Bond Ionicity Research Articles

Effects of phase transition on the structure and microwave dielectric properties of (1-x)CeO2-0.5xSm2O3 (x=0-0.9) ceramics

(1-x)CeO2-0.5xSm2O3 (x=0-0.9) microwave dielectric ceramics are synthesized by the conventional solid-phase reaction method, with the effects of phase transition on structure and microwave dielectric properties being investigated in detail. The phase transition process is analyzed using the ionicity and lattice energy of (1-x)CeO2-0.5xSm2O3 ceramics, which is related to the coordination number and effective valence electron charge. With the increase of Sm3+ content, there are three phase regions: the cubic fluorite phase (0 ≤ x ≤ 0.4), the rare-earth C-type phase (0.4 <x ≤ 0.8) and the mixed phase (0.8 <x < 1.0). With the phase transition, the microstructures of (1-x)CeO2-0.5xSm2O3 (x=0-0.9) ceramics show a quasi-periodic change, where x=0.4 and x=0.8 are the critical points with similar grain size and distribution. When the phase transition occurs, the polarization, Q׃ values and the resonant frequency temperature coefficients (τƒ) of (1-x)CeO2-0.5xSm2O3 (x=0-0.8) ceramics are all mutated, which is due to the change of bond ionicity and effective valence electron charge caused by the phase transition. The microwave dielectric properties in each pure phase region vary regularly with bond ionicity and valence. Compared with the cubic fluorite phase (0 ≤ x ≤ 0.4), (1-x)CeO2-0.5xSm2O3 ceramics with rare-earth C-type phase (0.4 <x ≤ 0.8) have lower dielectric constant, with better Q׃ values and τƒ values.

Effect of binary ion substitution on the crystal structure and microwave dielectric properties of MgTa2O6 ceramics

AbstractMg1/3A11/3A21/3Ta2O6 ceramics (where A1 and A2 represent Co, Ni, or Zn) were prepared using the solid‐phase reaction route. The results of X‐ray diffraction and Raman tests showed that (Co1/2Ni1/2)2+, (Co1/ 2Zn1/2)2+, and (Ni1/2Zn1/2)2+ ions replaced Mg2+ ions in the lattice of MgTa2O6, resulting in the formation of a pure tri‐rutile structure. All three complex ions could significantly reduce the sintering temperature of the ceramics (by 150°C–200°C) and could broaden the sintering window. The large deviation (48.8%–69.2%) between the porosity corrected relative permittivity εr‐corr. and the theoretical relative permittivity εr‐theo. might be due to the overestimation of the ionic polarizability of Ta5+ by Shannon. (Co1/2Ni1/2)2+ has the most significant effect of increasing the bond energy of ceramic A–O bonds, reducing the τf value from 51 to 36 ppm/°C. In addition, the mechanisms affecting their dielectric properties are discussed based on bond ionicity (fi), full width at half maximum (FWHM) of Raman peak, and bond energy (E). Mg1/3Co1/3Ni1/3Ta2O6 ceramic sintered at 1350°C have the most excellent microwave dielectric properties: εr = 26.8, Q × f = 86,000 GHz, and τf = 36 ppm/°C.

Effect of chemical bond polarity on wafer and cleavage surface oxidation for GaAs, GaSb, InAs and InSb single crystals

Rapid development of III–V semiconductor device technologies and broadening of device applications are hindered by the high density of surface states produced in those materials by intrinsic complex oxides and surface contaminations. Surface oxides acting as non-radiating recombination centers produce a number of surface levels in the band gap. Oxidation mechanisms depend on the chemical composition of a specific III–V semiconductor compound because surface reactivities differ between materials. The effect of chemical bond polarity on surface oxidation processes in single crystal III–V semiconductor wafers is currently an important research topic. In this work, surface-sensitive XPS method has been used for studying oxidation regularities in GaAs, InAs, GaSb and InSb single crystals undergoing natural atmospheric oxidation and subjected chemical-mechanical polishing. A method of assessing oxidation levels based on chemical shifts of XPS spectral features has been developed. Oxidation level has been shown to depend on chemical bond ionicity degree, the latter being evaluated using the Sanderson and Phillips models. A decrease in surface oxidation level with an increase in bond ionicity degree has been observed for (110) single crystal wafer cleavage surfaces and for (100) single crystal wafer surfaces after single crystal slicing and chemical-mechanical polishing. We show that surface oxidation level increases in the InAs–GaAs–InSb–GaSb sequence, from arsenides to antimonides.

Mechanisms affecting microwave dielectric properties of Li2Mg2TiO5 ceramics based on lithium excess

Herein, the feasibility was assessed through first principles, and the effect of lithium excess on both the structure and dielectric properties of Li(1+x)2Mg2TiO5 (0.000 ≤ x ≤ 0.200) ceramics was investigated by solid-phase reaction method. An appropriate excess of lithium can moderately inhibit the volatilization of lithium and reduce the production of the secondary phase MgTi2O4, though it cannot completely inhibit its production. Li(1+0.050)2Mg2TiO5 exhibits the best dielectric properties with εr =14.55, Q × f =127,393 GHz (@9.6 GHz), and τf = −19.91 ppm/°C. The influence of non-inherent factors on the dielectric properties is dominant, with the increase in relative density to 93.53 % and the growth in average grain size to 28.67 μm contributing to the reduction of dielectric loss. The decrease in εr is due to the decrease in secondary phase content. Results from bond ionicity and Raman wavenumber variations in the intrinsic factors show that the intrinsic factors have little effect on the εr. Whereas, the increase in lattice energy as well as the decrease in FWHM contribute to the decrease in dielectric loss. In addition, a proper excess of lithium helps to increase the bonding energy thereby optimizing the temperature stability. The optimized Li(1+0.050)2Mg2TiO5 has great potential for application in microwave communications.

Novel single-phase Li2SiO3 microwave dielectric ceramic with low permittivity

Novel low-permittivity lithium metasilicate (Li2SiO3) ceramics were synthesized to explore the relationships between crystal structure parameters and dielectric properties. X-ray diffraction and Rietveld refinement indicated that the Li2SiO3 ceramics formed a single orthorhombic structure with a Cmc21 space group. Optimal microwave dielectric properties of εr = 6.20 ± 0.02, Q×f = 30,550 ± 250 GHz (f0 = 15.5 GHz), and τf = −40.95 ± 0.56 ppm/℃ were achieved through sintering at 1025 ℃. Raman spectroscopy analysis showed that the values of εr and Q×f were negatively correlated with the Raman shift and full width at half maximum of the strongest peak at 982 cm−1, respectively. Meanwhile, the Phillips–Vechten–Levine theory revealed that the εr values and the Q×f values were closely related to the average bond ionicity and the lattice energy of the Si-O bonds, respectively. Moreover, the increase in τf values depended on the increase in oxygen bond valence and the distortion of the [LiO4] tetrahedron. These results on the Li2SiO3 ceramics could provide some theoretical information for developing high-performance microwave dielectric ceramics.

A novel microwave dielectric ceramic CaB2O4 with low dielectric constant

A novel microwave dielectric ceramic CaB2O4 with low dielectric constant was synthesized by the solid-state reaction method. The impact of diverse Ca/B ratios on the phase composition, microstructure and dielectric properties of CaB2+xO4+3x/2 (x = 0–0.6) ceramics was investigated. The bond ionicity and the lattice energy of CaB2+xO4+3x/2 ceramics were calculated by the P-V-L theory. The appropriate excess of boron facilitated the densification of CaB2O4 ceramics, resulting in the stable crystal structure and enhanced dielectric properties. CaB2.3O4.45 ceramic sintered at 950 °C had excellent properties: εr = 6.03, Q×f = 37,962 GHz, and τf = −24.7 ppm/°C. The influence of TiO2 on the microwave dielectric properties of CaB2O4 was investigated, 87 wt% CaB2O4 - 13 wt% TiO2 ceramics sintered at 950 °C showed a zero τf value.

Unraveling the electronic properties in SiO2 under ultrafast laser irradiation

First-principles simulations were conducted to explore various electronic properties of crystalline SiO2 (α-quartz) under ultrafast laser irradiation. Employing Density Functional Perturbation Theory and the many-body (GW) approximation, we calculated the impact of thermally excited electrons on the electronic specific heat, electron pressure, effective mass, deformation potential, electron-phonon coupling and electron relaxation time of quartz, covering a wide range of electron temperatures, up to 100,000 K. We show that the electron-phonon relaxation time of highly-excited quartz becomes twice faster compared to low-excited states. The deformation potential, which dictates atomic displacement, has a non-monotonic behavior with a well-pronounced minimum at around 16,000 K (2.7 × 1021 cm−3 of excited electrons) where the bond ionicity of the Si-O starts decreasing followed by a cohesion loss at 35,000 K due to the pressure exerted by the excited electrons on the lattice. Consequently, our calculated data, illustrating the evolution of physical parameters, can facilitate simulations of laser-matter interactions and provide predictive insights into the behavior of quartz under experimental conditions.

Microwave dielectric properties of Li2Mg2Ga2Ti3O12 ceramics with spinel-type

Novel spinel-type Li2Mg2Ga2Ti3O12 ceramics were prepared by the solid-state method. Rietveld refinement revealed that the Li2Mg2Ga2Ti3O12 ceramics possess a cubic spinel structure with the Fd3‾m(227) space group. Raman spectrum analysis indicated that the Q × f and FWHM of ceramics exhibit an inverse relationship. According to the P-V-L theory, the Ti-O bond has the highest percentage of contribution to both of bond ionicity and lattice energy, indicating a significant impact on εr and Q × f, respectively. The Li2Mg2Ga2Ti3O12 ceramics showed the best microwave dielectric properties (εr = 15.72, Q × f = 69,633 GHz, and τf = −20 ppm/°C), sintering at an optimum temperature of 1200 °C. Additionally, a dielectric resonance antenna, on the basis of Li2Mg2Ga2Ti3O12 ceramics was designed and simulated.

First-Principle Characterization of Structural, Electronic, and Optical Properties of Tin-Halide Monomers.

The growing interest in tin-halide semiconductors for photovoltaic applications demands in-depth knowledge of the fundamental properties of their constituents, starting from the smallest monomers entering the initial stages of formation. In this first-principles work based on time-dependent density-functional theory, we investigate the structural, electronic, and optical properties of tin-halide molecules SnXn 2-n, with and X=Cl, Br, I, simulating these compounds in vacuo as well as in an implicit solvent. We find that structural properties are very sensitive to the halogen species while the charge distribution is also affected by stoichiometry. The ionicity of the Sn-X bond is confirmed by the Bader charge analysis albeit charge displacement plots point to more complex metal-halide coordination. Particular focus is posed on the neutral molecules SnX2, for which electronic and optical properties are discussed in detail. Band gaps and absorption onset decrease with increasing size of the halogen species, and despite general common features, each molecule displays peculiar optical signatures. Our results are elaborated in the context of experimental and theoretical literature, including the more widely studied lead-halide analogs, aiming to contribute with microscopic insight to a better understanding of tin-halide perovskites.

On the issue of comparing the immobilization characteristics of matrix materials based on Nb–Ta–Ti-oxides of the types AB2O6 and A2B2O7

In the study presented here, the initial (that is, before the start of the process of natural hydrochemical influence) mineral formula of metamict polycrase in the composition of granite pegmatites of the Baltic Shield, applying an uranium natural half-leaching period, was calculated. To investigate the characteristics of immobilization of actinides in the studied polycrase, the absolute and relative uranium contents are compared with the corresponding uranium contents in the original betafite of the same deposit and age. It has been shown that over its geological history, betafite has lost up to 80% of its original uranium content. The proportion of uranium preserved in polycrase is twice as high. It is concluded that the difference in the relative content of uranium (27.3 wt% in polycrase and 31.6 wt% in betafite) cannot be the only reason for the complete oxidation of uranium in betafite, given that in polycrase 30% of uranium is preserved in the tetravalent state. It is more likely that the oxidation of uranium in betafite was primarily a result of the low ionicity of the chemical bonds compared to that in polycrase. This allows us to consider minerals of the euxenite group to be quite promising as matrix materials for the immobilization of actinides. At the same time, an opinion was expressed on the advisability of further comparative studies of Nb–Ta–Ti-oxides of the mineral groups AB2O6 and A2B2O7 for their use at the final stage of the nuclear fuel cycle.

Mitigating Sodium Ordering for Enhanced Solid Solution Behavior in Layered NaNiO2 Cathodes.

The O-type layered nickel oxides suffer from undesired cooperative Jahn-Teller distortion stemming from Ni3+ ions and undergo multiple biphasic structural transformations during the insertion/extraction of large Na+ ions, posing a significant challenge to stabilize the structural integrity. We present here a systematic investigation of the impact of substituting 5 % divalent (Mg2+) or trivalent (Al3+ or Co3+) ions for Ni3+ to alleviate Na+ion ordering and perturb the Jahn-Teller effect to enhance structural stability. We gauge a fundamental understanding of the Mg-O and Na-O or Mg-O-Na bonding interactions, noting that the ionicity of the Mg-O bond deshields the electronic cloud of oxygen from Na+ ions. Furthermore, calculations of the Van Vleck distortion modes reveal a relaxation of NiO6 octahedra from Jahn-Teller distortion and a reduced electron density at the interlayer with Mg2+ substitution. Long-range (operando X-ray diffraction) and short-range (magic angle spinning nuclear magnetic resonance) structural analyses provide insights into reduced ordering, allowing a stable continuous solid solution. Overall, Mg-substitution results in a high-capacity retention of ~96 % even after 100 cycles, showcasing the potential of this strategy for overcoming the structural instabilities and enhancing the performance of sodium-ion batteries.

First-principles definition of ionicity and covalency in molecules and solids.

The notions of ionicity and covalency of chemical bonds, effective atomic charges, and decomposition of the cohesive energy into ionic and covalent terms are fundamental yet elusive. For example, different approaches give different values of atomic charges. Pursuing the goal of formulating a universal approach based on firm physical grounds (first-principles or non-empirical), we develop a formalism based on Wannier functions with atomic orbital symmetry and capable of defining these notions and giving numerically robust results that are in excellent agreement with traditional chemical thinking. Unexpectedly, in diamond-like boron phosphide (BP), we find charges of +0.68 on phosphorus and -0.68 on boron atoms, and this anomaly is explained by the Zintl-Klemm nature of this compound. We present a simple model that includes energies of the highest occupied cationic and lowest unoccupied anionic atomic orbitals, coordination numbers, and strength of interatomic orbital overlap. This model captures the essential physics of bonding and accurately reproduces all our results, including anomalous BP.

Microwave dielectric properties and chemical bond of novel spinel-structure Zn3Ga2SnO8 ceramics

A novel low-εr Zn3Ga2SnO8 ceramic was prepared, and the phase composition and microstructural evolution were investigated. According to Rietveld refinement, TEM, and Raman spectroscopy, Zn3Ga2SnO8 ceramics had a cubic spinel structure in which Zn1 occupied the tetrahedral sites, and Zn2/Ga1/Sn1 occupied the octahedral sites. The relative density and packing fraction significantly influence the εr and Q × f of Zn3Ga2SnO8 ceramics, respectively. Meanwhile, the P–V–L theory indicates that the contribution rate of Sn–O bonds to bond ionicity is 29.2 %, and that of Ga–O bonds to lattice energy is 37.5 %. The Zn3Ga2SnO8 ceramics exhibit the best microwave dielectric properties (εr = 10.6, Q × f = 88,725 GHz, and τf = −25.1 ppm/°C) when sintered at 1320 °C. Given these advantages, Zn3Ga2SnO8 ceramic has certain potential applications in millimeter-wave technology.

Dominant mechanisms of thermo-mechanical properties of weberite-type RE3TaO7 (RE=La, Pr, Nd, Eu, Gd, Dy) tantalates toward multifunctional thermal/environmental barrier coating applications

Weberite-type RE3TaO7 (RE=La, Pr, Nd, Eu, Gd, Dy) tantalates have been studied as multifunctional thermal/environmental barrier coating materials with working temperatures above 1500 K. This study proposes the dominant mechanisms of their thermo-mechanical properties. The relative ionicity of the RE-O bonds increased with increasing RE+3 ionic radius, which weakened the bond strength and led to decreases in Young's modulus and hardness. A model was proposed to derive the high-temperature thermal conductivity based on the relationship between the phonon thermal conductivity and temperature, and the thermal radiative conductivity was estimated. RE3TaO7 exhibited a lower thermal conductivity than other fluorite-related oxides because its weak bond strength could slow down the phonon speed and enhance the an-harmonic vibrations of the lattices. At low temperatures, the RE atomic weight was a better descriptor of the thermal conductivity than the RE+3 ionic radius, and which could aid in further regulating the thermal conductivity. The thermal expansion coefficients of the different axes were affected by both RE-O and Ta-O bonds when weak RE-O bonds dominated the linear thermal expansion. The low oxygen ion conductivity derived from the strong covalent Ta-O bonds, which increased the activation energy for oxygen hopping in lattices of RE3TaO7. This study elucidates the dominant mechanisms of properties from the characteristics of crystals and chemical bonds, which are significant for high-temperature applications of tantalates.

Dispersion properties of (La0.06Ga0.94)2O3:Eu thin films

The dispersion of the refractive index of (La0.06Ga0.94)2O3:Eu thin films obtained by radio-frequency ion-plasma sputtering has been studied. It was found that the films have a polycrystalline structure corresponding to the monoclinic structure of β- Ga2O3. It is shown that the freshly deposited films are characterised by an abnormal dispersion, and after annealing in argon, a normal dispersion of the refractive index is observed. It was found that at normal dispersion, the spectral dependence of the refractive index in the visible region of the spectrum is mainly determined by electronic transitions from the 2p-state oxygen band, which form the upper filled level of the valence band to the bottom of the conduction band formed by hybridised 2p-states of oxygen and 4s-states of gallium. Two single-oscillator approximation models were analysed and compared for the films annealed in argon, and the approximation parameters, dispersion energy, degree of chemical bond ionicity, coordination number, and static refractive index were determined.

Effects of F− substitution on crystal structure, band characteristics and microwave dielectric properties of Li3Mg2NbO6 ceramics for high frequency applications

The structure–performance mechanism is essential to achieve comprehensive characteristics for advanced communication system. In the present work, F− ions were adopted to fabricated Li3Mg2NbO6-x/2Fx ceramics at a nitrogen atmosphere to modify dielectric characteristics. Correlation between bond characteristics, crystal structure and enhanced microwave dielectric characteristics was investigated through vibration analysis, P–V-L theory, Rietveld refinement, and First-principle calculations. Raman analysis and Rietveld refinement show that F− substitution led to a single phase. First-principle calculations indicates a high energy gap of 3.676 eV and p orbit contributed to Fermi energy. F− substitution led to the increase of Nb O bond ionicity and lattice energy, accounting for the variation in Q×f value and dielectric constant. The τf value was related to the expansion coefficient of the Li–O bond. Remarkably, the samples (x = 0.08) exhibited comprehensive dielectric characteristics with εr = 15.9, Q×f = 143,000 GHz and near-zero τf = −2.1 ppm/°C, proving an effective strategy for collaborative regulation. Based on structure simulation, 5G filter with ultra-low loss and high-suppression was designed through introducing transmission zeros from cross coupling. More specifically, a dielectric filter exhibited an interpolation loss of 0.2 dB at 3.3–3.6 GHz and high attenuations of 15 dB for 20 MHz outside the passband, verifying the feasibility for 5G applications.

Effects of oxygen vacancy on bond ionicity, lattice energy, and microwave dielectric properties of CeO 2 ceramics with Yb 3+ substitution

Effects of oxygen vacancy on bond ionicity, lattice energy, and microwave dielectric properties of CeO ₂ ceramics with Yb ³⁺ substitution

Breaking Through the Luminescence Stability Bottleneck of Oxyfluoride Phosphor for Sun‐Like Led Lighting

AbstractSunlight like optical source is the primary objective of healthy lighting. In contrast, achieving full spectrum lighting with high stability remains challenging, mostly due to the scarcity of purple chip‐pumped cyan‐green phosphors and their inherent emission instability. In this study, a novel strategy is presented that realizes simultaneous enhancements in both humidity stability (increased by 2.9 times) and luminescence quantum efficiency (enhanced by 1.5 times) for cyan‐emitting Sr3AlO4F:Ce3+ phosphor by employing Si/N doping. Theoretical modelling shows that these improvements are attributed to the regulation of energy bands and structural rigidity as well as modulation of the ionicity of chemical bonds. Furthermore, blending the other violet chip‐pumped phosphors, sun‐like LEDs that accurately replicate solar spectra in different times of the day with an optimal color rendering index of 99 is successfully engineered. Moreover, the universality of this strategy on other oxyfluoride phosphors has also been validated. This groundbreaking outcome offers a practical solution to overcome the inherent luminescence stability limitations in oxyfluoride phosphors, thus catalyzing the application of these cyan‐green candidates in sun‐like LED lighting.

Effects of oxygen vacancy on bond ionicity and microwave dielectric properties of YxCe1−xO2−0.5x (x = 0–0.7) ceramics

AbstractNovel YxCe1−xO2−0.5x (x = 0–0.7) ceramics, designed by replacing Ce4+ with Y3+, were prepared using a conventional oxide reaction. The oxygen vacancies, measured by X‐ray photoelectron spectroscopy and analyzed through the electronic structure calculated via the first‐principles method, were employed to investigate the effective valence electron charge, which plays a decisive role in calculating bond ionicity using P–V–L theory. After the substitution of Y3+ ions, the effective valence electron charge of the Ce–O bond changed because of an increase in oxygen vacancies, ultimately leading to a decrease in the Ce/Y–O bond ionicity of the YxCe1−xO2−0.5x ceramics. For microwave dielectric properties, when the YxCe1−xO2−0.5x (x = 0−0.5) ceramics were in the pure phase, porosity‐corrected permittivity and Q × f values depended on the bond ionicity, and the temperature coefficient of the resonance frequency was analyzed using the bond valence. When the YxCe1−xO2−0.5x (x = 0.6 and 0.7) ceramics were in multiple phases, the microwave dielectric properties were associated with the phase composition.

Mn–Si synergistically co-doping adjusting microstructures and microwave-millimeter-wave dielectric properties of Y3Al5O12 garnet ceramics: Lattice occupancy and ligand distortion perspective

Y3-xMnxAl5-xSixO12 (x = 0.1–0.5) ceramics were fabricated through the high-temperature reaction method. The effects of Mn2+-Si4+ co-doping on lattice occupancy, ligand distortion and microwave dielectric properties of Y3Al5O12 garnet were inquired. Mn2+ ions successively occupy the lattice sites of Y3+ dodecahedron and Al3+ octahedron along with the increase of distortion degree of [AlO6] octahedron. In Y3-xMnxAl5-xSixO12, εr predominantly correlates with ionic polarizability, average bond ionicity (△fi) and Mn2+ ionic lattice occupation. The Q × f is primarily influenced by factors like relative density, lattice energy (U) and atomic packing fraction. Meanwhile, τf is associated with the distortion of octahedra, bond energy (E), and the bond valence between Y/Mn and O. Ultimately, the microwave and millimeter-wave dielectric properties of the Y2.8Mn0.2Al4.8Si0.2O12 ceramics excel with values of εr = 10.89, Q × f = 56,135 GHz@11 GHz (Q × f = 88200 GHz@25 GHz), and τf = -39.35 ppm/°C.