Ionothermal synthesis and characterization of alkali metal polyoxometallates: Structural trends in the (emim)m[An(Mo8O26)] (emim = 1-ethyl-3-methylimidazolium; m = 2,3; n = 1,2; A = K,Rb, Cs) group of compounds

Three novel octamolybdates (emim)₃K(Mo₈O₂₆) (1), (emim)₃Rb(Mo₈O₂₆) (2) and (emim)₂Cs₂(Mo₈O₂₆) (3) (emim = 1-ethyl-3-methylimidazolium) have been prepared by ionothermal reactions using (emim)Br as a solvent. The structures contain octamolybdate anions [β-Mo₈O₂₆]^4–. In compounds 1 and 2, [β-Mo₈O₂₆]^4– units are linked by alkali metal cations to form 1D chain structures, while Cs cations in the compound 3 link to polyoxoanions to form a 2D layered structure.     

(MMgBO3)n (n = 1, M = Li,Na, K,Rb and n = 4, M = Cs): An investigation on the structure transition and optical properties

In crystal engineering, modification of the crystal structure by cation substitution is one of the most important and efficient methods. This paper reveals the origin of the structure transition of a series of (MMgBO₃)_n (n = 1, M = Li, Na, K, Rb and n = 4, M = Cs) compounds. A force-equilibrium model was established with the comparison of different structures of these compounds. After substituted by other alkali metal cations with different atomic radii, the changed bond between oxygen and cations leads to structural transition. The electronic and optical properties of NaMgBO₃, KMgBO₃ and RbMgBO₃ were also investigated by using DFT methods to give more comprehension of these compounds.     

A universal HF-free synthetic method to highly efficient narrow-band red-emitting A2XF6:Mn4+ (A = K,Na, Rb,Cs; X = Si,Ge, Ti) phosphors

Mn⁴⁺-activated fluoride red-emitting narrow-band phosphors have been successfully used in wide color-gamut white LEDs for liquid crystal display (LCD) backlights. However, highly concentrated and toxic HF is usually used in their synthesis, causing environment and safety issues. In this work, we proposed a HF-free green method, that is, using NH₄F/HCl instead of HF, to synthesize a series of A₂XF₆:Mn⁴⁺ (A = K, Na, Rb, Cs; X = Si, Ge, Ti) phosphors. The microstructure, photoluminescence (PL) properties, thermal quenching, and applications of the synthesized phosphors were investigated. Using the proposed approach, the phosphors generally showed a pure phase, a particle size ranging from 5 to 45 μm, and some characteristic sharp emission lines of Mn⁴⁺ in the red spectral range. The internal quantum efficiency was varied in a broad range of 69%-94% under the 460 nm excitation, depending on the composition of the fluoride host. Among these compositions, K₂XF₆:Mn⁴⁺ (X = Ge and Ti) phosphors even had a similar external quantum efficiency (>60%) with commercial ones. By combining K₂GeF₆:Mn⁴⁺ (narrow-band red) and β-sialon:Eu²⁺ (narrow-band green) with a blue LED, a white light-emitting diode (wLED) backlight with a color gamut of 87.7% National Television System Committee Standard, color temperature of 8423 K, and a luminous efficacy of 110.8 lm/W was demonstrated. These results indicate that the synthetic method proposed in this work is universal for preparing highly efficient fluoride phosphors used in wLEDs.     

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.     

Crystal Structure and Electrical Properties of Lead‐Free (1 − x)BaTiO3–x(Bi1/2A1/2)TiO3 (A = Ag,Li, Na,K, Rb,Cs) Ceramics

Ceramics of solid solutions (1  ?  x)BaTiO₃–x(Bi_1/2A_1/2)TiO₃ (A = Ag, Li, Na, K, Rb, Cs, x ≤ 0.20) were prepared and their crystal structures, dielectric, ferroelectric, and piezoelectric properties were investigated. It was found that (Bi_1/2A_1/2)TiO₃‐type doping compounds broadened the temperature range of the tetragonal phase in BaTiO₃ and all the compositions examined displayed a tetragonal structure at room temperature. The Curie temperature (T_C) was observed to increase with respect to pure BaTiO₃ to the range 140°C–210°C through solid solution. Remanent polarization (P_r) tended to decrease with increased content of doping compound, whereas the coercive field (E_C) rose and piezoelectric coefficient (d₃₃) fell. The highest d₃₃ value in the solid solutions was observed in 0.97BaTiO₃–0.03(Bi_1/2Ag_1/2)TiO₃ at 90 pC/N.     

Microwave dielectric properties of ATiO3 (A = Ni,Mg, Co,Mn) ceramics

Relationships between structural characteristics and microwave dielectric properties of ATiO₃ (A = Ni, Mg, Co, Mn) with ilmenite structure were investigated. The oxygen octahedral distortion was dependent on the type of A-site ions which affected to the temperature coefficient of the resonant frequency (TCF) of ATiO₃ ceramics. The quality factor (Qf) of ATiO₃ (A = Mn, Ni, Co) specimens was appreciably lowered than that of MgTiO₃ specimens due to the degree of covalency of cation–oxygen ion bond and the ordering of A-site ions. Also, the dielectric constant (K) was dependent on the electronic oxide polarizabilities of ATiO₃ ceramics.     

Synthesis and crystal structures of n = 3 Ruddlesden–Popper phase CaSr3Mn3−xFexO10 (x = 1.5, 1.0)

The triple layer Ruddlesden–Popper phases (n = 3 R–P phase) CaSr₃Mn_3−xFe_xO₁₀ (x = 1.5, 1.0) were stabilized by solid state reaction in air atmosphere and their crystal structures were refined with neutron diffraction data obtained at room temperature and at 5 K by means of Rietveld method. Both phases adopt space group I4/mmm and revealed no magnetic reflection at low temperature. Magnetic susceptibility data of CaSr₃Mn_1.5Fe_1.5O₁₀ and CaSr₃Mn₂FeO₁₀ compound showed spin glass transition at 25 and 12 K, respectively.     

Intraligand fluorescence of M(III)(oxinate)3 under ambient conditions with M = Fe,Ru, Os and oxinate = 8-quinolinolate

The electronic spectra of the complexes M(III)(oxinate)₃ with M = Fe, Ru, Os and oxinate = 8-quinolinolate are dominated by oxinate to M(III) LMCT transitions. Nevertheless, these complexes are also characterized by a fluorescence at higher energies, which originates from the oxinate ligands. This green luminescence appears in solution as well as in the solid state under ambient conditions. The electronic coupling between the IL and the LMCT states is apparently not strong enough to lead to a complete quenching of the oxinate IL fluorescence.     

Crystal structural refinement of corundum-structured A4M2O9 (A = Co and Mg,M = Nb and Ta) microwave dielectric ceramics by high-temperature X-ray powder diffraction

The microwave dielectric properties and crystal structure of corundum-structured Mg₄Nb₂O₉ (MN), Co₄Nb₂O₉ (CN) and Mg₄Ta₂O₉ (MT) compounds were investigated in this study. The crystalline phases of the ceramics were characterized by using the high-temperature X-ray powder diffraction and the crystal structures of the compounds were refined in terms of Rietveld analysis. The temperature coefficient of resonant frequency (τ_f) varied from −70 to −10 ppm/°C with increasing the composition x when Mg was substituted by Co. From the calculation of covalency of cation–oxygen bonds, it is found that the covalency of CoO bond is lower than that of MgO bond and the difference in the covalency of these cation–oxygen bonds influences on the temperature dependence of dielectric constant on the compounds. As for the dielectric constant (ɛ_r) and the quality factor (Q·f) of the ceramics, these values ranged from 10 to 16 and from 210,000 to 5000 GHz, depending on the composition x.     

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.     

Polymorphism of M3AlX Phases (M = Ti,Zr, Hf; X = C,N) and Thermomechanical Properties of Ti3AlN Polymorphs

M₃AlX (M = Ti/Zr/Hf, X = C/N) compounds are promising high‐temperature structural ceramics. However, their interesting polymorphism, thermomechanical stabilities, and thermal and mechanical properties were not fully understood. In this work, the polymorphisms of M₃AX phases are investigated by combining first‐principles and lattice dynamics calculations. Only Ti₃AlN shows polymorphic transition between the cubic and orthorhombic phases at around 1105 K; but other M₃AlX phases do not display similar polymorphic phase transition. Furthermore, the temperature‐dependent heat capacity, thermal expansion, and elastic stiffness of Ti₃AlN polymorphic phases are reported for the first time to explore the relationship between crystal structures, and mechanical and thermal properties. Ti₃AlN polymorphs show anisotropic thermal expansion and elastic stiffness; and the orthorhombic Ti₃AlN is suggested as a promising damage tolerant nitride, which has similar properties with the previously reported Zr₃AlN and Hf₃AlN.     

Syntheses,structural characterization and luminescent properties of M2(ATPA)3(DMF)2(H2O)2 (M = Nd,Sm, Eu,Gd, Tb,Dy; ATPA = 2-aminoterephthalate,DMF = N,N-dimethylformamide)

Six isomorphous metal–organic frameworks: M₂(ATPA)₃(DMF)₂(H₂O)₂ (M = Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Dy (6); ATPA = 2-aminoterephthalate, DMF = N,N-dimethylformamide), were synthesized by the self-assembly of lanthanide ions, 2-aminoterephthalate, DMF and H₂O. Their crystal structures determined by X-ray crystallography reveal interpenetrating frameworks. Compounds 2, 3, 5 and 6 exhibit mainly ligand luminescence at room temperature while 3 exhibits enhanced Eu³⁺ emissions at 77 K. Magnetic studies indicate 4 is paramagnetic with slight Gd³⁺–Gd³⁺ couplings.     

The structural evolution and superconductivity under pressure for superconductor AFe2As2 (A = Ba,Sr)

In collinear antiferromagnetic (AFM) order, the structural evolution and electronic structure of AFe₂As₂ (A = Ba, Sr) under hydrostatic and uniaxial pressures have been studied. The Fe-Fe distance is closely related to the AFM suppression as well as the Fe-As bond. Through analysis of Fermi surface and band diagram, the uniaxial pressure along c-axis can make the band near the Γ point to produce Lifshitz transition under the condition of lower pressure, and it is easier to suppress Fe magnetic moment. If the pressure component along c-axis is larger in the experiment, it is easier to cause the superconducting. The superconductivity of BaFe₂As₂ and SrFe₂As₂ under pressure may be due to the increasing of Fe 3d- and As 4p-hybridized bands width near the Fermi level.     

Cadmium(II) chloride complexes of imidazole-based ligands: Solvothermal syntheses,crystal structures and luminescent properties

Presented are the solvothermal syntheses, structures, characterizations, and luminescent properties of four novel organic-modified cadmium chloride complexes constructed from CdCl₂ and imidazole (or its derivatives), namely [CdCl₂(Im)₄] (1, Im = imidazole), [CdCl₄(HAPI)₂] (2, API = N-(3-Aminopropyl)-imidazole), [Cd(μ-Cl)₂(1-Mim)₂]_∞ (3, 1-Mim = 1-methyl imidazole) and [CdCl₃(HAPI)]_∞ (4). 1 and 2 feature a zero dimensional (0D) mononuclear structure with different mole ratios of Cd: Cl: ligand, while compound 3 and 4 bear infinite chains constructed from different subunits and connection modes. Worthy of note is that 4 represents a new structural type in the family of organic modified cadmium halides. The fluorescence spectra showed that compounds 1–4 exhibited emission peaks around 450 nm with the quantum yields of 23.01%, 5.84%, 27.31%, 5.71%, respectively.     

Composition-dependent intrinsic defect structures in pyroclore RE2B2O7 (RE = La,Nd, Gd; B = Sn,Hf, Zr)

Point defects are closely correlated with various properties of pyrochlore oxides and therefore play a key role on their engineering applications. Here, the native point defect complexes in RE₂B₂O₇ (RE = La, Nd, Gd; B = Sn, Hf, Zr) under stoichiometric and nonstoichiometric compositions are studied by first-principles calculations. The O Frenkel defect complex is predicted to be the predominant defect structure in stoichiometric zirconates and hafnates, whereas the cation antisite defect complex is the predominant one in stannates. In the case of BO₂ excess, the formation of the B-RE antisite defect together with the RE vacancy and the oxygen interstitial is energetically favorable, whereas the RE-B antisite defect together with the oxygen vacancy and the RE interstitial is preferable under the RE₂O₃ excess environments. Additionally, the formation energies of the native defect complexes are greatly affected by the B-site and/or RE-site cations. The strategy on tailoring the intrinsic defect structures of these pyrochlore oxides is proposed. It is expected to guide the experiments on the defect-related property optimization through stoichiometric and nonstoichiometric compositions, so as to meet the specific engineering requirements and promote their commercial applications.     

Microstructural characterization and crystallization behaviour of (1 − x)TeO2–xWO3 (x = 0.15, 0.25, 0.3 mol) glasses

DTA, XRD and SEM investigations were conducted on the (1 − x)TeO₂–xWO₃ glasses (where x = 0.15, 0.25 and 0.3). Whereas the 0.75TeO₂–0.25WO₃ and 0.7TeO₂–0.3WO₃ glasses show no exothermic peaks, an indication of no crystallization in their glassy matrices, two crystallization peaks were observed on the DTA plot of the 0.85TeO₂–0.15WO₃ glass. On the basis of the XRD measurements of the 0.85TeO₂–0.15WO₃ glass samples heated to 510 °C and 550 °C (above the peak crystallization temperatures), α-TeO₂ (paratellurite), γ-TeO₂ and WO₃ phases were detected in the sample heated to 510 °C and the α-TeO₂ and WO₃ phases were present in the sample heated to 550 °C. SEM micrographs taken from the 0.85TeO₂–0.15WO₃ glass heated to 510 °C showed that centrosymmetrical crystals were formed as a result of surface crystallization and were between 3 μm and 15 μm in width and 12 μm and 30 μm in length. On the other hand, SEM investigations of the 0.85TeO₂–0.15WO₃ glass heated to 550 °C revealed the evidence of bulk massive crystallization resulting in lamellar crystals between 1 μm and 3 μm in width and 5 μm and 30 μm in length. DTA analyses were carried out at different heating rates and the Avrami constants for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C were calculated as 1.2 and 3.9, respectively. Using the modified Kissinger equation, activation energies for crystallization were determined as 265.5 kJ/mol and 258.6 kJ/mol for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C, respectively.     

Microwave dielectric properties of low loss microwave dielectric ceramics: A0.5Ti0.5NbO4 (A = Zn,Co)

The microwave dielectric properties of low-loss A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics prepared by the solid-state route had been investigated. The influence of various sintering conditions on microwave dielectric properties and the structure for A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics were discussed systematically. The Zn_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as ZTN) showed the excellent dielectric properties, with ɛ_r = 37.4, Q × f = 194,000 (GHz), and τ_f = −58 ppm/°C and Co_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as CTN) had ɛ_r = 64, Q × f = 65,300 (GHz), and τ_f = 223.2 ppm/°C as sintered individually at 1100 and 1120 °C for 6 h. The dielectric constant was dependent on the ionic polarizability. The Q × f and τ_f are related to the packing fraction and oxygen bond valence of the compounds. Considering the extremely low dielectric loss, A_0.5Ti_0.5NbO₄ (A = Zn and Co) ceramics could be good candidates for microwave or millimeter wave device application.     

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.