Synergistic effect of La and Co co-doping on room-temperature ferromagnetism enhancement of TiO2 nanoparticles

The synergistic influence of lanthanum and cobalt co-doping on room temperature ferromagnetism (RTFM) of TiO₂ system is investigated. A series of Ti_0.97?xCo_0.03La_xO₂ nanoparticles were prepared and their structures and properties were systematically studied with X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy, UV–vis spectrophotometer, Raman spectra and magnetic measurement techniques, respectively. Detailed experimental characterizations indicate that the as-prepared La and Co co-doped samples exhibit single anatase phase, and all the samples exhibit strong visible photoluminescence associated with oxygen vacancies and a clear ferromagnetic hysteresis loop, both of which were dramatically enhanced with La and Co co-doping, and the maximum saturation magnetization (M_s) reaches 1.38 emu/g at the La content of 6 mol%. It is speculated that oxygen vacancies modulated by ionic La play an important role in the enhanced RTFM, which can be attributed to the bound magnetic polarons (BMPs) formed via ferromagnetic coupling between two neighboring Co²⁺ ions mediated by oxygen vacancy (F⁺ center). Our results present an alternative method to obtain high performance RTFM.     

Structural and electrical properties of La3+-doped Na0.5Bi4.5Ti4O15-Bi4Ti3O12 inter-growth high temperature piezoceramics

New lead-free inter-growth piezoelectric ceramics, Na_0.5Bi_8.5-xLa_xTi₇O₂₇ (NBT-BIT-xLa, 0.00≤x≤1.00), were prepared by the conventional solid-state method. Structural and electrical properties of NBT-BIT-xLa were studied. All the NBT-BIT-xLa samples exhibited a single inter-growth structured phase. XRD and Raman spectroscopy revealed a reduced orthorhombicity, which strongly supports the variation of dielectric and ferroelectric properties. Plate-like grains were found to decrease with the increasing x contents. Impedance spectra analysis indicated that oxygen vacancy defects dominated the contributions to the electrical conductivity. The increased activation energies for dc conductivity evidenced the reduction of oxygen vacancy concentration after La substitution, inducing the enhancement in piezoelectric constant (d₃₃) and remanent polarization (2P_r). The studies of thermal depoling indicated that the optimal d₃₃ of NBT-BIT-0.50La ceramics still remained 22 pC/N at 500 °C, implying that this ceramics could be potentially applied into high temperature devices.     

Electrical properties of La2-xPrxTi2O7 (x = 0–0.3) ceramics

The electrical properties of La₂Ti₂O₇ (LTO) ceramics have been enhanced through the substitution of La³⁺ ions by Pr³⁺ ions. Almost all doped Pr³⁺ ions will get at A - site without causing a change on monoclinic phase of LTO. The average grain size is 17.8 μm for La_1.9Pr_0.1Ti₂O₇ ceramics. The relaxation activation energy which is contributed by defect dipoles that are formed from TiO₆ oxygen octahedrons’ distortions in grains is 1.6 eV for La_1.8Pr_0.2Ti₂O₇ ceramics. This kind of defects will be activated from 520 °C and completely be activated until 650 °C. The piezoelectric coefficient d₃₃ = 3.0 pC/N of La_2-xPr_xTi₂O₇ ceramics maintains stable when the Pr³⁺ doping content x ranging from 0.1 to 0.3.     

La(Mg1/2Ti1/2)O3–La2/3TiO3 microwave dielectric ceramics

Structure and microwave dielectric properties were studied in the (1−x)La(Mg_1/2Ti_1/2)O₃–xLa_2/3TiO₃ system. Ceramics with this composition in the 0⩽x⩽0.5 range were processed from powders obtained by a citrate-based chemical route. Structure of these perovskite solid solutions changed from orthorhombic for x=0.1 and 0.3 to pseudocubic for x=0.5. Microwave and radio frequency measurements revealed increase in permittivity and temperature coefficient of the resonant frequency τ_f with increasing of La_2/3TiO₃ content. Close to zero τ_f value was found near to x=0.5 composition of (1−x)La(Mg_1/2Ti_1/2)O₃–x La_2/3TiO₃.     

The effect of second phase La0.67TiO2.87 on the phase structure and impedance spectroscopy of La2Ti2(1 + x)O7 piezoelectric ceramics

The Ti excess La₂Ti_{2 (1+x)} O₇ (x = 0, 0.005, 0.01, 0.02, 0.05, 0.1) piezoelectric ceramics have been prepared by sol-gel technology and solid state synthesis method. Through refinement analysis, the phase structure of the ceramics varies with Ti content. Most monoclinic phase (∼93%) and a handful of orthogonal phase (∼7%) coexist in La₂Ti_{2 (1+0)} O₇ ceramics. Pure monoclinic phase La₂Ti₂O₇ with space group P2₁ appears in La₂Ti_{2 (1+0.005)} O₇ and La₂Ti_{2 (1+0.01)} O₇ ceramics. Monoclinic phase La₂Ti₂ O₇ and a certain proportion of tetragonal phase La_0.67TiO_2.87 coexist in La₂Ti_{2 (1+0.02)} O₇, La₂Ti_{2 (1+0.05)} O₇ and Ti_{2 (1+0.1)} O₇ ceramics. With the excess of Ti content, the monoclinic phase ratio and distortion angles in a-b projection plane of the ceramics increase first and then decrease, which is consistent with the variation tendency of piezoelectric constant d₃₃. The excellent piezoelectric constant for Ti_{2 (1+0.01)} O₇ ceramics is 2.8 pC/N.Impedance analysis shows that the conductive mechanisms of all samples include both grain and grain boundary conductivity at temperature range T ≥ 500 °C. The formation of tetragonal phase La_0.67TiO_2.87 derives from Ti excess in pure monoclinic phase La₂Ti₂O₇. The existence of tetragonal phase La_0.67TiO_2.87 can obviously increase the capacitance of ceramics at x ≥ 0.05. All prepared piezoelectric La₂Ti_{2 (1+x)} O₇ ceramics have highly frequency stability and are candidates for ultrahigh temperature piezoelectric application.     

Superior hybrid improper ferroelectricity and enhanced ferromagnetism of Ca3(Ti1-xCox)2O7 ceramics through the superexchange interaction

Ca₃(Ti_1-xCo_x)₂O₇ ceramics were prepared by a tartaric acid sol-gel method and sintered in an oxygen atmosphere. The introduction of Co²⁺/Co³⁺ as acceptor dopants leads to the formation of more oxygen vacancies and defect dipoles in Ca₃(Ti_1-xCo_x)₂O₇ ceramics. Oxygen vacancy and defect dipoles lead to the transition of dielectric, leakage, and ferroelectric behaviors of Ca₃(Ti_1-xCo_x)₂O₇ ceramics. The coexistence of hybrid improper ferroelectricity and ferromagnetism at room temperature in Ca₃(Ti_1-xCo_x)₂O₇ ceramics has been successfully realized through the superexchange interaction of Co–O–Co. Ca₃(Ti_1-xCo_x)₂O₇ ceramics exhibit superior ferroelectricity (the remnant polarization is 3.29 μC/cm²) and enhanced ferromagnetism (the remnant magnetization reaches 6.4×10^?3 emu/g). This strategy based on the introduction of transition metal ions with unfilled 3d shells at B sites is an important approach to realize novel room-temperature single-phase multiferroic materials for Ca₃Ti₂O₇-based materials.     

Stable anatase phase with a bandgap in visible light region by a charge compensated Ga–V (1:1) co-doping in TiO2

A series of charge compensated Ga–V co-doped TiO₂ samples (Ti_(1-x)(Ga_0.5V_0.5)_xO₂) have been synthesized by a modified sol-gel process. X-ray diffraction pattern shows that the anatase to rutile (A→R) onset temperature (T_O) shifts to a higher temperature, whereas the complete phase transformation temperature (T_C) shifts to a low-temperature region as compared to pure TiO₂, due to Ga–V incorporation. Ga–V co-doping helps in the transformation of some smaller sized Ti⁴⁺ to a relatively larger Ti³⁺. In the anatase phase, oxygen content also increases with increasing doping concentration, which along with the larger size of Ti³⁺ results in lattice expansion and thereby delays the T_O. In the rutile phase, oxygen vacancy increases with increasing doping concentration, which results in lattice contraction and accelerates phase transition. Grain growth process is hindered in the anatase phase (crystallites size reduces from ~15 nm (x = 0.00) to 8 nm (0.10)), whereas it is accelerated in the rutile phase as compared to pure TiO₂. In both phases bandgap (E_g) reduces to the visible light region (anatase: E_g = 3.16 eV (x = 0.00) to 2.19 eV (x = 0.10) and rutile: 3.08 eV (x = 0.00) to 2.18 eV (x = 0.10)) in all co-doped samples. The tail of the absorption edge reveals lattice distortion and increase of Urbach energy proofs the same due to co-doping. All these changes (grain growth, phase transition, and optical properties) are due to lattice distortion created by the combined effect of substitution, interstitials, and oxygen vacancies due to Ga–V incorporation in TiO₂.     

Dielectric and optical properties of Ni doped Na0.5Bi0.5TiO3

Polycrystalline Ni doped Na_.5Bi_0.5TiO₃ samples (Na_0.5Bi_0.5)Ti_1-xNi_xO_3, (x?=?0.5, 0.10, 0.15) have been prepared by solid state reaction. The appearance of the additional peak in X-ray diffraction pattern indicates the ordering of Ti⁴⁺ and Ni²⁺ ions. Polygonal grains are converted into flakes with an increase of Ni concentration. Replacement of Ti⁴⁺ by Ni²⁺ strongly modifies the relative contribution of two peaks in the Raman bands within 200–400?cm^?1. Oxygen vacancy is observed in X-ray photoelectron spectrum to maintain charge neutrality due to aliovalent doping. Broad diffuse phase transition centered at the dielectric constant maximum indicates relaxor behaviour. Comparison between impedance and electric modulus spectrum suggests non-Debye relaxation. The ac conductivity follows the power law with the frequency exponent lies 0.52???0.72. The generation of holes by divalent Ni dopant at tetravalent Ti sites enhances optical band gap.     

Phase stability and equilibria in the La2O3–TiO2 system

XRD, electron probe wavelength and energy dispersive X-ray analyses were used to reexamine the phase relations in the La₂O₃–TiO₂ system. The diagram was redrawn to include the compound La₄Ti₃O₁₂ in addition to La₄Ti₉O₂₄, La₂Ti₂O₇ and La₂TiO₅. Above 1455°C a cation deficient perovskite La_2/3TiO₃ is stabilized by a small number of Ti³⁺ ions and remains stable on cooling in air. The proposed diagram represents a section through the system at the normal oxygen pressure in air at 1 atm, compositions being expressed in terms of the oxide components stable at room temperature.     

Crystalline phases and oxygen permeation properties of mixed conductive (La,Ca)FeO3-δ

A typical mixed conductive oxide with high oxygen permeability is La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF), which is applicable to oxygen separation membrane and to cathodes for solid oxide fuel cells. However, Sr and Co included in the LSCF lower its stability when kept at elevated temperatures. La_1-xCa_xFeO_3-δ(LCF) is a good candidate of mixed conductive oxides with no Sr and Co ions. This study investigated systematic details of crystalline phase, electrical conductivity, and oxygen permeability as functions of the Ca content in the LCF. The x = 0.3–0.4 samples with perovskite structure reveal higher electrical conductivity and higher oxygen permeation flux J_O2 among the investigated LCF. Particularly, the J_O2 of the x = 0.35 and 0.4 are higher than that of the LSCF, proposing that the samples replace the LSCF. Larger specific free volumes in x = 0.3–0.4 are a possible main reason for higher vacancy mobility, resulting in higher J_O2.     

Influence of the structural characteristics of Ti1−xSnxO2 nanoparticles on their photocatalytic activity for the elimination of methylcyclohexane vapors

In the present work, we have used Ph₃SnOH as precursor for the synthesis of highly active Ti_1−xSn_xO₂ nanosized photocatalysts. This new preparation route is based on the reaction of TiCl₄ with the organometallic compound in an organic medium to yield an amorphous precipitate of TiO₂ which contains adsorbed tin species. Subsequent crystallisation of the precipitate under thermal or hydrothermal conditions determines the phase composition (rutile/anatase ratio) and the particle size of the final material. Characterisation data reveal that the incorporation of Sn promotes the anatase to rutile transformation but reduces the size of rutile crystallites. The activity of these samples has been tested for the photocatalytic oxidation of methylcyclohexane (MCH) vapors in an oxygen flow. This hydrocarbon can be considered representative of the volatile organic chemicals (VOC) present in urban atmospheres. The results obtained indicate that the Ti_1−xSn_xO₂ materials obtained under thermal conditions present higher specific photoactivity than the reference material TiO₂ P25, especially when the reaction is performed in a stream of humid oxygen. On the other hand, the comparison with undoped TiO₂ prepared in similar conditions shows that the incorporation of Sn significantly increases the photocatalytic oxidation rate. High crystallinity and an adequate anatase to rutile ratio seem to be beneficial for the removal of MCH. In contrast, pure rutile Ti_1−xSn_xO₂ nanoparticles prepared by autoclaving the amorphous precursor in HCl shows a quite limited photoactivity, despite its high surface area.     

Preparation,characterization, and giant dielectric permittivity of (Y3+ and Nb5+) co–doped TiO2 ceramics

The microstructure and giant dielectric properties of Y³⁺ and Nb⁵⁺ co–doped TiO₂ ceramics prepared via a chemical combustion method are investigated. A main rutile–TiO₂ phase and dense ceramic microstructure are obtained in (Y_0.5Nb_0.5)_xTi_1-xO₂ (x = 0.025 and 0.05) ceramics. Nb dopant ions are homogeneously dispersed in the microstructure, while a second phase of Y₂O₃ particles is detected. The existence of Y³⁺, Nb⁵⁺, Ti⁴⁺ and Ti³⁺ as well as oxygen vacancies is confirmed by X–ray photoelectron spectroscopy and X–ray absorption near edge structure analysis. The sintered ceramics exhibit very high dielectric permittivity values of 10⁴–10⁵ in the frequency range of 40–10⁶ Hz. A low loss tangent value of ≈0.08 is obtained at 40 Hz. (Y_0.5Nb_0.5)_xTi_1-xO₂ ceramics can exhibit non–Ohmic behavior. Using impedance spectroscopy analysis, the giant dielectric properties of (Y_0.5Nb_0.5)_xTi_1-xO₂ ceramics are confirmed to be primarily caused by interfacial polarization.     

Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents Study of (1−x)Ba0.6Sr0.4La4Ti4O15–xTiO2 Ceramics

Microwave dielectric properties and thermally stimulated depolarization currents (TSDC) of (1?x)Ba_0.6Sr_0.4La₄Ti₄O₁₅–xTiO₂ (x = 0, 0.01, 0.02, 0.05, and 0.1) ceramics were studied. X‐ray diffraction analysis indicates that the specimens show a hexagonal perovskite structure; however, with an increase of x to 0.1, TiO_2?δ as a secondary phase can be detected in the ceramics. The variation of TiO₂ content has a significant effect on the dielectric properties of (1?x)Ba_0.6Sr_0.4La₄Ti₄O₁₅–xTiO₂ at microwave frequency. The dielectric permittivity of ceramics increases from 44 to 49 with the increase of TiO₂ content. The Qf value is in the range of 39 300–53 400 GHz. However, the temperature coefficient of resonant frequency (τ_f) changes from ?7.5 to–9.4 ppm/°C, and then turns to +3.9 ppm/°C. A near zero τ_f value can be obtained by tuning the content of TiO₂ addition. TSDC was also employed to analysis the extrinsic loss mechanism. Utilizing a fixed polarization condition, the TSDC relaxation peaks are present, which are generated by oxygen vacancies. And the concentration of oxygen vacancies increases with the increase of TiO₂ content. It can be concluded that the extrinsic dielectric loss is dominated by microstructure and oxygen vacancy defects.     

Enhanced thermal stability of piezoelectricity in lead-free (Ba,Ca)(Ti,Zr)O3 systems through tailoring phase transition behavior

Good thermal stability in lead-free BaTiO₃ ceramics is important for their applications above room temperature. In this study, thermal stable piezoelectricity in lead-free (Ba,Ca)(Ti,Zr)O₃ ceramics was enhanced by tailoring their phase transition behaviors. Comparison between (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.65Ca_0.35)TiO₃ and (1-y)Ba(Ti_0.8Zr_0.2)O₃-y(Ba_0.95Ca_0.05)TiO₃ revealed that latter system at y?=?0.80 had much better thermal stable piezoelectric coefficient than the former at x?=?0.45. Both systems crystalized in tetragonal to orthorhombic phase boundary at room temperature. The phase transition temperature and degree of diffusion were adjusted by Ca and Zr ions contents and demonstrated great influence on temperature dependent dielectric permittivity, hysteresis loops, and in-situ domain structures. The improved thermal stability of (1-y)Ba(Ti_0.8Zr_0.2)O₃-y(Ba_0.95Ca_0.05)TiO₃ prepared at y?=?0.80 was linked to its higher paraelectric to ferroelectric phase transition temperature (T_m?=?115.7?°C) and less degree of diffusion (degree of diffusion constant γ?=?1.35). By comparison, (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.65Ca_0.35)TiO₃ prepared at x?=?0.45 revealed T_m?=?81.3?°C and γ?=?1.65. Overall, these findings look promising for future stimulation of phase transition behaviors and design of piezoelectric materials with good thermal stabilities.     

Enhancement of the dielectric permittivity and magnetic properties of Dy substituted strontium titanate ceramics

Structural, dielectric and magnetic properties of dense Dy-substituted strontium titanate ceramics are investigated. In the Sr_1-1.5xDy_xTiO₃ system, incorporation of Dy onto the Sr site is confirmed by a linear decrease of the lattice parameter up to x = 0.05. Dielectric spectroscopy analysis of Sr_1-1.5xDy_xTiO₃ ceramics reveals four relaxations. Two relaxations observed below 55 K are attributed to dipoles formed by off-centre displacement of Dy³⁺ ions on the Sr sites. Other two dielectric relaxations found at higher temperatures are attributed to the oxygen vacancy related mechanisms. As result, very high dielectric permittivity of ～33500 at 28 K and of ～9600 around room temperature at moderate dissipation factor of ～0.02 are obtained for Sr_0.985Dy_0.01TiO₃ ceramics, making it a promising material for capacitor electronic applications. Paramagnetic behaviour observed for Sr_1-1.5xDy_xTiO₃ as well as for Sr_1-xDy_2xTi_1-xO₃ ceramics indicates impossibility to induce a magnetic order and hence magnetoelectric coupling in strontium titanate by Dy substitution.     

Phase transition induced morphotropic phase boundary behavior and the electric properties in strontium modulated Bi4.5Na0.5Ti4O15 lead-free ceramics

Lead-free (Bi_0.5Na_0.5)_1-xSr_xBi₄Ti₄O₁₅ ceramics (x = 0–0.9) are fabricated by solid state reaction process. XRD analysis shows the symmetry divergence from tetragonal to orthorhombic phase accompanied by morphotropic phase boundary with increasing strontium content. Raman spectra confirm the incorporation of strontium into (Bi_2.5Na_0.5Ti₄O₁₃)^2- layers. SEM graphs exhibit the typical plate-like morphology with regular variation of grain size and crystallization as strontium increases. Multistage ferroelectric transition is observed with x = 0.2–0.4. Piezoelectric performance measurements present the well thermal stability at x = 0.4. The dielectric properties display a shifting of Curie temperature towards low temperature with increasing strontium ions. It can be due to the crystal lattice distortion by larger radius of strontium and the increasing tolerance factor. ac conductivity and impedance measurements suggest that electron hopping mainly contributes to the low temperature region. Ionization conductivity by oxygen vacancy migration including first-ionization and double-ionization plays the dominating role in the middle and high temperature region. The controllable properties indicate the potential applications for electric devices of (Bi_0.5Na_0.5)_1-xSr_xBi₄Ti₄O₁₅ ceramic.     

Phase characteristics,microstructure, and electrical properties of (1-x)BaZr0.2Ti0.8O3-(x)(Ba0.7Ca0.3)0.985La0.01TiO3 ceramics

In this study, (1-x)BaZr_0.2Ti_0.8O₃-(x)(Ba_0.7Ca_0.3)_0.985La_0.01TiO₃ ((1-x)BZT-(x)BCLT) ceramics, where x = 0.3, 0.4, 0.5, and 0.6, were prepared employing a conventional solid-state sintering technique. X-ray diffraction patterns and dielectric measurements indicated three phase regions at room temperature, including a single rhombohedral (x = 0.3), a phase coexistence of rhombohedral and tetragonal (x = 0.4), and a single tetragonal structure (x ≥ 0.5). X-ray photoemission spectra at the surface of ceramics confirmed the oxidation state of Ba²⁺, Ca²⁺, Ti⁴⁺, and Zr⁴⁺ ions. Upon BCLT addition, the reduction of the average grain size and the presence of the tetragonal structure significantly affected the dielectric, ferroelectric, and piezoelectric properties of these ceramics. With these results, the composition x = 0.3 showed maximum ε_r′ and ε_m′, whereas the composition x = 0.5 showed maximum P_r, E_c, d₃₃, k_p, and d¹₃₃ factors. These results suggest a new phase diagram for the (1-x)BZT-(x)BCLT system, which could be tuneable by BCLT concentration and might be useful as an alternative material in dielectric, ferroelectric, and piezoelectric devices.     

Enhanced relative permittivity in niobium and europium co-doped TiO2 ceramics

The appearance of colossal permittivity materials broadened the choice of materials for energy-storage applications. In this work, colossal permittivity in ceramics of TiO₂ co-doped with niobium and europium ions ((Eu_0.5Nb_0.5)_xTi_1-xO₂ ceramics) was reported. A large permittivity (ε_r ～ 2.01?×?10⁵) and a low dielectric loss (tanδ ～ 0.095) were observed for (Eu_0.5Nb_0.5)_xTi_1-xO₂ (x?=?1%) ceramics at 1?kHz. Moreover, two significant relaxations were observed in the temperature dependence of dielectric properties for (Eu, Nb) co-doped TiO₂ ceramics, which originated from defect dipoles and electron hopping, respectively. The low dielectric loss and high relative permittivity were ascribed to the electron-pinned defect-dipoles and electrons hopping. The (Eu_0.5Nb_0.5)_xTi_1-xO₂ ceramic with great colossal permittivity is one of the most promising candidates for high-energy density storage applications.     

Microstructure,optical and magnetic properties of TiO2/CuO nanocomposites synthesized by a two-step method

TiO₂/CuO composites in different ratios were prepared via a two-step method. X-ray diffraction and transmission electron microscopy results indicated that part of Cu²⁺ substituted Ti⁴⁺ in the TiO₂ lattice in the composite, leading to Cu²⁺-substituted sites in the TiO₂ lattice as well as Cu²⁺ species located in the CuO lattice. Scanning electron microscopy revealed a morphology change in the sample from a three-dimensional structure to a two-dimensional structure while forming an interface between TiO₂ and CuO. X-ray photoelectron spectroscopy and Raman spectra showed that there are oxygen vacancies (V_O) and Ti³⁺ in the lattice. UV–vis absorption spectra exhibited a widening of the absorption range and a decrease in the bandgap with increasing amount of CuO in the TiO₂/CuO composites. Additionally, the composites exhibited room-temperature ferromagnetism (RTFM), as can be explained by the indirect double-exchange model, which is related to V_O and the exchange interaction between the 3d orbitals of Ti³⁺ and Ti⁴⁺ at the interface.     

Phase relations and electrical properties in the pseudo-ternary La2O3–TiO2–Mn2O3 system in air

Scanning electron microscopy (SEM), electron-probe microanalysis, energy- and wavelength-dispersive X-ray analysis and X-ray powder diffraction were used to investigate the subsolidus phase relations in the pseudo-ternary La₂O₃–TiO₂–Mn₂O₃ system in air (oxygen partial pressure p_O2=0.21   atm) at 1275 °C. The addition of Mn₂O₃ to the starting La₂O₃:3TiO₂ mixture led to the formation of a La-deficient perovskite La_2/3TiO₃ compound. The oxides form two new compounds with the proposed compositions: (i) La_1.7Ti_13.0Mn_6.3O_38−x, with a davidite-like crystal structure, and (ii) La₄₉Ti₁₈Mn₁₃O₁₂₉. There were also several solid solutions existing over a wide range of concentrations.