Synthesis and Properties of Multiferroic La-Modified BiFeO3 Ceramics

Multiferroic Bi_{(1− x )}La _x FeO₃ was synthesized by a modified Pechini approach. Optimal conditions for the synthesis of single-phase Bi_{(1− x )}La _x FeO₃ powder were reported. A conventional sintering process was used to fabricate Bi_{(1− x )}La _x FeO₃ ceramics. Ferroelectric and magnetic loops measured at room temperature indicate the coexistence of ferroelectricity and magnetism. The La modification of BiFeO₃ has a beneficial effect on the ferroelectric and magnetic properties. The ceramic samples show a stable dielectric constant and quite low dielectric loss between 100 Hz and 10 MHz. The piezoelectric constants of Bi_{(1− x )}La _x FeO₃ ceramics were first reported.     

Electrical and Magnetic Properties of the System SrFeO3–BiFeO3

Perovskite phases containing iron in both the tetravalent and trivalent states were prepared by equilibration at high oxygen pressures. Small bismuth additions to SrFeO₃ resulted in high conductivity (10¹ to 10³ (ohm-cm)^–1 at 26°C) and antiferromagnetic order below 130°K. Distortion to tetragonal symmetry occurs approximately at the composition Bi_0.3Sr_0.7FeO_2.87, but higher bismuth concentrations (up to 80 mole %) give rise to a second cubic phase region which shows decreased conductivity (∼10^–5 (ohm-cm)^–1) and weak ferromagnetism with ordering temperatures higher than 26°C. Rhombohedral specimens Bi_0.9Sr_0.1FeO_2.96 and BiFeO_3.01 are anti-ferromagnetic and have comparatively low conductivity.     

Ferroic Properties of Highly Dense Multiferroic Bi1−xLa0.05TbxFeO3 Ceramics Via Sheltered Spark Plasma Sintering

Multiferroic Bi_{0.95 −x} La_0.05Tb _x FeO₃ (BLTFO) ceramics were prepared by spark plasma sintering. The protection of CeO₂ powders in the spark plasma sintering process can effectively restrain the valence fluctuation of iron ions and high-dense BLTFO ceramics with good dielectric and ferroelectric properties are fabricated. The BLTFO ceramics have low loss (tanδ∼5%) between 10² and 10⁶ Hz. The doping of Tb can increase the dielectric and ferromagnetic properties, but decrease the ferroelectricity of BLTFO ceramics.     

Dielectric Relaxation of La3+-Modified Bi3TiNbO9 Aurivillius Phase Ceramics

Single Aurivillius phase Bi_{3− x} La _x TiNbO₉ (LBTN- x , x =0.00, 0.50, 0.75, and 1.00) ceramics were prepared by the conventional solid-state reaction method. When x ≤0.50, the ceramics are normal ferroelectrics, while LBTN-0.75 and LBTN-1.00 showed typical relaxor behavior, which could probably be attributed to the disordering induced by the La³⁺ partly substituting for Bi³⁺ in Bi₂O₂ layers. The dielectric relaxation of LBTN-0.75 and LBTN-1.00 were fitted using the Vögel–Fulcher relationship, and the results suggest that both of them are analogous to a spin glass with thermally activated polarization fluctuations above a freezing temperature.     

A Novel Temperature-Compensated Microwave Dielectric (1−x)(Mg0.95Ni0.05)TiO3−xCa0.6La0.8/3TiO3 Ceramics System

The microstructure and microwave dielectric properties of a (1− x )(Mg_0.95Ni_0.05)TiO₃− x Ca_0.6La_0.8/3TiO₃ ceramics system have been investigated. The system was prepared using a conventional solid-state ceramic route. In order to produce a temperature-stable material, Ca_0.6La_0.8/3TiO₃ was added for a near-zero temperature coefficient (τ_f). With partial replacement of Mg²⁺ by Ni²⁺, the dielectric properties of the (1− x )(Mg_0.95Ni_0.05)TiO₃− x Ca_0.6La_0.8/3TiO₃ ceramics can be promoted. The microwave dielectric properties are strongly correlated with the sintering temperature and the composition. An excellent Q × f value of 118,000 GHz can be obtained for the system with x =0.9 at 1325°C. For practical application, a dielectric constant (ɛ_r) of 24.61, a Q × f value of 102,000 GHz, and a temperature coefficient of resonant frequency (τ_f) of −3.6 ppm/°C for 0.85(Mg_0.95Ni_0.05)TiO₃−0.15Ca_0.6La_0.8/3TiO₃ at 1325°C are proposed. A parallel-coupled line band-pass filter is designed and simulated using the proposed dielectric to study its performance.     

Effect of La Doping on the Phase Conversion, Microstructure Change, and Electrical Properties of Bi2Fe4O9 Ceramics

Undoped and La-doped Bi₂Fe₄O₉ ceramics were synthesized using a soft chemical method. It is observed that in calcining La-doped Bi₂Fe₄O₉, Bi(La)FeO₃ phase rather than Bi_{2− x} La _x Fe₄O₉ gradually increases with increasing La doping content. The phase conversion from mullite-type structure of Bi₂Fe₄O₉ to rhombohedrally distorted perovskite one of Bi(La)FeO₃ with increasing La doping content indicates that La doping can stabilize the structure of BiFeO₃. This is further evidenced that Bi₂Fe₄O₉ can be directly converted to Bi(La)FeO₃ by heating the mixtures of nominal composition of Bi₂Fe₄O₉/ x La₂O₃. Furthermore, the microstructure changes and the room temperature hysteresis loops and leakage current for Bi_{2− x} La _x Fe₄O₉ with x =0 and 0.02 were characterized.     

Domain Structure and Fatigue Behavior of La3+-Doped SrBi2Ta2O9 Thin Films

Ferroelectric SrBi_1.4La_0.6Ta₂O₉ (SBLT) thin films were grown onto Pt/Ti/SiO₂/Si substrates by pulsed-laser deposition. With the aid of X-ray diffractometry, piezoresponse scanning probe microscopy, and ferroelectric-property measurements, a correlation between microstructure, as well as domain structure and ferroelectric properties, was established. Excluding the effect of preferential orientation on ferroelectric properties, the increase in remanent polarization was attributed to distortion of the perovskite-like sublattice and atom displacement. Despite the co-instantaneous observation of a 90° domain and slight fatigue behavior in the SBLT films, the 90° domain-wall clamping did not seem to account for the fatigue in the SBLT films. Instead, strain-stress aggravation of the SBLT sublattice, due to the substitution of La³⁺ into Bi³⁺ sites, decreased the self-regulated flexibility of the (Bi₂O₂)²⁺ layers and caused fatigue in the SBLT.     

Ferroelectric and Ferromagnetic Properties of Hot-Pressed Bi0.95−xLa0.05TbxFeO3 Ceramics

Multiferroic Bi_{0.95− x} La_0.05Tb _x FeO₃ (BLTFO) ceramics were prepared by hot-pressing. It was found that their dielectric properties were very sensitive to sintering temperature. The samples hot-pressed at 740°C show stable dielectric constant and rather low loss (∼1%) in the frequency range up to 1 MHz. When the nominal content of Tb is <0.08, the atomic arrangement keeps the R 3 c symmetry, while a phase transition appears at x ≈0.08–0.10. The introduction of Tb enhances the coercive field of the BLTFO ceramics and then restrains the ferroelectricity gradually. The doping of Tb at x ≤0.10 increases the ferromagnetization.     

Effects of Excess Bi2O3 on the Ferroelectric Behavior of Nd-Doped Bi4Ti3O12 Thin Films

Effects of excess Bi₂O₃ content on formation of (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNT) films deposited by RF sputtering were investigated. The microstructures and electrical properties of BNT thin films are strongly dependent on the excess Bi₂O₃ content and post-sputtering annealing temperature, as examined by XRD, SEM, and P – E hysteresis loops. A small amount of excess bismuth improves the crystallinity and therefore polarization of BNT films, while too much excess bismuth leads to a reduction in polarization and an increase in coercive field. P – E loops of well-established squareness were observed for the BNT films derived from a moderate amount of Bi₂O₃ excess (5 mol%), where a remanent polarization 2P _r of 25.2 μC/cm² and 2E _c of 161.5 kV/cm were shown. A similar change in dielectric constant with increasing excess Bi₂O₃ content was also observed, with the highest dielectric constant of 304.1 being measured for the BNT film derived from 5 mol% excess Bi₂O₃.     

Phase Relations in the Na0.5Bi0.5TiO3–Li3xLa(2/3)−x□(1/3)−2xTiO3 System

The phase relations and the mechanism of solid-state synthesis for the Na_0.5Bi_0.5TiO₃–Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} TiO₃ system were investigated using X-ray powder diffraction, scanning electron microscopy, and thermal analysis. The study revealed that the extent of the homogeneity range—which is related to the A-site substitution between (Na_0.5Bi_0.5)²⁺ and (Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} )²⁺ pseudo cations of a perovskite structure—depends strongly on the ordering of the (Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} )²⁺ species. The solid-state reaction of the compounds in the homogeneity range is completed only after multiple high-temperature firings. However, the system is also subjected to a slow thermal decomposition; this is particularly so for the compounds with a high × value and an increased Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} TiO₃ concentration.     

Sintering and Dielectric Characterization of Pseudoternary Compounds from the Bi2O3–TiO2–TeO2 System

Using X-ray diffraction analysis, scanning electron microscopy, thermogravimetry, and measurements of the dielectric properties up to the MW frequency range, the characterization of Bi₂Ti₃TeO₁₂, Bi₂TiTeO₈, and Bi₆Ti₅TeO₂₂ compounds, which all include Te⁶⁺, was performed. As the processes of Te⁶⁺ reduction and the evaporation of TeO₂-containing species contribute to the presence of secondary phases, the preparation of single-phase ceramics is rather difficult. To minimize the amount of secondary phases during the firing process, the pellets were muffled in a corresponding compound and then fired in an autoclave furnace under 10 bars of oxygen pressure. By sintering the Bi₂Ti₃TeO₁₂, Bi₂TiTeO₈, and Bi₆Ti₅TeO₂₂ between 840° and 1010°C, ceramics with ɛ_r ranging from 36 to 350, Q × f values from 220 to 12 500 GHz, and τ_f from +41 to +2600 ppm/K were obtained.     

Populations and Emission Properties of the 5I6 and 5I7 Levels in Ho3+ Doped into PbO–Bi2O3–Ga2O3 Glasses

Emission properties of PbO–Bi₂O₃–Ga₂O₃ glasses doped with Ho³⁺ were investigated for fiber-optic amplification at the 1.18 μm wavelength region. When the glasses were doped with Ho³⁺ ions only, there was a weak emission at 1.18 μm with a lifetime of ∼200 μs. However, when Yb³⁺ ions were codoped, the lifetime of the 1.18 μm emission increased to 630 μs together with a significant increase in intensity. A similar enhancement in the intensity and lifetimes was realized for the 2.05 μm emission. These effects are due to energy transfer from the Yb³⁺:²F_5/2 to the Ho³⁺:⁵I₆ level. Devitrification of the ternary PbO–Bi₂O₃–Ga₂O₃ glasses was efficiently suppressed by adding 10 mol% GeO₂. Optimum Ho³⁺ concentration was ∼0.4 mol%, whereas Yb³⁺ ions can be added up to the solubility limit.     

Pyrochlore Phase Formation in the System Bi2O3–ZnO–Ta2O5

The subsolidus phase diagram of the system Bi₂O₃–ZnO–Ta₂O₅ in the region of the cubic pyrochlore phase has been determined at 1050°C. This phase forms a solid solution area that includes the ideal composition P, Bi₃Zn₂Ta₃O₁₄; possible solid solution mechanisms are proposed, supported by density measurements of Zn-deficient solid solutions. The general formula of the solid solutions is Bi_{3+ y} Zn_{2− x} Ta_{3− y} O_{14− x − y} , based on the creation of Zn²⁺, O²⁻ vacancies in Zn-deficient compositions and a variable Bi/Ta ratio.     

Phase Formation and Dielectric Characterization of the Bi2O3–TeO2 System Prepared in an Oxygen Atmosphere

Solid-state synthesis of compositions from the Bi₂O₃–TeO₂ system show that, under an oxygen atmosphere, Te⁴⁺ oxidizes to Te⁶⁺ and yields four room-temperature stable compounds: Bi₂Te₂O₈, Bi₂TeO₆, Bi₆Te₂O₁₅, and new a compound with the nominal composition 7Bi₂O₃·2TeO₂. Dense ceramics can be prepared from all these compounds by sintering between 650° and 800°C under an oxygen atmosphere. The permittivity of these compounds varies from ∼30 to ∼54, the Q × f value from 1.100 to 41.000 GHz (∼5 GHz), and the temperature coefficient of resonant frequency from −43 to −144 ppm/K. Bi₆Te₂O₁₅ and 7Bi₂O₃·2TeO₂ do not react with silver, and, therefore, they have the potential to be used for applications in low-temperature cofired ceramic (LTCC) technology.     

Novel Fast Lithium Ion Conduction in Garnet-Type Li5La3M2O12 (M = Nb, Ta)

Lithium metal oxides with the nominal composition Li₅La₃M₂O₁₂ (M = Nb, Ta), possessing a garnetlike structure, have been investigated with regard to their electrical properties. These compounds form a new class of solid-state lithium ion conductors with a different crystal structure compared with all those known so far. The materials are prepared by solid-state reaction and characterized by powder XRD and ac impedance to determine their lithium ionic conductivity. Both the niobium and tantalum members exhibit the same order of magnitude of bulk conductivity (∼10⁻⁶ S/cm at 25°C). The activation energies for ionic conductivity (<300°C) are 0.43 and 0.56 eV for Li₅La₃Nb₂O₁₂ and Li₅La₃Ta₂O₁₂, respectively, which are comparable to those of other solid lithium conductors, such as Lisicon, Li₁₄ZnGe₄O₁₆. Among the investigated materials, the tantalum compound Li₅La₃Ta₂O₁₂ is stable against reaction with molten lithium. Further tailoring of the compositions by appropriate chemical substitutions and improved synthesizing methods, especially with regard to minimizing grain-boundary resistance, are important issues in view of the potential use of the new class of compounds as electrolytes in practical lithium ion batteries.     

Transformational Superplasticity of Bi2WO6 and Bi2MoO6

Transformational Superplasticity was studied in the compounds Bi₂WO₆ and Bi₂MoO₆. The magnitudes of transitional strain are related to the ( T_t/T_mP ), s of the phase transitions and are proportional to the externally applied stresses. Strain-rate sensitivities were similar, 0.85 and 0.86; however, the Bi₂WO₆ exhibited a strain-axis intercept and the Bi₂MoO₆ a stress-axis intercept. The grain-size effect present in the Bi₂WO₆ supports an accommodated grain-boundary sliding mechanism for the superplastic deformation process.     

Phase Formation and Crystal-Structure Determination in the Bi2O3–TiO2–TeO2 System Prepared in an Oxygen Atmosphere

Using X-ray diffraction analysis and scanning electron microscopy it was revealed that in an atmosphere of flowing oxygen in the temperature range 700°–800°C, three new compounds are formed in the Bi₂O₃–TiO₂–TeO₂ pseudoternary system. These compounds are Bi₂Ti₃TeO₁₂, Bi₂TiTeO₈, and Bi₆Ti₅TeO₂₂, and all the compounds include Te⁶⁺. All three crystal structures were solved and refined using X-ray powder diffraction data. Based on the results of the phase formation, a solid-state compatibility diagram is proposed.     

Lanthanum-Modified (1 – x)(Bi0.8La0.2)(Ga0.05Fe0.95)O3·xPbTiO3 Crystalline Solutions: Novel Morphotropic Phase-Boundary Lead-Reduced Piezoelectrics

(1 – x )(Bi_0.8La_0.2)(Ga_0.05Fe_0.95)O₃· x PbTiO₃ (BLGF-PT) crystalline solutions have been fabricated by solid-state reactions. BLGF-PT has single perovskite phase structure with a rhombohedral–tetragonal (FE_r-FE_t) morphotropic phase boundary (MPB) at a PT content of x = 0.43. Lanthanum substitution has been found to increase the insulation resistance and decrease the coercive field down to 20 kV/cm, which results in significant improvements in dielectric and piezoelectric properties of BLGF-PT. The dielectric constant, loss tangent, Curie temperature, remnant polarization, piezoelectric d ₃₃ constant, and planar coupling factor of 1760, 0.05, 264°C, 33 μC/cm², 295 pC/N, and 0.36, respectively, have been achieved for BLFG-PT in the vicinity of the MPB. Compared with conventional Pb(Zr,Ti)O₃ (PZT) piezoelectric ceramics, the BLGF-PT is a competitive alternative piezoelectric material with decreased lead content.     

Phase Relations in the System Bi2O3-WO3

Phase relations in the system Bi₂O₃-WO₃ were studied from 500° to 1100°C. Four intermediate phases, 7Bi₂O₃· WO₃, 7Bi₂O₃· 2WO₃, Bi₂O₃· WO₃, and Bi₂O₃· 2WO₃, were found. The 7B₂O · WO₃ phase is tetragonal with a ₀= 5.52 Å and c ₀= 17.39 Å and transforms to the fcc structure at 784°C; 7Bi₂O₃· 2WO₃ has the fcc structure and forms an extensive range of solid solutions in the system. Both Bi₂O₃· WO₃ and Bi₂O₃· 2WO₃ are orthorhombic with (in Å) a ₀= 5.45, b ₀=5.46, c ₀= 16.42 and a ₀= 5.42, b ₀= 5.41, c ₀= 23.7, respectively. Two eutectic points and one peritectic exist in the system at, respectively, 905°± 3°C and 64 mol% WO₃, 907°± 3°C and 70 mol% WO₃, and 965°± 5°C and 10 mol% WO₃.     

Formation of Infinite-Layered (Ca1-xSrx) CuO2 and NaCuO2-type (Ca1-yNay)0.85CuO2 in Tartrate Route

Both NaCuO₂-type Ca_0.85CuO₂ and infinite-layered (Ca_{1- x} Sr _x )CuO₂ could be prepared much more easily by firing the dried solids from mixed acetate aqueous solutions titrated with tartaric acid than by normal calcination. The presence of a narrow solid-solution composition range of 0.10 < x > 0.16 was confirmed in infinite-layered (Ca_{1- x} Sr _x )CuO₂ in the preparation using the tartrate route. The calcium could also be substituted by sodium in a range of y > 0.15 in NaCuO₂-type (Ca_{1- y} Na _y )_0.85CuO₂ using the same route. Further substitution of Ca²⁺ with Y³⁺ might also be possible in infinite-layered (Ca_{1- x} Sr _x )CuO₂, but resulted in the NaCuO2-type compound in the substitution with Na⁺.