the morphotropic phase boundary (MPB). Using a PbO:Bi2O3 flux, and a cooling rate of 0.5°C/h over the temperature range where crystal growth is expected, our aim was to produce highly pure, good size, crack free rhombohedral single crystals. Energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM) was used to determine the chemical composition and structure of the crystals, and powder X-ray diffraction (XRD) confirmed the crystal structure.