Hexagonal BaTiO3 Research Articles

Study the Phase Transition and Dielectric Property of Sr-Doping Hexagonal Ba(Ti,Ga)O3 Ceramic

In this work, we study the phase transition and structure properties of hexagonal Ba1-xSrxTi0.8Ga0.2O3 (x = 0.01, 0.03, 0.05, 0.07, and 0.09) systems, and also investigate the microwave dielectric property of this system. The all samples are prepared by the mixed oxide method, the structure of samples are transferred from hexagonal phase to mixed phases then tetragonal phase through the Sr-doping process, which is consistent with the thermodynamic prediction on literature. The samples are included the tetragonal phase, whose dielectric property is poorer than the hexagonal phase samples, so it also points out that the hexagonal BaTiO3 material has very good dielectric property and a large potential for application.

Dielectric characteristics of B-site-modified hexagonal-barium titanate

Cr, Sb co-substituted hexagonal BaTiO3 perovskite ceramics with a composition of BaTi1−2xCrxSbxO3 (x = 0–0.2) were synthesized via the conventional solid-state reaction method. Their structures transitioned from tetragonal (x = 0) with space group P4 mm to hexagonal (x = 0.1) with space group P63/mmc and finally to hexagonal containing impurities (X = 0.2). The refinement results of the sample (x = 0.1) revealed that it is possible to stabilize the hexagonal polymorph of BaTiO3 at room temperature by the partial replacement of Ti4+ with Cr3+ and Sb5+. Moreover, the structural changes of BaTiO3 depend on sintering method and temperature of sintering. The ceramic dielectric properties were studied, and the results showed that with increased doping, the ceramic dielectric constant gradually decreased. All ceramics materials with a composition of BaTi1−2xCrxSbxO3 (x ≥ 0.075) are dielectric insulators at room temperature with permittivity values between 30 and 60.

Structure, electrical and magnetic property investigations on Fe-doped hexagonal BaTiO3

A powder mixture of BaCO3 and TiO2 was mechanochemically treated in a planetary ball mill in an air atmosphere for up to 80h, using tungsten carbide balls as the milling medium. Mechanochemical reaction leads to the gradual formation of BaTiO3 phase. Phase evolution during synthesis and changes in powder size and morphology were monitored by XRD, VSM, BDS and FE-SEM analysis and it was shown that the formation of BaTiO3 phase. Extended time of milling directed to formation of higher amount of BaTiO3 perovskite phase. Barium titanate with good nanopowered was formed after 6h sintering without pre-calcination step was performed at 1000°C for 6h and 1200°C for 6h within heating rate 10°C/min.

Fine-grained BaTiO3–MgFe2O4 composites prepared by a Pechini-like process

The synthesis of BaTiO3–MgFe2O4 composite powders by a Pechini-like one-pot process and resulting ceramic bodies is described herein. Phase formation during the decomposition of homogenous (Ba, Ti, Fe, Mg)-gels was monitored up to 1200°C. Composite powders consisting of BaTiO3 and MgFe2O4 were obtained after decomposition at 700°C for 1h resulting in crystallite sizes of about 10nm. The shrinkage and sintering behaviour of compacted powders were examined. Sintering at ⩾1200°C leads to the formation of hexagonal BaTiO3 only for composites with a MgFe2O4 content of ⩾30wt.%. SEM images of ceramic bodies reveal that the MgFe2O4 particles are surrounded by BaTiO3 crystallites. Magnetic measurements of both powders and corresponding ceramic bodies show ferrimagnetic behaviour with low coercivities. At high-field and low temperatures an additional small paramagnetic contribution was observed which increases with decreasing MgFe2O4 content and should be considered by the calculation of the saturation magnetization.Finally, the frequency dependent variation of the relative permittivities of the composites was investigated.

Oxygen-deficient BaTiO3−x perovskite as an efficient bifunctional oxygen electrocatalyst

Perovskite oxide catalysts have emerged as the most promising bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts for electrochemical energy conversion and storage. In this work, a new type of oxygen-deficient BaTiO3−x has been synthesized using a sol–gel method followed by a reductive heat treatment at 1300°C in vacuum. The prepared perovskite nanoparticles have an average particle size on the order of 100nm with uniform size distribution. X-ray diffraction shows that this perovskite catalyst consists of a significant amount of hexagonal BaTiO3−x. State-of-the-art IrO2 nanoparticles were also prepared in this work, which were used for reference and has excellent OER activity. Importantly, the oxygen-deficient perovskite catalysts exhibited high catalytic activity simultaneously for the ORR and the OER in alkaline electrolyte. The more challenged OER activity measured with the perovskite exceeds the IrO2 catalyst at relatively low potentials (<1.6V) evidenced by a much reduced onset potential (1.32V) and increased current density. In order to clearly elucidate the structure of the oxygen-deficient BaTiO3−x catalysts, X-ray and neutron diffraction experiments were further carried out, indicating that the hexagonal phase in the best performing BaTiO3−x catalyst is oxygen-deficient with a stoichiometry of BaTiO2.76. The oxygen vacancies in the perovskite crystal structure may lead to vastly enhanced electrocatalytic activity toward the ORR and OER. This work demonstrates a new type of highly efficient perovskite bifunctional catalyst for electrochemical energy technologies relying on oxygen electrocatalysis.

Correlation among disorder, electronic and magnetic phases of SrRuO3

Electric and magnetic properties of Sr1−xBaxRu1−xTixO3 (0 ⩽ x ⩽ 0.8) have been investigated to find the interrelationship between metallicity and ferromagnetism in SrRuO3 (SRO). The simultaneous doping of Sr and Ru with Ba and Ti results in single phase SRO at x = 0.1 and mixed phase of SRO and hexagonal BaTiO3 (h-BTO) at x ⩾ 0.2. Co-doping at Sr and Ru sites gives rise to oxygen vacancy and mixed valency of Ru (Ru3+ and Ru4+). Room temperature resistivity increases due to modification of p(O)-d(Ru) hybridization and phase segregation. Temperature dependent resistivity reveals metal-insulator transition around 232 K at x = 0.1 and insulator down to 2 K at x ⩾ 0.2. The insulating state (x = 0.1) at low temperature is well described by weak localization and electron–electron interaction. Temperature dependence of resistivity (x ⩾ 0.2) follows Mott's three dimensional variable range hopping model. Localization length and average hopping distance decrease with the increase of x, indicating the presence of more disorder. Ferromagnetic transition temperature decreases to 149 K at x = 0.1 and remains constant up to x = 0.5. The Curie–Wiess (CW) temperature (ΘCW) decreases monotonically and becomes negative at x = 0.5. The effective magnetic moment estimated from CW law is smaller than that of pure SRO due to the formation of Ru3+ ions. The saturation magnetization diminishes, suggesting the demagnetization factor owing to diamagnetic h-BTO. The coercivity increases from 6700 Oe (x = 0) to 12 500 Oe (x = 0.4) and then decreases to 3700 Oe (x = 0.5). Ferromagnetic cluster comprising of doped SRO gives rise to the formation of a Griffith-like phase. The co-occurrence of high jump in resistivity ratio and disappearance of ferromagnetism suggests an interplay between transport process and magnetism at low temperature.

Local geometric and electronic structures and origin of magnetism in Co-doped BaTiO3 multiferroics

We have prepared polycrystalline samples BaTi1−xCoxO3 (x = 0–0.1) by solid-state reaction. X-ray diffraction and Raman-scattering studies reveal the phase separation in crystal structure as changing Co-doping content (x). The samples with x = 0–0.01 are single phase in a tetragonal structure. At higher doping contents (x &amp;gt; 0.01), there is the formation and development of a secondary hexagonal phase. Magnetization measurements at room temperature indicate a coexistence of paramagnetic and weak-ferromagnetic behaviors in BaTi1−xCoxO3 samples with x &amp;gt; 0, while pure BaTiO3 is diamagnetic. Both these properties increase with increasing x. Analyses of X-ray absorption spectra recorded from BaTi1−xCoxO3 for the Co and Ti K-edges indicate the presence of Co2+ and Co3+ ions. They locate in the Ti4+ site of the tetragonal and hexagonal BaTiO3 structures. Particularly, there is a shift of oxidation state from Co2+ to Co3+ when Co-doping content increases. We believe that the paramagnetic nature in BaTi1−xCoxO3 samples is due to isolated Co2+ and Co3+ centers. The addition of Co3+ ions enhances the paramagnetic behavior. Meanwhile, the origin of ferromagnetism is due to lattice defects, which is less influenced by the changes caused by the variation in concentration of Co2+ and Co3+ ions.

Multiferroic properties of hexagonal Ba3Ti2MnO9

Noncentrosymmetric hexagonal Ba3Ti2MnO9 presents multiferroic behavior.

Effect of fabrication routes on the microstructure, the dielectric and ferroelectric properties of the Mn-doped BaTiO3 ceramics

The effect of fabrication routes on the microstructure, the dielectric and ferroelectric properties of the Mn-doped BaTiO3 ceramics was systematically studied in the present study. It can be concluded that the ways of doping manganese into BaTiO3 matrix had a strong impact on the obtained ceramics. Doping manganese after the calcination of BaTiO3 (BT + Mn) would inhibit the grain growth and cause the hexagonal BaTiO3 when sintered at 1,400 °C in the air. While doping manganese at the initial stage [BaTi(1−x)MnxO3], no hexagonal BaTiO3 is detected in the same sintering temperature. As the macroscopic properties, the dielectric properties showed permittivity diffusion with the frequency in the temperature range of −10 to 150 °C only in the ceramics fabricated by the former route (BT + Mn). Moreover, the dielectric relaxation process disappeared after aging treatment only in the ceramics fabricated by the later route [BaTi(1−x)MnxO3]. The ferroelectric properties showed strong aging effect in the sample fabricated by the latter route [BaTi(1−x)MnxO3] both with fine grain and coarse grain, while the sample fabricated by the former route (BT + Mn) showed slight aging phenomenon ever after aging at room temperature for 10 days. The electrostrain also showed big difference within different samples: The recoverable electrostrain were 0.11 % (BT + Mn) and 0.21 % [BaTi(1−x)MnxO3] at the same sintering temperature of 1,320 °C and aging in the room temperature for 10 days. Such results were analyzed from the viewpoint of the differences in solubility of manganese and the consequent manganese–oxygen vacancy defect dipoles in the different fabrication routes.

Fabrication of Textured Ceramics Using Mn and Nb-doped Hexagonal BaTiO&lt;sub&gt;3&lt;/sub&gt; by an Electrophoretic Deposition in a High Magnetic Field

Fabrication of Textured Ceramics Using Mn and Nb-doped Hexagonal BaTiO&lt;sub&gt;3&lt;/sub&gt; by an Electrophoretic Deposition in a High Magnetic Field

Effects of Hexagonal BaTiO3 Addition on Textured BaTiO3 Thick Films Prepared by Screen Printing

The effects of hexagonal BaTiO3 addition to BaTiO3 thick films were examined. A- and c-axis-preferred BaTiO3 thick films were prepared by hexagonal BaTiO3 addition. The addition of the hexagonal Ba(Ti0.96Mn0.04)O3 was more effective for preparing the a- and c-axis-preferred thick films than the addition of the hexagonal BaTiO3 prepared by reducing BaTiO3 in a 3% H2–N2 atmosphere. According to increases in the firing time at 1370 °C, the Mn of Ba(Ti0.96Mn0.04)O3 in the Ba(Ti0.96Mn0.04)O3-added BaTiO3 thick films was diffused, and the hexagonal phase in the thick films changed to the tetragonal phase. The degree of the preferred orientation of the Ba(Ti0.96Mn0.04)O3-added BaTiO3 thick films increased with increasing firing time. The hexagonal-phase ratio of the reduced BaTiO3-added BaTiO3 thick films decreased markedly when the firing temperature was above 1000 °C. The existence of the hexagonal phase at 1370 °C in which the grain growth advanced could be important in obtaining highly oriented thick films. The tan δ values of the Ba(Ti0.96Mn0.04)O3-added BaTiO3 thick films were lower than 3%. The remanent polarizations of the thick films were greater than 7 µC/cm2.

Decoration of In2O3 nanowires with BaTiO3 nanoparticles for enhancement of magnetic properties

We have fabricated the composite nanowires consisting of In2O3 core and BaTiO3 shell, via a two-step method. The BaTiO3 shell has been deposited on core In2O3 core nanowires by means of pulsed laser deposition technique. X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy were employed to analyze the structure and morphology of the products. While SEM confirmed that the products maintained the one dimensional morphology, the shell layer corresponded to the nanoparticles of the hexagonal BaTiO3 phase. With bare In2O3 nanowires exhibiting weak ferromagnetism, the ferromagnetic behavior was enhanced by decorating the In2O3 nanowires with BaTiO3 nanoparticles. The shell coating enhanced the ferromagnetic behavior at both 5 and 300 K, in terms of the increase of coercive field, remanent magnetization, and saturation magnetization values in the hysteresis curves. We suggested that the generation of BaTiO3 nanoparticles and oxygen vacancies during the high-temperature shell coating process played a role in enhancing the ferromagnetic properties of the nanowires.

Ferromagnetic antiphase domain boundary in Mn-doped hexagonal BaTiO3 multiferroics

The magnetic properties of hexagonal Ba(Ti1−xMnx)O3 (x = 1/9, 1/6) samples specifically treated under different thermal conditions are investigated. A magnetic transformation from paramagnetism of as-grown samples to weak ferromagnetism of post-annealed samples is observed at room temperature. Associated with the structural modulation characteristics and dielectric behaviors, our experimental results reveal that the high-temperature ferromagnetism in Mn-doped hexagonal BaTiO3 derives from RIII types of antiphase domain boundaries. Spin glass transitions are observed at Tsg = 48 K and 44 K in the densely doped as-grown and post-annealed samples, respectively, but the transitions are not intrinsic properties of the hexagonal phases.

Synthesis and Characterization the Dielectric Properties of Cobalt Doping Hexagonal BaTiO3

BaTi1−xCoxO3 ceramics (x = 0.05∼0.25) have been synthesized with the solid state method and the Co-doping samples were formed hexagonal structure with x < 0.25. The XPS spectra show the two Co2+2p3/2 and Co2+2p1/2 peaks in full doping range, which indicates hexagonal BaTi1−xCoxO3 are stable by Co2+, not Co3+-ion. Although the radius of Co2+-ion is larger than the radius of Ti4+-ion, the volume of unit cell is still decreasing with Co concentration increasing due to oxygen vacancy increasing, which can induce the hydrostatic pressure in this system. The Qxf value and K value are both decreasing with Co-doping level increasing due to hydrostatic pressure increasing.

Study of the structure and mechanical properties of hexagonal BaTiO3 thin films prepared by sol–gel processing

Hexagonal barium titanate (HBT) thin films were prepared on borosilicate plate substrates via sol–gel method using the dip-coating process. The structure, texture and morphology of the thin film were analyzed by X-ray diffraction, atomic force microscopy, nanoindentation technique, and transmission electron microscopy. The results showed that the thin film annealed at 700 °C crystallized with BaTiO3 hexagonal phase and traces of Ba2TiO4 (secondary phase). The nanoparticles and the RMS roughness of the sample treated at 700 °C presented high values when compared with those thermally treated at lower temperatures. The hardness and Youngs’ modulus of the thin films increased with increasing in grain size, and the thin film annealed at 700 °C with crystallite size about 10 nm presented multiple “pop-in” events during nano-indentation loading curves. The annealing temperature, growth size and surface roughness were discussed in connection with the HBT mechanical properties.

Structural phase separation and optical and magnetic properties of BaTi1−xMnxO3 multiferroics

Our work studies the influences of the Mn doping on structural characterization and optical and magnetic properties of BaTi1−xMnxO3 (x = 0.0–0.12) prepared by conventional solid-state reaction. Detailed analyses of XRD patterns and Raman scattering spectra indicate an incorporation of Mn dopants into the Ti sites of BaTiO3 host lattices, and the tetragonal-hexagonal transformation taking place at a threshold concentration of xc ≈ 0.01. An increase of Mn-doping content in BaTi1-xMnxO3 introduces more electronic levels associated with lattice defects and Mn ions to the forbidden gap and thus reduces luminescent intensity in the visible region. Magnetic data recorded at room temperature reveal that all the samples exhibit ferromagnetic order, and there is a phase separation in magnetism as varying x values. Particularly, the samples with x = 0.5–0.7 have a coexistence of two ferromagnetic phases with different coercivities, which are associated with tetragonal and hexagonal Mn-doped BaTiO3 structures. The nature of ferromagnetism is explained by means of the results of structural and optical analyses, where Mn3+ and Mn4+ ions play a crucial role.

Hexagonal BaTiO3/Ni0.8Zn0.2Fe2O4 composites with giant dielectric constant and high permeability

Hexagonal BaTiO3/Ni0.8Zn0.2Fe2O4 (BT/NZO) composites were synthesized by solid–state reaction method. Phase composition, microstructure, dielectric and magnetic properties of the composites were investigated. The BT/NZO composites had giant dielectric constants and very high permeabilities simultaneously. According to Cole–Cole plots, the composites showed a polydispersive nature of the Maxwell-Wagner relaxation. The forming mechanism of hexagonal BaTiO3 in the composites was also investigated.

Structure, electrical and magnetic property investigations on dense Fe-doped hexagonal BaTiO3

Hexagonal Ba(Ti1−xFex)O3−δ (x = 1/6, 1/3) ceramics treated with post annealing are specifically synthesized to explore the origin mechanism of the unusual ferromagnetism in the doped system. X-ray diffraction refinements and transmission electron microscope experiments reveal that their structures are incommensurately modulated owing to simultaneous oxygen vacancies at both O1 and O2 sites. Consequently, coexisting weak ferrielectricity and weak ferromagnetism are presented at room temperature. Analysis on their leakage current plot reveals that their conduction follows trap-filled limit model. In combination with the magnetism studies on 5 mol% Fe-doped crystals [Phys. Rev. B 83, 144407 (2011)], the reduction of ferromagnetism with an increase of conductivity suggests that dynamic exchanges of trapped electrons among the bound magnetic polarons attribute to the intrinsic ferromagnetism.

Structural Transformation of Hexagonal (0001)BaTiO3 Ceramics to Tetragonal (111)BaTiO3 Ceramics

A ceramic slurry that contains a 6H-type Ba(Ti0.95Mn0.05)O3 powder was casted into a plaster mold under 10 T magnetic field to form a green compact of (0001)-oriented Ba(Ti0.95Mn0.05)O3. After sintering the green compact at 1300 °C in air, it was confirmed that the (0001)-oriented 6H-type perovskite structure transformed to a (111)-oriented 3C-type perovskite structure. The structural transformation was again examined using hexagonal BaTiO3 prepared by reducing pseudo-cubic BaTiO3 powder in H2 atmosphere. In this case, the preferred (0001) orientation was not confirmed for the green compacts. After sintering the green compacts at 1300 °C in air, mixed crystal orientations of (100)/(001) and (111) were observed for the resultant tetragonal BaTiO3 ceramics. This (100)/(001) orientation was suppressed by annealing the hexagonal BaTiO3 powder at 1000 °C before slip-casting, leading to highly (111)-oriented ceramics. It was found that the green compacts of (0001)-oriented hexagonal BaTiO3 can transform into (111)-oriented tetragonal BaTiO3 ceramics, maintaining the macroscopic crystal orientations due to a similar atomic stacking along [0001] of 6H-type BaTiO3 and [111] of 3C-type BaTiO3.

Defect-induced magnetism: Test of dilute magnetism in Fe-doped hexagonal BaTiO3single crystals

Single crystalline Fe-doped hexagonal BaTiO${}_{3}$ samples with varying oxygen content are created by specifically intended post-growth annealing treatments, in order to check the influence of defects on the unusual high temperature ferromagnetism observed in this system. The various defects have been shown to play a crucial role in dilute magnetic systems and therefore, it is important to carry out this check for the Fe-doped BaTiO${}_{3}$ system also, in which unusual ferromagnetism was reported even in its bulk single crystalline form. The x-ray diffraction and dielectric studies carried out here have confirmed that the Fe doping of Ti is intrinsic, while the high resolution transmission electron microscopy (HRTEM) and x-ray photoemission spectroscopy (XPS) studies proved the absence of unwanted magnetic metal clusters in the sample. The transport studies show that the oxygen concentrations could be varied substantially by the thermal treatments. Finally, magnetization measurements on the samples demonstrated that ferromagnetism is stronger in samples with higher oxygen deficiency, which could interestingly be retreated under high oxygen atmosphere and reversibly be taken back to a lower magnetic state. The vacancy-induced ferromagnetism is further confirmed by EPR measurements, which is consistent with earlier studies and, consequently, put the doped BaTiO${}_{3}$ in the list of true dilute magnetic oxide (DMO) systems.