Site-selective substitution and resulting magnetism in arc-melted perovskite ATiO3-δ (A = Ca,Sr, Ba)

Magnetic properties in perovskite titanates ATiO_3-δ (A = Ca, Sr, Ba) were investigated before and after arc melting. Crystal structure analysis was conducted by powder synchrotron X-ray diffraction with Rietveld refinements. Quantitative chemical element analysis was carried out by X-ray photoelectron spectroscopy. Magnetic measurements were conducted by vibrating sample magnetometer and X-ray magnetic circular dichroism (XMCD). The magnetic properties are found to be affected by impurities of 3d elements such as Fe, Co, and Ni. Depending on the composition and crystal structure, the occupation of the magnetic ions in perovskite titanates is selectively varied, which is interpreted to be the origin of the different magnetic behaviors in arc-melted perovskite titanates ATiO_3-δ (A = Ca, Sr, Ba). In addition, both formation of oxygen vacancies and the reduction of Ti⁴⁺ to Ti³⁺ during arc-melting also play a role as proven by XMCD. Nevertheless, preferential site occupation of magnetic impurities is dominant in the magnetic properties of arc-melted perovskite ATiO_3-δ (A = Ca, Sr, Ba).     

The red-emitting phosphors of Mn4+-activated A2MgWO6 (A = Ba,Sr, Ca) for light emitting diodes

A series of Mn⁴⁺ ions activated A₂MgWO₆ (A = Ba, Sr, Ca) phosphors, showing bright red emission peaks appeared around 700 nm under the excitation of 355 nm, were synthesized by the solid-state reaction. The crystal structures and photo-luminescent (PL) properties of these synthesized phosphors were deeply investigated with the aids of X-ray diffraction measurement (XRD), and the temperature dependent PL/decay curves in detail. The optimum doping concentration of Mn⁴⁺ ions in A₂MgWO₆ (A = Ba, Sr, Ca) lattices were studied through the relationship between the Mn⁴⁺ ions doping concentrations and the luminescent intensities. The thermal stability of the synthesized red-emitting phosphors was checked based on the temperature-dependent PL intensities ranging from 7 to 510 K. Comparative studies of the luminescent properties for Mn⁴⁺ ions in isostructural A₂MgWO₆ (A = Ba, Sr, Ca) lattice with double perovskite structure were studied. The results indicate that the synthesized red-emitting phosphors are the ideal choice for white light emitting diodes (W-LEDs).     

Zirconium Incorporation into CaTiO3 Perovskite Prepared from Xerogels and Implication for the Fate of (Ca,Sr)TiO3 Nuclear Waste Ceramics

CaTiO₃ perovskite has been proposed as a ceramic waste form for immobilization of ⁹⁰Sr. Nonradioactive coprecipitated xerogel powders with nominal atomic ratios of Ca:Zr:Ti = 0.75:0.25:1.00 were synthesized to mimic the fate of (Ca_0.75⁹⁰Sr_0.25)TiO₃ solid solution after complete decay of the Sr and its intermediate product Y to stable Zr when an excess B⁴⁺ (Ti and ⁹⁰Zr) cations will present. Ca:Ti = 1.00:1.00 samples were used as a reference. The powders were heated to various conditions to explore the thermodynamic stability of its oxides. The heated Ca:Zr:Ti = 0.75:0.25:1.00 samples formed a major orthorhombic Ca(Zr_1?xTi_x)O₃ perovskite phase. The Ti/(Ti + Zr) ratio of the perovskite preserves its nominal ratio at 600°C. The Zr rejects from the Ca(Zr_1?xTi_x)O₃ with further increasing the temperature, following the formation of Ca–Ti–Zr–O secondary phases. This study indicates a tendency of the Zr to segregate from an original (Ca,Sr)TiO₃ waste form when the stoichiometry is controlled by the conversion of Sr to Zr (in normal oxidation states).     

Short-range dissolution–precipitation crystallization of hydrothermal barium titanate

A modified autoclave was used to investigate the crystallization mechanism of BaTiO₃ during the hydrothermal reaction of Ba(OH)₂ and TiO₂ anatase. An uneven distribution of the crystallized BaTiO₃ particles was observed: more than 99 wt.% of total BaTiO₃ particles remained where the precursor TiO₂ was put; less than 1 wt.% was collected from the areas away from the TiO₂. According to the experimental observations in this work and proofs reported in the literature, we propose that the crystallization mechanism is dissolution–precipitation in nature, but the soluble Ti⁴⁺ species can only redisperse in a short distance away from TiO₂ particles before precipitation. In other words, the nucleation of hydrothermal BaTiO₃ starts at a low concentration of Ti⁴⁺. The mechanism of the Ba(OH)₂–Ti(OH)₄ reaction is a fast dehydration process.     

Computational Analysis of Oxide Ion Conduction in Orthorhombic Perovskite Structured La0.9A0.1InO2.95 (A = Ca,Sr and Ba)

Oxide ion conduction in orthorhombic perovskite structured oxides, La_0.9A_0.1InO_2.95 (A = Ca, Sr and Ba) is analyzed using molecular dynamics simulation. Factors influencing oxide ion conductivity of the compositions considered are analyzed using radial distribution function, bond energies between dopant and oxide ions, and the diffusion path. It is known that perovskite oxides with smaller ion size mismatch between host and dopant ions have higher electrical conductivities. However, exceptions exist, such as a La_0.9A_0.1InO_2.95 (A = Ca, Sr and Ba) system, where high electrical conductivities occur with large ion size mismatches. Based on this study, a dopant with smaller ion than host ion results in the formation of strong ionic bonds with oxide ions, suggesting that the A‐site dopant should be larger than the host ion for forming weaker O–A bonds. Consequently, the trade‐off between ion size mismatch and O–A bond needs to be considered for enhancing oxide ion conductivity of perovskite oxides.     

Influence of Se and Te substitutions at Tl-site of Tl(Ba,Sr)CaCu2O7 superconductor on the AC susceptibility and electrical properties

The Sr and Ba bearing Tl-1212 phase, Tl(Ba,Sr)CaCu₂O₇ is an interesting superconductor. The Sr only bearing TlSr₂CaCu₂O₇ is not easily prepared in the superconducting form. The Ba only bearing TlBa₂CaCu₂O₇ on the other hand does not show improvement in the transition temperature with elemental substitution. In this work the influence of multivalent Se (non-metal) and Te (metalloid) substitutions at the Tl-site of Tl_1-xM_x(Ba,Sr)CaCu₂O₇ (M = Se or Te) superconductors for x = 0–0.6 was studied. The samples were prepared via the conventional solid-state reaction method. XRD patterns showed a single Tl-1212 phase for x = 0 and 0.1 Se substituted samples. The critical current density at the peak temperature, T_p of the imaginary (χ”) part of the AC susceptibility (χ = χ’ +χ”), J_c-inter(T_p) for all samples was between 15 and 21 A cm⁻². The highest superconducting transition temperature was shown by the x = 0.3 Se-substituted sample (T_c-onset = 104 K, T_c-zero = 89 K, T_cχ’ = 104 K and T_p = 80 K). Te suppressed the superconductivity of Tl-1212 phase. The order of highest transition temperatures are as follows: Tl_1-xTe_x(Ba,Sr)CaCu₂O₇<Tl(Ba,Sr)CaCu₂O₇<Tl_1-xSe_x(Ba,Sr)CaCu₂O₇. This work showed that Se was better than Te in improving the transition temperature and flux pinning of the Tl-1212 phase. The roles of ionic radius of Se and Te on the superconductivity of Tl(Ba,Sr)CaCu₂O₇ are discussed in this paper.     

Reactive Hydrothermal Liquid‐Phase Densification (rHLPD) of Ceramics – A Study of the BaTiO3[TiO2] Composite System

A densification process called reactive hydrothermal liquid‐phase densification (rHLPD), based on principles of hydrothermal reaction, infiltration, reactive crystallization, and liquid‐phase sintering, is presented. rHLPD can be used to form monolithic ceramic components at low temperatures. The densification of barium titanate–titania composite monoliths was studied to demonstrate proof of concept for this densification model. Permeable, green titania (anatase) compacts were infiltrated with aqueous barium hydroxide solutions and reacted under hydrothermal conditions in the temperature range 90°C–240°C. The effects of reaction time and temperature on the conversion of titania (anatase) into barium titanate were studied. Utilizing a 72 h reaction at 240°C between l.0 M Ba(OH)₂, an anatase (TiO₂) powder compact, and a corresponding Ba/Ti ratio of 1.5, it was possible to crystallize a composite 95 wt% (88 mol%) BaTiO₃ and 5 wt% (12 mol%) TiO₂. The composite had a relative density of ~90% with a compressive strength of 172 ± 21 MPa and a flexural strength of 49 ± 4 MPa.     

A comparison study on the substitution of Y3+−Al3+ by M2+−Si4+(M = Ba,Sr, Ca,Mg) in Y3Al5O12: Ce3+ phosphor

Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ (M = Ba, Sr, Ca, Mg) phosphors were successfully prepared through a classic solid-state reaction method. The crystal structures, photoluminescence spectra, quantum yields, and thermal stabilities of the phosphors were investigated in detail. The results indicate that all Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ phosphors maintain the crystal structure of garnets. The emission peaks of Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ (M = Ba, Sr, Ca, Mg) phosphors are located at 537, 538, 554, and 565 nm, respectively. A red-shift trend of emission peak is observed with decreasing M radius, which can be ascribed to the increase in the crystal-field splitting in the Ce³⁺ 5d level owing to the co-doping of M²⁺−Si⁴⁺. Under 460 nm excitation, the luminescence quantum yields and thermal stabilities of the Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ phosphors decreased with the decrease of M radius. The IQE of the Y_1.94BaAl₄SiO₁₂:0.06Ce³⁺ phosphor is 92.89%, and the resistance to thermal quenching is improved to be 93.32% at 150°C. In addition, the color shifts of Y_1.94MAl₄SiO₁₂: 0.06Ce³⁺ phosphors with increasing temperature are all tiny, which also demonstrates good resistance to thermal quenching of luminescence. The linear shrinkage of Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ phosphors is significantly improved compared with that of YAG: Ce³⁺, which is expected to generate Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ transparent/translucent ceramics and fabricate high-powder w-LEDs for high-quality solid-state lighting in the future.     

ZnBO-doped (Ba, Sr)TiO3 ceramics for the low-temperature sintering process

ZnBO-doped (Ba, Sr)TiO₃ ceramics were investigated for low-temperature co-fired ceramics (LTCCs) applications. Until now, B₂O₃ and Li₂CO₃ dopants have been commonly employed as the low-temperature sintering aids. In this paper, we suggest ZnBO as an alternative dopant to the B₂O₃ and Li₂CO₃. To reduce the sintering temperature of (Ba, Sr)TiO₃, we have added 1–5 wt.% of ZnBO to (Ba, Sr)TiO₃. ZnBO-doped (Ba, Sr)TiO₃ ceramics were respectively sintered from 750 to 1350 °C by 50 °C to confirm the sintering temperature with different dopant contents. By adding 5 wt.% of ZnBO to the (Ba, Sr)TiO₃ ceramics, the sintering temperature of (Ba, Sr)TiO₃ ceramics can be reduced to 1100 °C. From the XRD analysis, ZnBO-doped (Ba, Sr)TiO₃ has no pyro phase. By adding ZnBO dopants to (Ba, Sr)TiO₃ ceramics, both of relative dielectric permittivity and loss tangent were decreased. From the frequency dispersion of dielectric properties, the relative dielectric permittivity and loss tangent of 5 wt.% ZnBO-doped (Ba, Sr)TiO₃ were 1180 and 3.3 × 10⁻³, while those of BST were 1585 and 4.8 × 10⁻³, respectively.     

Stoichiometry control and structure evolution in hydrothermally derived (Ba,Sr)TiO3 films

(Ba,Sr)TiO₃ films were synthesized on the titanium metal substrates in solution of Ba(OH)₂ and Sr(OH)₂ by hydrothermal method. Crystallinity and microstructure of the films changed with time, concentration and temperature. Effects of the mole ratio of barium and strontium in solution on the composition of film have been studied. The barium contents in the BST films are fairly lower than those in the original solutions. This indicates that strontium is more readily incorporated into the BST films, relative to barium. The results of narrow-scan of XPS spectrum confirm that the valences of Ba, Sr, Ti and O elements of hydrothermally prepared BST films are +2, +2, +4, and −2, respectively. SEM photographs show that the BST films are dense and well-compact. AFM analyses show that the average surface roughness of the films is 40–50 nm. It is concluded that BST films of different mole ratio of barium and strontium with thickness of up to 2 μm have been prepared successively by the environmentally benign hydrothermal method.     

Thermodynamics of Nanoscale Calcium and Strontium Titanate Perovskites

The surface enthalpies of nanocrystalline CaTiO₃ and SrTiO₃ perovskites were determined using high‐temperature oxide melt solution calorimetry in conjunction with water adsorption calorimetry. The nanocrystalline samples were synthesized by a hydrothermal method and characterized using powder X‐ray diffraction, FTIR spectroscopy, and Brunauer–Emmett–Teller surface area measurements. The integral heats of water vapor adsorption on the surfaces of nanocrystalline CaTiO₃ and SrTiO₃ are ?78.63 ± 4.71 kJ/mol and ?69.97 ± 4.43 kJ/mol, respectively. The energies of the hydrous and anhydrous surfaces are 2.49 ± 0.12 J/m² and 2.79 ± 0.13 J/m² for CaTiO₃ and 2.55 ± 0.15 J/m² and 2.85 ± 0.15 J/m² for SrTiO₃, respectively. The stability of the perovskite compounds in this study is discussed according to the lattice energy and tolerance factor approach. The energetics of different perovskites suggest that the formation enthalpy becomes more exothermic and surface energy increases with an increase in ionic radius of the “A” site cation (Ca, Sr, and Ba), or with the tolerance factor. PbTiO₃ shows a lower surface energy, weaker water binding, and a less exothermic enthalpy of formation than the alkaline‐earth perovskites.     

Cyclic voltammetric studies of nickel hydroxide and cobalt hydroxide thin films in alkali and alkaline earth metal hydroxides

Cyclic voltammetric studies of thin films of electrosynthesized (ES)-Ni(OH)₂ and Co(OH)₂ in different alkaline electrolytes suggest that the mechanism of oxidation is different from the mechanism of reduction. While the metal ion (alkali or alkaline earth metal) intercalation-deintercalation from the electrolyte into the film provides the driving force, the reduction reaction takes place heterogeneously independent of the electrolyte concentration, whereas oxidation takes place homogeneously across a nebulously defined electrode-electrolyte interphase. ES-Ni(OH)₂ permits facile intercalationdeintercalation of alkali metal ions Li⁺, Na⁺ and K⁺, irrespective of their ionic size, while the reactions of ES-Co(OH)₂ are sensitive to ionic size, requiring larger potentials in KOH compared to LiOH. In the alkaline earth metal hydroxides, both Ni(OH)₂ and Co(OH)₂ films show greater reversible characteristics in the order Ba(OH)₂ > Sr(OH)₂ > Ca(OH)₂. This may be trivially related to the order of the solubilities of the three hydroxides.     

Effect of Variations in Annealing Temperature and Metallic Cations on Nanostructured Molybdate Thin Films

Crystalline molybdate thin films were prepared by the complex polymerization method. The AMoO₄ (A = Ca, Sr, Ba) films were deposited onto Si wafers by the spinning technique. The Mo–O bond in the AMoO₄ structure was confirmed by FTIR spectra. X-ray diffraction revealed the presence of crystalline scheelite-type phase. The mass, size, and basicity of A²⁺ cations was found to be dependent on the intrinsic characteristics of the materials. The grain size increased in the following order: CaMoO₄ < SrMoO₄ < BaMoO₄. The emission band wavelength was detected at around 576 nm. Our findings suggest that the material’s morphology and photoluminescence were both affected by the variations in cations (Ca, Sr, or Ba) and in the thermal treatment.     

Single-phase (Ba,Ca)2ScAlO5 impregnant for dispenser cathodes with enhanced thermionic emission performance

(Ba,Ca)₂ScAlO₅ compounds are synthesized and improved emission performance is achieved by virtue of their single-phase structure and low work function. The substitution of Ca²⁺ for Ba²⁺ effectively lowers the melting points of (Ba,Ca)₂ScAlO₅ from over 1800 °C–1700 °C when the Ca²⁺ substitution reaches 20 at%. During sintering, the co-precipitation precursor first reacts to form (Ba,Ca)₃Al₂O₆ near 912 °C before reacting with BaCO₃ and Sc₂O₃ at the higher temperature of 1186 °C to generate (Ba,Ca)₂ScAlO₅. Emission tests reveal that the maximum full space charge current density is 9.6 A cm^?2 at 1000 °C_B (brightness temperature) for the cathode impregnated with single-phase (Ba,Ca)₂ScAlO₅. Low work function (~1.5 eV) and good emission uniformity make such a cathode promising for applications in high-power microwave devices.     

Synthesis and characterization of bioinspired nano-hydroxyapatite by wet chemical precipitation

In this work, nanometric hydroxyapatite was added with mineral traces naturally present in mammalian bones. It was synthesized by wet chemical precipitation using Ca(OH)₂ obtained from eggshells as a precursor. Subsequent calcination was performed at a low temperature of 250 °C to remove reaction by-products but avoiding the crystal size transition to micrometric sizes. Synthetic hydroxyapatite crystals exhibited irregular-spherical morphologies with a Feret's diameter ranging from 10 to 90 nm. Most importantly, this hydroxyapatite contains levels of Ca, Mg, P, Na, S, Sr, K, Si, Fe, Al, and Ba, close to those reported for mammalian bones. The analysis of the commercial synthetic hydroxyapatite revealed that its crystals form micrometric-like-flakes clusters that do not have additional ions but only Ca and P.     

Chromium poisoning for prolonged lifetime of electrodes in solid oxide fuel cells - Review

Solid Oxide Fuel Cells (SOFCs) offer low carbon emission and high efficient energy conversion systems. For the wide commercial distribution of this system, one of the technological issues and challenges is prolonged durability: the SOFC systems should have a long lifetime of more than 10 years. The volatile chromium species poisoning is the key degradation factor to overcome at the functional ceramics of air electrode (cathode)/interlayer/electrolyte interfaces in the SOFC system among many degradation factors. This paper reports recent degradation mechanisms, especially on the chromium (Cr) vapors poisoning at the perovskite oxide cathode. The Cr-concentration levels at cathodes were evaluated from the reported data at small cells and practical cell-stacks. The interactions of volatile Cr species and perovskite oxide cathode surface were evaluated by the chemical reaction of cathode materials with Cr-vapors to form SrCrO₄ and the electrochemical induced Cr-vapors reduction (Cr⁶⁺ to Cr³⁺) to form Cr₂O₃ at (La,Sr,Ca)MnO₃-based and (La,Sr)(Co,Fe)O₃-based materials. Recovery mechanism from Cr-poisoning was reanalyzed at the (La,Sr)(Co,Fe)O₃/ceria-based interlayer/YSZ electrolyte interfaces by Cr-cleaning reaction with the evaporation of Cr₂O₃/SrCrO₄ and nano-meter level cation migration/rearrangement effects with phase separation and new phases formation. This paper is covering not only the elucidation of degradation mechanism but also the fundamentals of physical and chemical analyses on perovskite oxide cathode surface and interfaces. An insight for new materials combination for the next-generation SOFCs is also included.     

Alkaline earth metal oxides as solid bases for ligand-free Heck reaction

Alkaline earth metal oxides (MO, M = Mg, Ca, Sr and Ba) are investigated as solid bases for ligand-free Heck reaction (Pd concentration: 0.05 mol%). SrO is efficient to promote Heck arylation of styrene with bromobenzene (BB). However, aryl amination of BB to form N,N-dimethylaniline is also observed with SrO. CaO-n particles (n: calcination temperature) are prepared by calcination of dried Ca(NO₃)₂ aqueous solution at 700 or 900 °C. The CaO-n particles outperform other alkaline earth metal oxides considering both BB conversion and selectivity to Heck products. The CaO-900 particles show better performance than the CaO-700 particles. This is attributed to their platelet morphology with sharp edges and corners and the preferential growth along the (1 1 1) crystal plane.     

Formation energies and site preference of substitutional divalent cations in carbonated apatite

First-principles calculations were performed to examine defect formation energies and site preference of substitutional divalent cations M²⁺ (M = Mg, Cu, Zn, Cd, Sr, Pb, and Ba) in hydroxyapatite (HAp, Ca₁₀(PO₄)₆(OH)₂) and carbonated apatite (CAp). All inequivalent substitutional sites of and M²⁺ were investigated to determine their most preferential sites. For all M²⁺ studied, their defect formation energies for the most stable substitutional sites were lower in CAp than in hydroxyapatite (HAp), demonstrating that M²⁺ are preferentially substituted into CAp over HAp. For Ca sites in CAp, correlations between the defect formation energies and Ca-O bond lengths showed that bigger and smaller M²⁺ than Ca²⁺ are preferentially substituted for Ca sites with longer and shorter bond lengths than those in HAp, respectively. In addition, Ca sites with lower coordination numbers than 6 are preferentially substituted by Zn²⁺ and Cu²⁺ that originally tend to form 4- or 5-fold coordination in their phosphate crystals. substitution is therefore likely to effectively stabilize substitutional foreign ions by modifying bond lengths and coordination numbers of Ca sites from those in pure HAp. These effects may play an important role in enhancing the M²⁺ solubility into CAp.     

Dielectric Relaxation and Collective Vibrational Modes of Double‐Perovskites A2SmTaO6 (A = Ba,Sr and Ca)

The crystal structures of the double‐perovskite oxide A₂SmTaO₆ series (AST, A = Ba, Sr, Ca) synthesized by solid‐state reaction technique are determined from X‐ray powder diffraction data. Rietveld refinements of the X‐ray diffraction data of the samples show that Ba₂SmTaO₆ (BST) crystallizes in cubic phase and Sr₂SmTaO₆ (SST) and Ca₂SmTaO₆ (CST) crystallize in monoclinic phase. Fourier transform infrared spectra show two primary modes of the samples at around 370 and 600 cm^?1. The vibrational properties of the samples are studied by Raman spectroscopy taken at 488‐nm wavelength. Group‐theoretical study is performed to assign the different vibrational modes of the samples in accordance with structural symmetry. The observed shifts of some vibrations in the SST and CST w.r.t. BST upon changing the A cation are tentatively explained. Dielectric spectroscopy is applied to investigate the ac electrical conductivity of AST in different temperatures between 303 and 673 K and in a frequency range of 42 Hz–1 MHz. The complex impedance plane plots show that the relaxation (conduction) mechanism in these materials is purely a bulk effect arising from the semiconductive grains. The relaxation mechanism of the samples is modeled by Cole–Cole equation. The frequency‐dependent conductivity spectra are found to follow the power law.     

First-principles study of phase-transition temperature and optical properties of alkaline earth metal (Be,Mg, Ca,Sr or Ba)-doped VO2

Vanadium dioxide (VO₂) is an attractive material for energy-saving smart windows due to its metal-to-insulator reversible phase transition near ambient temperature, accompanied by large changes in its optical properties. We conducted first-principles calculations to study the phase-transition temperature and optical properties of alkaline earth metal (Be, Mg, Ca, Sr or Ba)-doped VO₂. The results show that the Be atom prefers to locate at the octahedral interstitial site, while Mg, Ca, Sr and Ba atoms prefer to substitute for the V atom in VO₂. Be, Mg, Ca, Sr and Ba doping reduces the phase-transition temperature of VO₂ 0by 51.4, 59.7, 61.5, 58.4 and 58.3?K, respectively, when the doping concentration is set at one atomic percentage. In addition, the introduction of alkaline earth metal scales the band structures of VO₂, which enhances the ability to block the infrared light (in the order of Be > Mg > Ca > Sr > Ba) and promotes the transmission of visible light (in the order of Be > Mg ≈ Ca > Sr > Ba).