Microwave dielectric properties of scheelite (A = Ca,Sr, Ba) and wolframite (A = Mg,Zn, Mn) AMoO4 compounds

The microwave dielectric properties of scheelite (A = Ca, Sr, Ba) and wolframite (A = Mg, Mn, Zn) AMoO₄ compounds and their relations with structure have been examined using a network analyzer and X-ray powder diffraction. The Mo ion polarizability has been also investigated from AMoO₄ compounds using a least square refinement technique in conjunction with the Clausius–Mosotti equation. It was found that dielectric properties such as dielectric constant, temperature coefficient of resonant frequency, and quality factor were found to be correlated with the size of A-cations and the structure of compounds. The well sintered AMoO₄ samples (>95% of theoretical density) exhibited dielectric constant of 7–11, quality factor of 37,000–90,000 GHz and temperature coefficient of resonant frequency of −57 to −87 ppm/°C, respectively. These investigations showed that AMoO₄ ceramic could be selected as a possible candidate for microwave dielectric ceramics because of its low dielectric constant and high quality factor.     

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).     

Novel AMoO4:Eu3+ (A = Ca and Ba) optical thermometer: Investigation of effect of local ionic coordination environment on optical performance and temperature measurement sensitivity

A range of Eu³⁺-doped AMoO₄ (A = Ca and Ba) phosphors were successfully synthetized, and their crystal structures, optical performance, and temperature measurement sensitivities were investigated in detail. Peak doping concentration of CaMoO₄:Eu³⁺ phosphor was 0.18, while peak doping concentration of BaMoO₄:Eu³⁺ phosphor may be greater than 0.18. Then, temperature-dependent photoluminescence emission spectra of representative CaMoO₄:0.09Eu³⁺ and BaMoO₄:0.03Eu³⁺ phosphors were recorded. CaMoO₄:0.09Eu³⁺ phosphor exhibited abnormal thermal quenching, which was attributed to defects caused by heterovalent substitution of ions and increase in the temperature, and good thermal stability. Finally, the possibility of using both phosphors as optical thermometers was discussed, which exhibited good temperature sensitivity. However, CaMoO₄:0.09Eu³⁺ phosphor exhibited two peak absolute (Sa, 1.28 %K⁻¹ and 1.39 %K⁻¹) and relative sensitivities (Sr, 1.21 %K⁻¹ and 1.20 %K⁻¹). In addition, variation trend of Sr value with temperature was considerably peculiar. Two optimum Sa and Sr values were attributed to abnormal thermal quenching of CaMoO₄:0.09Eu³⁺ phosphor. Peak Sa and Sr values of BaMoO₄:0.03Eu³⁺ phosphor was 12.39 %K⁻¹ and 0.89 %K⁻¹, respectively. In addition, Sa of AMoO₄:Eu³⁺ phosphor was negatively related to Eu³⁺ central asymmetry, while peak Sr value was more inclined to appropriate ionic central asymmetry.     

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).     

Cation Field Strength Effects on Boron Coordination in Binary Borate Glasses

The field strength of modifier cations in boron‐containing oxide glasses has important but complex effects on boron coordination, and has long been known to have major effects on glass and liquid properties. With well‐constrained compositional and fictive temperature information in three binary borate glass series, we report how different modifier cations (Na⁺, Ba²⁺, Ca²⁺) affect boron coordination (¹¹B MAS NMR), as well as glass transition temperatures and configurational heat capacities (DSC). Using estimated reaction enthalpies for converting a ^[4]B to a ^[3]B with an NBO from previous studies, we compare boron coordinations in glasses with different modifier cations on an isothermal basis. Temperature and modifier cation effects can thus be isolated. At low modifier contents [R = (Na₂,Ca,Ba)O/B₂O₃<0.45], N₄ is systematically higher in the order Na>Ba>Ca, suggesting the enhanced stabilization of NBO for the divalent cations, especially for the smaller Ca²⁺. At higher R values, N₄ for Na borates drops below values for Ca and Ba borates. The trend in N₄ with modifier field strength reverses at high R values (~ R > 0.7), with Ca > Ba > Na. The transition may be related to the enhanced stabilization of ^[4]B‐O‐^[4]B groups by higher field strength cations in NBO‐rich glasses in which boron is the primary network component.     

Hot-corrosion behavior of calcium-magnesium aluminosilicate melts on Ba1/3Sr1/3Ca1/3Al2Si2O8

Hot-corrosion behavior of Ba_1/3Sr_1/3Ca_1/3Al₂Si₂O₈ (BSCAS) in the presence of molten calcium-magnesium-aluminum-silicate (CMAS) is investigated in the temperature range of 1250–1350 °C. In comparison, the hot corrosion behavior of Ba_0.5Sr_0.5Al₂Si₂O₈ (BSAS) is also studied under the same conditions. The results indicate that CMAS corrosion of both BSCAS and BSAS is caused by the interdiffusion of Ba/Sr and Ca between CMAS and corroded samples. The presence of Ca cations in BSCAS lowers the diffusion driving force of Ca cations between CMAS and BSCAS, resulting in a reduced diffusion rate of Ca cations from CMAS into BSCAS. Moreover, the sluggish diffusion effect of multi-component cations hinders the outward diffusion of Ba/Sr cations from BSCAS. Thus, the BSCAS shows a better CMAS corrosion resistance than BSAS.     

Investigation of the relations between structure and microwave dielectric properties of divalent metal tungstate compounds

The microwave dielectric characteristics of AWO₄ (A = Mg, Zn, Mn, Ca, Sr, Ba) compounds and their relations with structure have been investigated as a function of A cations. Crystal structure was investigated by XRD; the structure changed from wolframite to scheelite with increasing size of A cation. Dielectric properties such as permittivity, temperature coefficient of resonant frequency (τ_f), and quality factors (Q × f) were found to be correlated with the size of A cations and the structure of compounds. Permittivity decreased with the size of A cations, and temperature coefficient of resonant frequency was correlated with unit cell volume. AWO₄ compounds are suitable for the applications of low-k dielectric materials; permittivity = 8–15, τ_f = −50 to −70, and Q × f = 60,000–70,000.     

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).     

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.     

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.     

Some aspects of glass formation in R′2O−P2O5 and R″O−P2O5 systems

The limits of the glass-formation regions are established for the binary R′₂−P₂O₅ and R″O₅−P₂O₅ systems (R′=Li, Na, K: R″=Mg, Ca, Sr, Ba). It is shown that these boundaries closely correlate with those reported by other investigators despite the difference in the conditions in which these binary phosphate glasses were synthesized. The fact that all of the systems under study have about the same upper limits of glass-formation regions is attributed to the ratio R′₂O(R′O): P₂O₅, which for the systems in question is 1.0–1.5 rather than the individual crystal-chemical characteristics of the R⁺ and R²⁺ cations. Translated from Steklo i Keramika, No. 2, pp. 13–15 February, 1997.     

Expanding gaudefroyite family to Sr2MBi(REEO)3(BO3)4 (M = Ca,Sr, Ba; REE = Y,Eu) borates with large second harmonic generation responses

The synthesis, crystal structure, and characterization of a few new noncentrosymmetric rare-earth borates of gaudefroyite family Sr₂MBi(REEO)₃(BO₃)₄ (M = Ca, Sr, Ba; REE = Y, Eu) are represented. Samples of Sr₂MBi(YO)₃(BO₃)₄ (M = Ca, Sr, Ba) borates were synthesized by multi-step solid state reactions. The crystal structures of Sr₃Bi(YO)₃(BO₃)₄, Sr₂CaBi(YO)₃(BO₃)₄ and Sr₂BaBi(YO)₃(BO₃)₄ were refined in P6₃ space group to R₁ 0.059, 0.058 and 0.018, respectively. Second-harmonic generation measured with Kurtz-Perry powder technique shows an increase in the nonlinear optical activity of materials by almost two times upon partial substitutions of Sr for Ca and Ba in the Sr₃Bi(REEO)₃(BO₃)₄ and its decrease if Y atoms are replaced by Eu. Thermal expansion coefficients for Sr₂MBi(YO)₃(BO₃)₄ (M = Ca, Sr, Ba) borates and Ca₄(MnO)₃(BO₃)₃(CO₃) gaudefroyite are calculated for the first time. Thermal expansion of these compounds is almost isotropic due to the arrangement of BO₃ units which are located in the ab and ac planes.     

Growth and microstructure of Ba β-alumina films by laser chemical vapor deposition

Ba β-alumina films were prepared by laser chemical vapor deposition. Mostly single-phase Ba β-alumina films were obtained at 1125–1200 K and for an Al/Ba molar ratio of 12.4–16.6. BaAl₂O₄ and α-Al₂O₃ were codeposited with Ba β-alumina under Ba- and Al-rich conditions, respectively. The Ba β-alumina films consisted of hexagonal grains, and the (1 1 0)-oriented Ba β-alumina films had a fin-like columnar structure. The highest deposition rate reached 120 μm h^?1 at around 1200 K. A thin layer of Ba-rich superstructure was formed on the surface of the (1 1 0)-oriented columnar grains.     

Sol-gel auto-combustion synthesis of Ba–Sr hexaferrite ceramic powders

We report the mechanism involved in sol-gel auto-combustion synthesis of Ba–Sr-hexaferrite (Ba_1-xSr_xFe₁₂O₁₉; x = 0, 0.25, 0.5, 0.75 and 1, BSFO) ceramic powders through the analysis of the phases evolved during annealing of the as-synthesized powders, along with their structure and morphological studies. The XRD patterns of the as-synthesized samples indicate the formation of barium/strontium monoferrite ((Ba/Sr)Fe₂O₄) and maghemite (γ-Fe₂O₃) phases along with a minute amount of hematite (α-Fe₂O₃) phase. Annealing of these samples facilitates formation of BSFO phase through the solid state reaction between BaFe₂O₄ and γ-Fe₂O₃ phase. Interestingly, after annealing the samples with x = 0, 0.5 and 1, at 1000 °C for 2 h, we observed that phase pure Ba–Sr hexaferrite structure forms, but for samples with x = 0.25 and 0.75, high amount of hematite (α-Fe₂O₃) phase is observed, especially for x = 0.75. The reason associated with this could be the large difference between the ionic radii of Ba²⁺ and Sr²⁺ ions occupying the oxygen site. Furthermore, our study on annealing dependent phase evolution confirms that, this difference in ionic radii forbids the formation of a single phase Ba–Sr hexaferrite. The growth of clear hexagonal-shaped plate-like particles with varied particle sizes was observed for all the samples. The particle size variation may be due to the influence of the ionic radii difference on the sinterability of the samples. Our study provides a better understanding of synthesis mechanism of Ba–Sr hexaferrite samples.     

Effect of crystal structures on nucleation mechanism in hydrothermal synthesis of MZrO3 (M=Ba,Sr, Ca)

Hydrothermal method is widely used in the synthesis of perovskite-type oxides, whereas few studies are reported for the nucleation mechanism, so that the relationship between the crystal structures and reactive activities of reactants and products is still unknown. Herein, the reaction processes are analyzed on the basis of XRD, SEM and Raman characterizations, and the nucleation mechanism is investigated for the hydrothermal synthesis of MZrO₃ (M = Ba, Sr, Ca). We propose that the negative charged cyclic tetramer complexes [Zr₄(OH)₈(OH)₁₆]^8- form in the hydrothermal reaction, which play major roles in the nucleation process. The tetramer complexes continually dehydrate and condensate to form substructural units composed of alkali-earth ions and 6-fold Zr tetramers; substructural units further dehydrate and distort to form perovskite structures. The reactive activation energy increases with the decreasing of M²⁺ (M = Ba, Sr, Ca) ionic radius because the incorporation of smaller A site ions in the perovskite structure is accompanied by greater rotation and distortion of the ZrO₆ octahedra, leading to the decrease of reactive activity accordingly. In a word, the proposed nucleation mechanism in this paper is of great significance for the study of perovskite.     

Photochemical synthesis of AZrO3−X thin films (A=Ba,Ca and Sr) and their characterization

(Ba, Ca, Sr)ZrO_3−X thin films were prepared by a photochemical method using thin films of β-diketonate complexes as precursors. The photolysis of these films induces the fragmentation of the 2,2,6,6-tetramethyl-3,5-heptanedionate ligand and the partial reduction of metal ion together with volatile organic compounds as sub-products. When the metallic complexes are irradiated in air, the final product of the reaction are ternary metal oxides. The photoreactivity of these films was monitored by FT-IR spectroscopy by a period of 72 h, followed by post-annealing at 950 °C. X-ray photoelectron spectroscopy and X-ray diffraction techniques were used to analyze the chemical composition and the crystalline structure of the films obtained. The results indicate that Ba, Ca, Sr, Zr and O are present in the form of perovskita, preferably adopting an amorphous structure. The surface morphology examined by atomic force microscopy revealed a rough and irregular surface. The UV–vis measurements suggest a slight increase in the optical band gap values, which promote a reduction of the intermediary energy levels or defects.     

Single crystal structure of fully dehydrated partially Co-exchanged zeolite X: Comparison with partially dehydrated partially Co-exchanged zeolites X

The crystal structure of the fully dehydrated Co, Na–X zeolite (Na₁₆Co₃₈Al₉₂Si₁₀₀O₃₈₄) is investigated and compared to that of three partially hydrated zeolites. In the fully dehydrated crystal, cobalt ions almost entirely occupy Site I, and partially Site II while the residual sodium cations partly occupy Site II and Site III. The dehydration induces a migration of Co²⁺ cations from Site I′ to Site I and severely affects the structure, the structural strain being released by changes of framework valence angles or, to a less extent, by distortions of TO₄ tetrahedra. Comparison with dehydrated M–X zeolite structures (M = Ba, Na, Ca, Tl, Li) confirms the relation between the size of the charge compensating cation and the stresses on the skeleton that are mainly explained by cation–oxygen electrostatic interactions.     

Effects of non-stoichiometric defects on the dielectric properties of (Ba0.5Sr0.5)xTiO3-ZnGa2O4 composite ceramics

In order to explore the effects of non-stoichiometric defects on the dielectric properties of composite ceramics, 70 wt% (Ba_0.5Sr_0.5)_xTiO₃-30 wt%ZnGa₂O₄ ((BS)_xT50-ZG, x = (Ba + Sr)/Ti = 0.99, 1.00, 1.01 and 1.05) composite ceramics were fabricated by the traditional sintering technique. The association between structure and dielectric properties has been studied. The results show that the distortion of the crystal lattice brought by the partial Schottky defects, namely [V″_Ba,Sr–V^˙˙_O]^× and [V′′′′_Ti–2V^˙˙_O]^×, induces a decrease in the Curie temperature of (BS)_xT50-ZG composite ceramics. The orientation of the elastic dipoles brought by oxygen vacancies causes the pinning of the domain walls, which reduces the dielectric loss at low frequencies. Tunability is related to the dipole polarization caused by [V″_Ba,Sr–V^˙˙_O]^× and [V′′′′_Ti–2V^˙˙_O]^× defect complexes. In addition, compared with the composite ceramic with x = 1, the Q values of the composite ceramics with x < 1 and x > 1 decreases due to the deterioration of the microstructure homogeneity and the enhancement of the disorder of the B-site cations in (BS)_xT50.     

Microwave dielectric properties and crystal structure of homologous compounds ALa4Ti4O15 (A = Ba,Sr and Ca) for base station applications

Homologous compounds ALa₄Ti₄O₁₅ (A = Ba, Sr and Ca) with high dielectric constant ϵ_r and quality factor Q × f are candidate materials for dielectric resonators in base station of telecommunication systems. We have investigated the relationship between the microwave dielectric properties and crystal structure of these new materials. Single crystals of CaLa₄Ti₄O₁₅ were synthesized to analyze the crystal structure precisely. The ceramic discs of the ALa₄Ti₄O₁₅ (A = Ba, Sr and Ca) were also synthesized and the microwave dielectric properties were measured. Ba-analogy showed the highest ϵ_r of 44.4 due to the large cationic movement. Ca-analogy showed the highest Q × f of 50,246 GHz due to resemblance in ionic radius between Ca²⁺ (r = 1.34 Å: 12-coordination) and La³⁺ (r = 1.36 Å: 12-coordination) ions. Sr-analogy showed near zero temperature coefficient of resonant frequency τ_f of −8.4 ppm/°C compared with the others. The relationships between their crystal structures and properties were discussed.     

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.