Microwave dielectric properties and crystal structures of Mg0.7Al2.2O4 and Mg0.4Al2.4O4 ceramics with defect structures

Mg_0.7Al_2.2O₄ and Mg_0.4Al_2.4O₄ ceramics with cation vacancies were synthesized using the molten salt method, and the relationships between the microwave dielectric properties and crystal structures of these materials were investigated. The ²⁷Al NMR spectra of these ceramics indicate that the preferential occupation of tetrahedral sites by Al³⁺ cations was enhanced by the introduction of cation vacancies. The λ values of Mg_0.7Al_2.2O₄ and Mg_0.4Al_2.4O₄ ceramics fired at 1600°C, which correspond to the fraction of Al³⁺ cations in tetrahedral sites, were 0.37 and 0.60, respectively. Crystal structure refinements using the Rietveld method suggest that cation vacancies are preferentially located at octahedral sites in both ceramics. The ε_r and Q·f values of a Mg_0.7Al_2.2O₄ ceramic fired at 1600°C were 7.7 and 201 111 GHz, respectively, while those of a Mg_0.4Al_2.4O₄ ceramic fired at 1600°C were 7.5 and 232 301 GHz, respectively. These data demonstrate that the preferential occupation of tetrahedral sites by Al³⁺ cations following the introduction of cation vacancies enhances the Q·f value.     

Diffraction,structure and phase stability studies on aluminium titanate

The crystal structure studies of aluminium titanate (Al₂TiO₅) in the temperature range of 25–1200 °C have been investigated by X-ray and neutron powder diffraction. The use of simultaneous refinement of the X-ray and neutron data has enhanced the refinement precision of the structure model of Al₂TiO₅ with the concomitant improvement in values such as atomic coordinates, site occupancies and unit-cell parameters. Results from Rietveld refinements showed that the disordering of Ti and Al atoms in the metal 1 and metal 2 sites. High-temperature neutron diffraction studies of the Al₂TiO₅ crystal structure revealed an increasing substitution of Al for the Ti atoms in the crystal structure. It is proposed that this increased substitution is responsible for the decomposition of Al₂TiO₅ that is observed between 900 and 1280 °C.     

Study of the changes in the structure and optical properties of MgxCo1-xCr2-yAlyO4 spinel caused by A/B site ion substitution

In this work, nanosubmicron blue-green pigment powder based on the composition of Mg_xCo_1-xCr_2-yAl_yO₄（0 = x ≤ 1, 0 = y ≤ 2）was prepared by a gel casting method. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Rietveld refinement with GSAS (General Structure Analysis System), and UV–Vis absorption spectroscopy were used to study the phase composition, grain size, morphology, cation distribution in the crystal structure and spectral absorption of the samples. Colour parameters were also studied by using a colour measurement spectrophotometer. The studies demonstrate that the distribution of cations in the crystal structure is disordered and that divalent and trivalent cations are mixed to occupy tetrahedral and octahedral sites. Furthermore, the substitution of ions at the A/B site leads to a change in the cation distribution ratio at the tetrahedral and octahedral sites. With increasing Mg²⁺ doping concentration, the inversion parameter of the spinel increases, while with increasing Al³⁺ doping concentration, the inversion parameter of the spinel decreases. In addition, changes in the calcining atmosphere lead to a change in the oxygen vacancy content in the structure. Under the condition of a reductive atmosphere, the oxygen vacancy content significantly increases, and the inversion parameter also increases. The colour difference for the synthesized Mg_xCo_1-xCr_2-yAl_yO₄ spinel powder is related to the proportion of chromophore ions occupying tetrahedral and octahedral sites and the number of oxygen vacancies.     

Preparation and cycling performance of Aland F co-substituted compounds Li4AlxTi5−xFyO12−y

Li₄Al_xTi_5−xF_yO_12−y compounds were prepared by a solid-state reaction method. Phase analyses demonstrated that both Al³⁺ and F⁻ ions entered the structure of spinel-type Li₄Ti₅O₁₂. Charge-discharge cycling results at a constant current density of 0.15 mA cm⁻² between the cut-off voltages of 2.5 and 0.5 V showed that the Al³⁺ and F⁻ substitutions improved the first total discharge capacity of Li₄Ti₅O₁₂. However, Al³⁺ substitution greatly increased the reversible capacity and cycling stability of Li₄Ti₅O₁₂ while F⁻ substitution decreased its reversible capacity and cycling stability slightly. The electrochemical performance of the Al³⁺-F⁻-co-substituted specimen was better than the F⁻-substituted one but worse than the Al³⁺-substituted one.     

Microwave dielectric properties and cation distributions of Zn1-3xAl2+2xO4 ceramics with defect structures

The spinel-structured Zn_1-3xAl_2+2xO₄ (x = 0–0.2) ceramics having defective structures were synthesized using the molten salt method, and their microwave dielectric properties and cation distributions were assessed. The ²⁷Al solid-state nuclear magnetic resonance spectra of these ceramics demonstrate that they have an intermediate spinel structure in which the tetrahedral site occupancy increases from 0.03 to 0.64 as x increases. Moreover, crystal structure refinements suggest that cation vacancies are located at octahedral sites for x = 0.1 and 0.2. Based on these data, the introduction of cation vacancies at octahedral sites appears to enhance the preferential occupation of tetrahedral sites by Al³⁺. The ε_r of these ceramics slightly decreased from 8.5 to 8.2 with increasing x, while the Q·f value increased significantly, from 127,532 to 202,468 GHz, upon the introduction of cation vacancies. An intermediate spinel structure with preferential occupancy of tetrahedral sites by trivalent cations exhibits an enhanced Q·f value.     

Layered monodiphosphate Li9V3(P2O7)3(PO4)2: A novel cathode material for lithium-ion batteries

Single phase Li₉V₃(P₂O₇)₃(PO₄)₂ is synthesized at 750 °C via solid-state reaction method for the first time. The Rietveld refinement results show that the trigonal system (space group: ) with the lattice parameters a = 0.9724 nm, c = 1.3596 nm are obtained. Its intrinsic electrical conductivity of 1.43 × 10⁻⁸ S cm⁻¹ is higher than that of LiFePO₄ and as the same order of Li₃V₂(PO₃)₄. The electrochemical measurement results show that there are two plateaus (3.77 V and 4.51 V) and three plateaus (3.77 V, 4.51 V and 4.75 V) in the potential ranges of 2.0–4.6 V and 2.0–4.8 V, respectively. In the range of 2.0–4.6 V, two discharge plateaus (4.46 V and 3.74 V) can be observed and 110 mAh g⁻¹ of discharge capacity is achieved. The Rietveld refinement result of the X-ray diffraction (XRD) data at the end of discharge after the first cycle suggests that the structural reversibility can be retained during electrochemical reactions in Li₉V₃(P₂O₇)₃(PO₄)_2. In the range of 2.0–4.8 V, almost six lithium ions are extracted and the trigonal structure is still recovered after 30 cycles. Therefore, this novel layered vanadium monodiphosphate offers a promising candidate as cathode material for lithium-ion batteries.     

Crystal structure and luminescence mechanism of novel Fe3+-doped Mg0.752Al2.165O4 deep red-emitting phosphors

Nonstoichiometric alumina-rich spinel provides diverse and changeable local environments for transition-metal dopants. In this contribution, novel Mg_0.752Al_2.165−xO₄:xFe³⁺ deep red-emitting phosphors were designed and prepared by the solid-state reaction method. The red emission presents an unexpected shift from 735 to 770 nm by comparing with Fe³⁺-doped MgAl₂O₄. The excitation spectrum of Mg_0.752Al_2.165−xO₄:xFe³⁺ is broadened in the UV region with a new strong peak at 320 nm. The crystal structure refinement and NMR spectra fitting reveal that the cation vacancies and disorder increase with excess Al³⁺ entering the spinel crystal lattice. According to the results of EPR, NMR, and PL/PLE measurements, it was proposed that the Fe³⁺ ions locate at the distorted octahedral coordination. The changes of the local structure of Fe³⁺ ions promote the doublet state's involvement in the d−d transition. It was proposed that the new excitation peak at 320 nm in Mg_0.752Al_2.165−xO₄:xFe³⁺ is associated with the transitions from the ground state ⁶A_1g(⁶S) to the ⁴A_2g(⁴F)/T_1g(⁴P) and doublet states. The transition between the lower energy excited state of ²T_2g(²I) and ⁶A_1g(⁶S) mainly contributes to the deep red emission and the red-shifting effect.     

Investigation of the microstructure,cation distribution and optical properties of nanoscale NixMg1-xAl2O4 spinel pigments

Nanoscale Ni_xMg_1-xAl₂O₄ spinel pigments were synthesized by a citric acid precursor combined with the gel-casting method. The microstructure, cation distribution and optical properties as a function of calcining temperature and nickel content were investigated by the X-ray diffraction (XRD) Rietveld refinement, transmission/field emission scanning electron microscopy (TEM/FESEM), X-ray photoelectron spectroscopy (XPS), colour measurement and UV–vis–NIR spectrophotometry. Upon increasing the calcining temperature, both Ni²⁺ and Mg²⁺ hindered the migration of Al³⁺ to octahedral sites. When the Ni content increased, the cation site percentage of Ni²⁺ in the tetrahedral and octahedral sites varied slightly while that of Al³⁺ and Mg²⁺ change substantially. The cation exchange resulted in an increase in the inversion parameters and a decrease in the lattice parameters with increasing temperature or Ni content. Furthermore, Rietveld refinement also showed a shrinkage of the tetrahedra and an expansion of the distorted octahedra in the spinel structure. Short-range information based on optical spectra suggests that variation in the splitting energy of tetrahedra and octahedra caused the change in the spectral absorption. This study may deepen the understanding of the structural-optical property relationship of Ni_xMg_1-xAl₂O₄ spinel, which is vital to the further colour modification of ceramics and glazes.     

Phase evolution,structure, and electrochemical performance of Al-, Ga- and Ta- substituted Li7La3Zr2O12 ceramic electrolytes by a modified wet chemical route

Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is one of the most promising solid-state electrolytes (SSEs) for advanced solid-state lithium batteries (SSLBs). In this work, Li_6.25Al_0.25La₃Zr₂O₁₂, Li_6.4Ga_0.2La₃Zr₂O₁₂, and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ ceramics are prepared by a modified wet chemical route. The composition of the black mixtures derived from the precursors is ascertained. The phase evolution and structural properties from the ceramic mother powders to the final ceramic electrolytes are discussed in detail. The characteristic of cubic LLZO with the space group I-43d arises in the Li_6.4Ga_0.2La₃Zr₂O₁₂ ceramic electrolyte pellet after the secondary higher-temperature (1200 °C) sintering. The Rietveld refinement reveals the roles of Al³⁺ substitution at the Li⁺ sites and Ta⁵⁺ substitution at the Zr⁴⁺ sites to adjust crystal structure. In addition, the electrochemical performance of the ceramic pellets is also investigated. Remarkably, the Li_6.4La₃Zr_1.4Ta_0.6O₁₂ ceramic electrolyte has the most outstanding electrochemical performance, showing the high ionic conductivity of 6.88 × 10^?4 S cm^?1 (25 °C), the low activation energy of 0.42 eV and an extremely low electronic conductivity of 1.77 × 10^?8 S cm^?1 (25 °C). Overall, it is supposed that this work may help to achieve high-quality modified LLZO ceramic electrolytes, especially using the wet chemical strategy.     

Compositional tailoring effect on crystal structure,mechanical and thermal properties of γ-AlON transparent ceramics

In view of the practical application of γ-AlON as a promising transparent structural ceramic, in-depth insight into its mechanical and thermal properties is essential. The solid-state MAS NMR technique was combined with XRD Rietveld refinement to confirm the crystal structure of Al_(8+x)/3O_4-xN_x (x = 0.299–0.575). These structural parameters were further applied to predict hardness and elastic properties based on theoretical exploration, which are in good agreement with the experimental values. A slight enhancement of mechanical properties with increasing nitrogen concentration is attributed to the stronger chemical bond in octahedra. The experimental thermal conductivity of γ-AlON transparent ceramics was improved slightly with the rise of x in the temperature range from 298 K to 1074 K. The intrinsic lattice thermal conductivity was determined by eliminating the extrinsic phonon scattering as well as the thermal radiation. The reason for the discrepancy between experimental and intrinsic thermal conductivity was revealed. The present methods provided powerful and accessible guidelines in optimizing the mechanical and thermal properties of oxynitride materials.     

Synthesis,crystal structure,and anti-thermal quenching performance of Lu2Sr(1−x)Al4SiO12:xEu2+ phosphors

With high-temperature solid-state reaction method, a series of Lu₂Sr_(1−x)Al₄SiO₁₂:xEu²⁺ phosphors have been synthesized. With Rietveld refinement method, the crystal structure of Lu₂SrAl₄SiO₁₂ has been refined. Under the excitation of the ultraviolet and violet band light, Lu₂Sr_(1−x)Al₄SiO₁₂:xEu²⁺ emits the Eu²⁺ characteristic blue broadband light. The photoluminescence properties of concentration quenching, emission peak shift, reflectance spectra, and luminescence decay have been investigated. With the structure analyses, the corresponding physical mechanisms have been discussed. With the increased temperature, this phosphor shows well thermal stabilities. For the x_Eu = 0.06, 0.08, and 0.1 phosphors, the strong anti-thermal quenching performance has been observed. The reason for the anti-thermal quenching of this phosphor has been discussed. The trap capture mechanism may be the suitable physical mechanism to explain the anti-thermal quenching of this phosphor. This phosphor shows the potential applications in the white LED lighting fields.     

Enhanced electrochemical performance of LiFePO4/C via Mo-doping at Fe site

A series of Mo-doped LiFe_1−3xMo_xPO₄/C (x = 0.000, 0.025, 0.050, 0.100, 0.150) cathode materials are synthesized by sol–gel method. XRD, ICP and Rietveld refinement results reveal that Mo doped in the crystal lattice and probably occupied Fe site. The structure benefits the transportation of Li⁺ and the diffusion of Li⁺ in the doped materials are enhanced remarkably than that of the undoped one, which leads to excellent electrochemical performance. The doped sample with x = 0.025 exhibits the best electrochemical performance, with the initial discharge capacity of 162.3 mAh g⁻¹ at 0.1 C rate.     

The structure and electrical properties of Ca0.6(Li0.5Bi0.5-xPrx)0.4Bi2Nb2O9 high-temperature piezoelectric ceramics

Ca_0.6(Li_0.5Bi_0.5-xPr_x)_0.4Bi₂Nb₂O₉ ceramics were prepared via a solid-state reaction method. The effect of the Pr content on the structural and electrical properties was systematically investigated. X-ray diffraction (XRD) combined with Rietveld refinement and X-ray photoelectron spectroscopy (XPS) demonstrated that a moderate amount of Pr³⁺ can be incorporated into the NbO₆ octahedra, while excess Pr³⁺ ions probably enter into the (Bi₂O₂)²⁺ layers, thus resulting in an increase in the tetragonality of the crystal structure. The introduction of Pr suppressed the generation of oxygen vacancies and improved the preferential grain growth along the c-axis, which might be responsible for enhancing the resistivity (ρ ~ 10⁶ Ω cm at 600°C). The replacement of Pr³⁺ for A-site Bi³⁺ enhanced the piezoelectric property, and the piezoelectric constant d₃₃ increased from 13.8 pC/N to 16.3 pC/N. The high depolarization temperature (up to 900°C) implied that CBN-LBP100x ceramics are promising candidates for ultrahigh-temperature application.     

Thermal decomposition and pore structure of pure and doped stannic oxide gel

The surface properties of a stannic oxide gel and its thermal dehydration products obtained both in vacuo and in the presence of air in the temperature range 100–600°C have been examined by N₂ adsorption. Phase and structural changes have been followed by differential thermal analysis and X-ray diffractometry. Complete pore structure analysis showed that samples dehydrated at or below 250°C were microporous. Above 250°C the pores were found to widen with increase of temperature, the widening occurring concurrently with the crystallisation process. Doping with cations of lower valency (Li⁺ and Al³⁺) than the host cation (Sn⁴⁺) had little effect on the pore structure and specific surface area for the low temperature samples (≥250°), whereas at higher temperatures, e.g. 600°C, it increased the specific area remarkably. The dope ions produce oxygen vacancies and hinder or retard sintering in SnO₂.     

Thermal decomposition and pore structure of pure and doped stannic oxide gel

The surface properties of a stannic oxide gel and its thermal dehydration products obtained both in vacuo and in the presence of air in the temperature range 100–600°C have been examined by N₂ adsorption. Phase and structural changes have been followed by differential thermal analysis and X-ray diffractometry. Complete pore structure analysis showed that samples dehydrated at or below 250°C were microporous. Above 250°C the pores were found to widen with increase of temperature, the widening occurring concurrently with the crystallisation process. Doping with cations of lower valency (Li⁺ and Al³⁺) than the host cation (Sn⁴⁺) had little effect on the pore structure and specific surface area for the low temperature samples (≤250°), whereas at higher temperatures, e.g. 600°C, it increased the specific area remarkably. The dope ions produce oxygen vacancies and hinder or retard sintering in SnO₂.     

Li3AlN2—a self‐activated yellow light emitting wide‐bandgap semiconductor used for LEDs

In allusion to the problems existing in the defect‐related luminescent materials, a series self‐activated light emitting semiconductors of Li₃AlN₂: R (R=0, Na⁺, Mg²⁺, Si⁴⁺, Tb³⁺, Eu³⁺) have been successfully synthesized by sample‐pressure sintering. Under the excitation at 422 nm, a yellow light peaked at 580 nm have been observed in the host lattice of Li₃AlN₂. The crystal structure and the electron structure of Li₃AlN₂ have been measured to investigate the defect‐related luminescent properties of Li₃AlN₂ using the Rietveld refinement on the basic of X‐ray diffraction data and the density functional theory (DET). The results show that Li₃AlN₂ crystallizes in cubic phase with full filled edge‐shared (Al/Li)N₄ tetrahedrons and is a wide‐bandgap semiconductor. The impurity defects produced by ions substitution have also been investigated, which leads to the red‐shift of the emission peak. Finally, the photoluminescence excitation (PLE) spectrum of Li₃AlN₂ with two excitation bands peaked at 300 nm and 422 nm has been detected, and the latter matches well with near‐UV LED chips. The thermal stability shows that integral intensity of Li₃AlN₂ at 150°C still has 60% of the initial intensity at room temperature. The results indicate their potential applications as the LED used phosphors.     

Influence of Ti and Fe doping on the structural and electrochemical performance of LiCo0.6Ni0.4O2 cathode materials for Li-ion batteries

The combination of lithium cobalt oxide (LCO) and lithium nickel oxide (LNO) property for Li-ion batteries (LIB) brings a very promising cathode material, LiCo_1?xNi_xO₂ with a high specific reversible capacity and good cycling behaviour. Nonetheless, high toxic Co content and an instability of Li⁺/Ni²⁺ interaction in LiCo_1?xNi_xO₂ crystal structure paved the way for some modification for the development of this potential material. In this research, the self-propagating combustion method is used to reduce 40% Co content of LCO by replacing it with 40% Ni content resulting in cathode material with the stoichiometry of LiCo_0.6Ni_0.4O₂ (LCN). To improve the stability of the LiCo_0.6Ni_0.4O₂ structure, 5% of Ti and Fe was substituted at the Co site of the LCN material. The effect of these different cation substitutions (Ti⁴⁺ and Fe³⁺) on the structural and electrochemical performance of layered LiCo_0.6Ni_0.4O₂ cathode materials was investigated. Rietveld refinement revealed that Fe doped material has the longest atomic distance Li–O in the structure that allow better Li⁺ diffusion during intercalation/deintercalation to give an excellent electrochemical performance (138 mAhg^?1). After 50th cycle, it is found that the discharge cycling for Ti and Fe substituted materials were improved by more than 5% compared to pristine material. Both Ti and Fe doped materials were also having less than 13% of capacity fading indicates that the substitution of some Co with Ti and Fe are stable and can retain their electrochemical properties.     

Enhancement of dielectric properties and energy storage density of bismuth and lithium co-substituted strontium titanate ceramics

Polycrystalline Bismuth and Lithium Co-Substituted Strontium Titanate Sr _(1-x)(Bi,Li)xTiO₃, was prepared using the solid-state method with microwave assisted heating of initial materials. The effect of Bi³⁺ and Li⁺ concentration on the crystal structure, microstructure, and permittivity and energy storage properties of SrTiO₃ ceramics are investigated. The phase and structure have been confirmed by XRD along with Rietveld refinement studies. Morphological investigations have been carried out using FESEM. Frequency and temperature dependence of dielectric permittivity was investigated using impedance spectroscopy. The sample with x?=?0.02 has shown dielectric relaxation behavior. The activation energy of relaxation is found to be 1.2?eV and relaxation time equals 1.12 $\times$ 10^?7 sec. The room temperature P-E loop has been investigated, and the result confirms that there is no signature of the ferroelectric phase in all samples. The energy storage density was theoretically estimated in the present study using a P-E loop. The results showed an astonishing ten-time increase in energy storage density with 8% co-substitution. With increasing x, the grain size steadily decreased, and dielectric breakdown strength increased, yielding a higher energy storage density. The obtained results herald a promising future in the development of electrical capacitors for energy storage applications.     

Discovery of new broadband yellow-emitting nitridoalumosilicate phosphor and its pc-WLED application

Developing a yellow phosphor with broadband emission covering more red-light areas is an effective approach to achieve high-quality solid-state lighting. In this study, a novel yellow-emitting nitride phosphor, Ca₅Si₂Al₂N₈:Ce³⁺, was successfully prepared at atmospheric pressure and lower temperatures (1300°C), and its structure-property relation was revealed using crystal refinement, photoluminescence (PL) spectra, time-resolved PL spectra, and density-functional theory calculations. The results demonstrate that Ca atoms occupy three different crystallographic sites in the lattice, which are substituted by Ce³⁺ to form multiple luminescence centers. Thus, Ca₅Si₂Al₂N₈:Ce³⁺ emits strong yellow light with a maximum peak at 585 nm and a wide emission band. Compared with YAG:Ce³⁺, Ca₅Si₂Al₂N₈:Ce³⁺ has a wider emission band with a FWHM of 150 nm, which can effectively cover the green and red areas. Moreover, the sample can be fully excited by a blue LED chip due to its broad excitation band. Notably, the Ca₅Si₂Al₂N₈'s tight crystal structure composed of edge-sharing AlN₄ and SiN₄ tetrahedra pairs guarantee its thermochemical stability and quantum efficiency. Furthermore, Ca₅Si₂Al₂N₈:Ce³⁺ exhibits better thermal stability than YAG:Ce³⁺. The results indicate that Ca₅Si₂Al₂N₈:Ce³⁺ is a promising yellow phosphor for WLEDs.     

Crystal structure and electrochemical performance of Li3V2(PO4)3 synthesized by optimized microwave solid-state synthesis route

To investigate the crystal structure and electrochemical performance of samples synthesized under different microwave solid-state synthesis condition, a series of Li₃V₂(PO₄)₃ samples has been synthesized at five different temperatures for 3-5 min and at 750 °C for various time. The as-synthesized Li₃V₂(PO₄)₃ samples are characterized and studied by ICP-AES analysis, X-ray diffraction (XRD), Rietveld analysis, scanning and transmission electron microcopy (SEM and TEM). At relatively lower temperature (650 °C) and very short reaction time (3 min), pure phase of Li₃V₂(PO₄)₃ could be synthesized in microwave irradiation field. The crystal structure and Li atomic fractional coordinate present a significant deviation upon the change of microwave irradiation temperature and time. Relatively, the diffusion ability of lithium cations and the electrochemical performance are affected. Under the proper reaction temperature and time, the carbon-free samples MW750C5m and MW850C3m show the best specific discharge capacity 126.4 and 132 mAh g⁻¹ at the voltage range of 3.0-4.3 V, near the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). At the voltage range of 3-4.8 V, the sample MW750C5m presents the best initial specific charge capacity of 197 mAh g⁻¹, equivalent to the reversible cycling of three lithium ions per Li₃V₂(PO₄)₃ formula unit (197 mAh g⁻¹). The initial discharge capacity, the samples MW750C5m and MW850C3m present high specific discharge capacity 183.4 and 175.7 mAh g⁻¹, respectively. The relationship among microwave irradiation condition, crystal structure, lithium atomic fractional coordinates and the electrochemical performance have been discussed in detail.