Hexagonal BaTiO3 Research Articles

Thermodynamic and dielectric properties of hexagonal barium titanate near the phase transitions

The critical behavior of the thermodynamic quantities is studied for the two phase transitions near the transition temperatures (74 K and 222 K) in the hexagonal (h) BaTiO3. A linear relationship has been established between the variations of the frequency shifts and of the dielectric constant by using the literature data in h-BaTiO3. Temperature and pressure dependence of the mode Grüneisen parameters ( γp and γT ) are also described by a power-law formula as the other thermodynamic parameters near Tpt in h-BaTiO3. Our results show that they explain the observed behavior adequately, in particular soft-mode frequency, dielectric constant, thermal expansion, the excess specific heat and the entropy near Tpt in h-BaTiO3. Temperature dependence of the isobaric mode Grüneisen parameter ( γp ), enthalpy and the Gibbs free energy, which we have evaluated, can be compared with experiments in h-BaTiO3.

The heterostructure topology of a chemiresistive sensor based on hexagonal BaTiO3 and 2D SnO for toluene detection

The SnO–BaTiO3 bi-layered heterostructured chemiresistive sensor exhibits promising potential for detecting volatile organic compounds (VOCs) under ambient conditions.

Multiferroic and half-metallic character of hexagonal BaTi0.5Fe0.5O3: DFT based calculation

ABSTRACT In this paper, we report a density functional theory (DFT) calculation study based on the full potential linear augmented plane wave (FP-LAPW) to investigate the effect of Fe incorporation in hexagonal BaTiO3 with a high Fe content : BaTi0.5Fe0.5O3.We discuss the structural, mechanical, electronic and magnetic properties as well as the spontaneous electrical polarisation P. The partial Fe substitution in BaTiO3 conserves and doubles its initial polarisation value from 0.12 to 0.25 C/m2. In contrast to nonmagnetic BaTiO3, we observe a net magnetic moment of 6.0 μB/formula unit. But, when the strong correlations among d electrons of Fe atoms are taken into account, the total magnetic is increasing and is equal to 10 μB/formula unit, due to a net increasing of magnetic moment of Fe atoms. Therefore, the coexistence of spontaneous electrical polarisation P along with a magnetic moment leads to a new and interesting multiferroic behaviour. We also observe a half-metallic behaviour in which spin down bands structure shows metallic character whereas spin up bands structure shows semi-conductor character with an energy gap of 1.48 eV. This reveals that the multiferroic BaTi0.5Fe0.5O3 could be potential candidate for spintronics devices.

Kinetic Control of Anion Stoichiometry in Hexagonal BaTiO3

The cubic oxyhydride perovskite BaTiO3−xHx, where the well-known ferroelectric oxide BaTiO3 is partially hydridized, exhibits a variety of functions such as being a catalyst and precursor for the synthesis of mixed-anion compounds by utilizing the labile nature of hydride anions. In this study, we present a hexagonal version, BaTi(O3−xHx) (x < 0.6) with the 6H-type structure, synthesized by a topochemical reaction using hydride reduction, unlike reported hexagonal oxyhydrides obtained under high pressure. The conversion of cubic BaTiO3 (150 nm) to the hexagonal phase by heat treatment at low temperature (950~1025 °C) using a Mg getter allows the introduction of large oxygen defects (BaTiO3−x; x − 0.28) while preventing the crystal growth of hexagonal BaTiO3, which has been accessible at high temperatures of ~1500 °C, contributing to the increase of the hydrogen content. Hydride anions in 6H-BaTiO3−xHx preferentially occupy face-sharing sites, as do other oxyhydrides.

Electric field‐assisted solid‐state reaction of BaCO3‐TiO2 system

AbstractWe report phase transition process during the solid‐state reaction of BaCO3‐TiO2 system under the assistance of electric field. Experiments were conducted at a constant heating rate with preset field strength and current limit. Solid‐state reaction was completed upon reactive flash sintering taking place at ~1002℃ under 200 V/cm. Hexagonal BaTiO3 phase, which rarely occurs at such temperatures, was obtained after flash sintering at a current density of 23.5 mA/mm2. It is speculated that oxygen deficiency during flash sintering triggered cubic‐to‐hexagonal transition of BaTiO3. Furthermore, X‐ray diffraction results show that solid‐state reaction takes place prior to flash sintering. Electric field could accelerate the reaction but did not alter the sequence of phase evolution.

Study of the Crystallization and structural behavior of Bismuth Barium Titanate Glass-Ceramics

Bismuth Barium titanate glass-ceramics with composition (100-x)Bi2O3-xBaTiO3 synthesized via controlled crystallization have been explored for their structural and optical properties. X-Ray Diffraction data clearly confirmed the presence of crystalline phases in the prepared glass-ceramics with orthorhombic Ba2Ti13O22 phase being the leading and stable one. Additionally, the commencement of hexagonal BaTiO3 phase has been found in higher compositions above x ≥ 20 with increase in BaTiO3 content. FTIR and Raman spectroscopy revealed the conversion of Non-bridging oxygen atoms (NBO's) to Bridging oxygen atoms (BO's) takes place when BaTiO3 content increases in the sample and thus resulting in compactness of the network. UV-Visible spectroscopy has been performed to calculate their optical band gap energies (Eg). An increase in optical band gap energy has been found and same has also been confirmed using Urbach energy approach. The lower values of Urbach energy propose the creation of (BO's), decrease in disorder and hence support structural stability of the system. The electronegativity and electronic polarizability values reveal the covalent character of glass-ceramic samples.

Spectroscopic investigations and magnetic measurements on iron-containing barium titanate glass-ceramics

Glass-ceramics with cubic BaTiO3 as a major and hexagonal BaTi1-xFexO3-δ as a second phase were prepared by crystallization at 560 °C in a glass with the composition 20.1Na2O/23TiO2/23.1BaO/3Al2O3/7.6B2O3/17.4SiO2/5.8Fe2O3. X-ray photoelectron spectroscopy revealed presence of Ti4+ and Fe3+ ions in tetrahedral and octahedral coordination. Ba2+ occurs in different environments depending on the thermal history of the samples. The numbers of bridging and non-bridging oxygens vary with the crystallization time. Electron paramagnetic resonance spectroscopy showed four types of Fe3+ ions that are non-uniformly distributed in the samples: Fe3+ ions in magnetic domains, isolated Fe3+ in a rhombic crystal field, magnetically coupled Fe3+ ions in BaTiO3 and Fe3+ ions located in a crystalline phase. The ratio between the described types of Fe3+ ions depends on the annealing time. Mössbauer spectroscopy detected Fe3+ tetrahedral and octahedral coordinated in the glass. Fe3+ was incorporated in distorted octahedral and pentahedral coordination in the Ti4+ sites of hexagonal BaTiO3 crystals, the electroneutrality being preserved by the occurrence of oxygen vacancies. Vibrating sample magnetometer measurements were used to determine ferromagnetic properties in samples crystallized for times ≥ 3 h. Maximum magnetization of 0.8 emu/g and a coercive force of 25 Oe were determined for the sample crystallized for 3 h at 560 °C.

Local Coordination Environments and Vibrational Dynamics of Protons in Hexagonal and Cubic Sc-Doped BaTiO3 Proton-Conducting Oxides

The proton local coordination environments and vibrational dynamics associated with the two order of magnitude change in proton conductivity in hydrated forms of hexagonal and cubic structured BaTi...

Microstructure driven magnetodielectric behavior in ex-situ combustion derived BaTiO3-ferrite multiferroic composites

The present work investigates the magnetodielectric behavior of ex-situ combustion derived 7BaTiO3@3ferrite and 3ferrite@7BaTiO3 multiferroic composites. Three different ferrites, such as CoFe2O4, ZnFe2O4, and Co0.5Zn0.5Fe2O4, have been selected for the preparation of multiferroic composites. Distinct microstructural features were observed in 7BaTiO3@3ferrite and 3ferrite@7BaTiO3 composites. X-ray diffraction study confirmed the presence of hexagonal and tetragonal BaTiO3 in 3ferrite@7BaTiO3 composites. Plate-like and spherical BaTiO3 grains were observed in these composites. However, only spherical or polyhedral BaTiO3 grains were found in 7BaTiO3@3ferrite composites, as confirmed from EDS mapping. Also, plate-like BaFe12O19 and polyhedral ferrite grains were detected in all composites. The presence of BaFe12O19 and its effect on magnetic and dielectric behavior of composites was also analyzed in this work. Maxwell-Wagner polarization phenomenon due to charge depletion at the BaTiO3-ferrite interface was found to be prominent in 7BaTiO3@3ferrite composites because of the maximum connectivity of ferrite phases. The main factors for the magnetocapacitance response in the present systems are due to the combined effects of interface-dominated magnetoresistance, magnetostriction of ferrites and microstructure of composites. The variation in magnetocapacitance response of these multiferroic composites may be suitable for potential applications in the field of sensors and devices.

Colossal dielectric constant and ferroelectric investigation of BaTiO3 nano-ceramics

Barium titanate (BaTiO3) nano-ceramics are prepared using nano-ball milling method. Nano-milled samples are calcined at 1200 °C for 6 h to 72 h. Transition from agglomerated growth to fine particles (diameter ~ 350 nm) is observed after nano-ball milling process. Calcination of milled powders at 1200 °C for 6 h and 12 h results in mixed hexagonal and Perovskite BaTiO3 phases. Perovskite phase pure structure is obtained after calcination of 24 h. Further, this phase pure structure is observed to be stable for longer periods of calcination time, i.e., up to 72 h with grain size of ~ 150 nm. Colossal dielectric constant of 1.38 × 106 at log f = 1.3 is observed after calcination for 24 h. Slight increase in dielectric constant, i.e., 2.13 × 106, is observed after calcination for 72 h. Temperature-dependent analyses of sample, calcined at 72 h, exhibit a transition in conduction mechanism at 120 °C. Activation energy of ~ 0.81–1.15 eV is observed for BaTiO3 nano-ceramics. Perovskite BaTiO3 ceramics exhibit saturated ferroelectric hysteresis curves with polarization of 10.28–57.6 μC/cm2. The presence of colossal dielectric constant along with ferroelectricity in BaTiO3 nano-ceramics has potential applications in modern microelectronic devices and for development of novel capacitive data storage devices.

Misidentification of hexagonal phase as barium carbonate during chemical synthesis of barium titanate nanopowders

Abstract Barium titanate (BaTiO 3 ) nanopowders were synthesized by combining two facile route namely so-gel and hydrothermal process. The samples synthesized at 100 and 200 °C are found to exhibit cubic crystalline structure and mixed tetragonal and orthorhombic phases, respectively, as analyzed from XRD studies. In addition, characteristic peaks of unknown phase often identified as barium carbonate (BaCO 3 ) was also observed. Post-annealing treatment suggests stabilization of hexagonal BaTiO 3 , confirmed by XRD and Raman studies.

Polymorphic phase transition in BaTiO3 by Ni doping

Abstract We demonstrate a phase transition between BaTiO3 polymorphs induced by Ni substitution experimentally. The structural evolution of tetragonal-to-hexagonal polymorph in Ni-doped BaTiO3 ceramics is observed macroscopically and then, visualized in an atomic scale. Under this structural transition, the long-range ferroelectric ordering in the undoped BaTiO3 with tetragonal symmetry is suppressed and eventually, vanishes in the heavily Ni-doped BaTiO3 with hexagonal symmetry and dielectricity. An oxygen vacancy defect can be created at a local Ni doping site with a decrease in the charge valence state from Ti4+ (3d0) to Ti3+ (3d1). With the reduction of the Ti4+ oxidation state, the polar Ti–O ionic displacement is degenerated in the tetragonal BaTiO3 and thereby, the short-range Coulomb repulsion may be restored in the hexagonal BaTiO3 resulting in a polymorphic phase transition. Our work is of potential interest for artificial realization of an exotic phase in complex oxide materials by doping.

Influence of Hexagonal Boron Nitride and BaTiO3 as Hybrid Fillers on the Structural, Morphological and Dielectric Behavior of Polyvinyl Alcohol Nanocomposites

Polyvinyl alcohol (PVA) nanocomposites were prepared using hexagonal boron nitride (h-BN) and Barium titanate (BTO) as hybrid fillers. PVA/h-BN/BaTiO3 nanocomposite films were prepared by simple solution casting method with different loadings of h-BN and BaTiO3. The nanocomposites were characterized using Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), Atomic Force Microscopy (AFM), thermo gravimetric analysis (TGA) and mechanical properties. The dielectric behaviour of nanocomposites were studied in the frequency range 50 Hz - 5MHz and temperature in the range from 35- 155˚C. The FTIR results indicate strong chemical interaction between h-BN and BaTiO3 with polymer systems. Sharp peaks in XRD studies reveals the crystalline nature of the fillers in the nanocomposites. The dielectric properties such as dielectric constant, dielectric loss and ac-conductivity of nanocomposites were obtained. The mechanical studies are also performed, and found some enhancement in the thermal stability for this polymer nanocomposites.

Controlling the phase transition in nanocrystalline ferroelectric thin films via cation ratio.

Traditionally, the ferroelectric Curie temperature can be manipulated by chemical substitution, e.g., in Ba1-xSrxTiO3 as one of the archetypical representatives. Here, we show a novel approach to tune the ferroelectric phase transition applicable for nanostructured thin films. We demonstrate this effect in nano-grained BaTiO3 films. Based on an enhanced metastable cation solubility with Ba/Ti-ratios of 0.8 to 1.06, a significant shift of the phase transition temperature is discovered. The transition temperature increases linearly from 212 K to 350 K with increasing Ba/Ti ratio. For all Ba/Ti ratios, a completely diffused phase transition is present resulting in a negligible temperature sensitivity of the dielectric constant. Schottky defects are identified as the driving force behind the off-stoichiometry and the shift of the phase transition temperature as they locally induce lattice strain. Complementary temperature dependent Raman experiments reveal the presence of the hexagonal polymorph in addition to the perovskite phase in all cases. Interestingly, the hexagonal BaTiO3 influences the structural transformation on the Ba-rich side, while on the Ti-rich side no changes for the hexagonal polymorph at the ferroelectric transition temperature are observed. This concerted structural change of both polymorphs on the Ba-rich side causes a broad phase transition region spanning over a wide range up to 420 K including the transition temperature of 350 K obtained from dielectric measurements. These findings are promising for fine adjustment of the phase transition temperature and low temperature coefficient of permittivity.

Electronic structure and magnetic properties of BaTi1-xMnxO3

The electronic structure and magnetic properties of polycrystalline BaTi1-xMnxO3 (x = 0–0.1) compounds prepared by solid-state reactions were studied. The results revealed that the increase in Mn content (x) did not change the oxidation numbers of Ba (+2) and Ti (+4) in BaTi1-xMnxO3. However, there is the change in Mn valence that Mn3+,4+ ions coexist in the samples with x = 0.01–0.04 while Mn4+ ions are almost dominant in the samples with x = 0.06–0.1. We also point out that Mn3+ and Mn4+ ions substitute for Ti4+ and prefer locating in the tetragonal and hexagonal BaTiO3 structures, respectively, in which the hexagonal phase constitutes soon as x = 0.01. Particularly, all the samples exhibit room-temperature ferromagnetism. Ferromagnetic order increases with increasing x from 0 to 0.02, but decreases as x ≥ 0.04. We think that ferromagnetism in BaTi1-xMnxO3 is related to lattice defects and/or exchange interactions between Mn3+ and Mn4+ ions.

New insights into the 6H-type hexagonal perovskite solid solution BaTiO3 − δ: Influence of acceptor and donor doping on crystal structure and electrical properties

Polycrystalline 6H-type hexagonal Barium Titanate BaTi0.5(Fe0.33M0.17)O3−δ, where M=Mo or W was prepared by the solid state reaction method. The effects of the cationic substitution of Fe3+, Mo6+ and W6+ for titanium in the B site for BaTiO3 perovskite lattice on the symmetry and the electrical properties were investigated. It was found that the substitution of Ti4+ by (Fe3+Mo6+) gives a hexagonal perovskite structure, whereas the substitution by (Fe3+W6+) cations results in a mixture of double cubic and hexagonal pervoskite phases. The difference in the average radius of cations in the B site (rB=0.50×rTi+0.33×rFe+0.17×rM, (M=Mo or W)) created the difference in cell parameters. The conductivity obtained for BaTi0.5(Fe0.33Mo0.17)O3-δ sample at a temperature of 200°C was 2.3×10−3Scm−1, which is much greater than that established by BaTi0.5(Fe0.33W0.17)O3-δ sample at a temperature of 328°C (0.26×10−3Scm−1) This means that a mixed ionic-electronic conductor (MIEC) character may be present in the first compound. Impedance Spectroscopy data collected for our samples over a range of temperatures and frequencies showed the appearance of a phenomenon of blocking at the grain boundary level, which means that the compounds are highly influenced by the oxygen stoichiometry. The correlation between the electrical properties of grain boundaries and their chemical composition is consistent with the interpretation in terms of the space charge model with a positive excess charge in the grain boundary core.

Ferromagnetic properties of barium titanate ceramics doped with cobalt, iron, and nickel

The influence of annealing in strongly reducing atmosphere on the magnetic properties of hexagonal BaTiO3 + 0.04 BaO + x/2 Co2O3 (0.0025 ≤ x ≤ 0.10) ceramics was investigated. The samples air-sintered at 1673 K were subsequently tempered at 1473 K in H2/Ar stream. While the as-sintered samples exclusively exhibit paramagnetic behavior, the annealed samples show distinct saturation in the field dependence of the magnetization at 300 K measured in the range between −90 and 90 kOe. Besides, the field dependence of the magnetization is hysteretic with coercive fields in the order of 100 Oe. Both properties point to ferromagnetic regions which were identified as precipitations of metallic cobalt by TEM and EDX. The cobalt precipitations were exclusively found in tetragonal grains which were completely Co-free outside the precipitations. Obviously, these tetragonal grains were formed during the annealing process when the Co content of the formerly hexagonal grains was concentrated into metallic Co particles by diffusion processes. Hence, the Co-free matrix of the grains transformed into the cubic phase which is the equilibrium phase of undoped BaTiO3 at the annealing temperature 1473 K and during cooling to room temperature into tetragonal phase. The size of the metallic precipitations ranges from about 20 to 100 nm. A reduction both of the annealing temperature to 1373 K and of the annealing time from 120 to 30 min did not change the minimum particle size, but now, the very rare precipitations only occurred at triple points or grain boundaries. EPR measurements confirmed the occurrence of the ferromagnetic precipitations. While at 300 K, the as-sintered samples did not show any Co EPR signal, the annealing in strongly reducing atmosphere caused a strong and broad line which is attributed to the ferromagnetic resonance signal of the Co precipitations. The investigations were extended for the dopants, iron or nickel, with a nominal sample composition of BaTiO3 + 0.04 BaO + 0.01 Fe2O3 or 0.02 NiO, respectively. The field dependence of the magnetization as well as the EPR spectra showed similar results compared to the case of Co-doped samples. Hence, also in Fe- or Ni-doped BaTiO3 ceramics, ferromagnetic properties are caused by annealing in strongly reducing atmosphere.

Symmetry breaking in hexagonal and cubic polymorphs of BaTiO3

BaTiO3 appears in cubic and hexagonal variants, both of which are centrosymmetric. Samples of cubic BaTiO3 are known to exhibit breaking of the centric symmetry locally and globally. It has been proposed that the local symmetry breaking originates in polar regions, the precursors of the ferroelectric phase. Origins of the macroscopic symmetry breaking, which are not well understood, have been previously tentatively correlated with inhomogeneities in the samples, such as strain gradients that may align or redistribute objects such as charged point defects or polar regions making material macroscopically polar. No such data are available for BaTiO3 with hexagonal symmetry. We compare dielectric, elastic, and pyroelectric properties of the two materials in polycrystalline form. In contrast to cubic BaTiO3, hexagonal BaTiO3 does not exhibit macroscopic pyroelectric response at room temperature. This is consistent with apparent absence of polar regions in the hexagonal material, and the fact that in hexagonal BaTiO3, strain rather than polarization is the order parameter for the phase transition into ferroelectric-ferroelastic phase. The thermally stimulated currents measured in hexagonal and cubic BaTiO3, however, show that both materials exhibit noncentric macroscopic symmetry. This result supports the idea that extrinsic factors, such as strain gradients, which are apparently common for both materials, may break the macroscopic symmetry, which may then lead to alignment and redistribution of polar regions or charged defects.

Structure and Properties of Ir-Containing Oxides with Large Spin-Orbit Coupling: Ba2In(2-x)Ir(x)O(5+δ).

In this work, the solid solution series Ba2In(2-x)Ir(x)O5+δ (x = 0-1.4, 2) was synthesized, and its structural, magnetic, and charge-transport properties were measured. With increasing Ir content, three transitions in the room-temperature structure were observed: orthorhombic to tetragonal to cubic to a monoclinic distortion of a hexagonal BaTiO3 structure. Neutron diffraction shows Ba2In(1.6)Ir(0.4)O5.4 to be cubic and Ba2InIrO6 to be monoclinic, the latter contrary to previously published X-ray diffraction refinements. Magnetization measurements show Curie-Weiss behavior for x = 0.2-0.6, which arises from nearly 50:50 ratio of Ir(V) and Ir(VI). To our knowledge, this is the first time Ir(VI) has been stabilized with standard solid-state methods under ambient conditions. The electrical resistivity measurements show all the compounds studied are semiconducting and that resistivity decreases with increasing Ir content, suggesting the proximity to a metal-insulator transition. A sign reversal in the high-temperature Seebeck coefficient is observed indicating both electron and hole charge transport.

Modification of the ferromagnetic properties of Fe-doped BaTiO3 polycrystalline ceramics by using Bi3+ doping

We study the crystal structure and the magnetic properties of Ba1−xBixTi0.9Fe0.1O3 (0 ≤ x ≤ 0.12) ceramics prepared by using a conventional solid-state reaction. X-ray diffraction and Raman scattering spectroscopy measurements show that Bi3+ substitution suppresses the formation of the hexagonal BaTiO3 phase and stabilizes the tetragonal BaTiO3 phase. The samples with x ≥ 0.08 demonstrate a single tetragonal BaTiO3 phase. Samples with x ≤ 0.1 show obvious magnetic orders at room temperature. The saturated magnetization, paramagnetic contribution and magnetic transition temperature are found to depend on the Bi doping concentration. Samples with 0.08 ≤ x ≤ 0.1 show a single tetragonal phase structure with a ferromagnetic order. These results indicate that the crystal structure and the ferromagnetic properties of Ba1−xBixTi0.9Fe0.1O3 (0 ≤ x ≤ 0.12) ceramics are intrinsic.