Bond Ionicity Research Articles

Structure, infrared spectra and microwave dielectric properties of the novel Eu2TiO5 ceramics

AbstractThe microwave dielectric properties of Eu2TiO5 ceramic prepared by a conventional solid‐state method were investigated for the first time. An orthorhombic structure with Pnam space group was obtained from x‐ray diffraction. On the basis of P‐V‐L chemical bond theory and refined lattice parameters, the bond parameters of bond ionicity, lattice energy, bond energy, and coefficient of thermal expansion of Eu2TiO5 were computed. The relationship between chemical bond characteristics and microwave dielectric properties was discussed. Besides, far‐infrared reflective spectra indicated the absorption of structural phonon oscillation might be the main contribution to polarization for Eu2TiO5 ceramic. Eu2TiO5 ceramic sintered at 1300°C for 6 hours possessed excellent microwave dielectric properties of εr ~ 14.4 ± 0.2, Q × f ~ 21 000 ± 500 GHz, and τf ~ −10 ± 2 ppm/°C.

Enhanced Catalytic Performance of Subnano Copper Oxide Particles.

Subnanoparticles (SNPs) with ultrasmall particle sizes (<1 nm) have potential to provide catalytic activity that is superior to that of nanoparticles. Size-controlled CunOx (n = 12, 28, and 60) materials supported on zirconia, prepared using a dendritic macromolecular reactor, exhibited increased ionicity of the Cu-O bonds with a decrease in size of the particles, which was suggested on the basis of the peak intensity in the Cu 2p3/2 region. The polarization of the Cu-O bonds in the ultrasmall copper oxides provides size-dependent catalytic activity in aerobic oxidation of the CH3 group bonded with aromatic rings. The smallest Cu12Ox materials achieved an excellent large turnover number (TON = 40 206) without any significant deactivation.

Chemical bonds in intercalation compounds CuxTiCh2 (Ch = S, Te).

A thorough study of the chemical bonding between intercalated copper and host lattice TiCh2 (Ch = S, Te) was performed. In order to separate the contributions of the copper, titanium, and chalcogen states into the electronic structure of the valence band, photoelectron spectroscopy in nonresonant and resonant (Cu 3p-3d and Ti 2p-3d) excitation modes was used. It is shown that the ionicity of the chemical bond between copper and host lattice is decreased in the TiS2 → TiSe2 → TiTe2 row. In CuxTiS2, copper atoms form the chemical bond with TiCh2 host lattice, while in CuxTiTe2 directly with tellurium atoms.

Sintering behaviour, lattice energy and microwave dielectric properties of melilite-type BaCo2Si2O7 ceramics

A novel melilite-type compound BaCo2Si2O7 with monoclinic structure (C2/c) was prepared via solid-state reaction method. The correlations amongst microstructure, crystal structure, bond ionicity, lattice energy and microwave dielectric properties were systematically analyzed. The microwave dielectric properties of BaCo2Si2O7 ceramics were determined by extrinsic (pores) and intrinsic (lattice energy) factors. The dielectric constant (εr), quality factor (Q × f) and temperature coefficient of resonant frequency (τf) were dominated by the relative density and the lattice energies of Si–O and Ba–O bonds. Optimum microwave dielectric properties for BaCo2Si2O7 ceramics were obtained when sintered at 1060 °C for 3 h: εr = 9.26, Q × f = 31 135 GHz and τf = −92.05 ppm C−1.

Structure, bond characteristics and microwave dielectric properties of new A0.75Ti0.75Ta1.5O6 (A[dbnd]Ni, Co, Zn and Mg) ceramics based on complex chemical bond theory

Tri-rutile structured A0.75Ti0.75Ta1.5O6 (ANi, Co, Mg, Zn) ceramics were synthesized using traditional solid reaction method. The crystal structures were studied by X-ray diffraction in conjunction with Rietveld refinement analysis. Based on the complex chemical bond theory and crystallographic data, some principle chemical bond characteristics such as bond ionicity, lattice energy, bond energy and coefficient of thermal expansion of complex A0.75Ti0.75Ta1.5O6 ceramics were obtained through quantitative calculation. The calculated results provided useful information to clarify the correlations between chemical bond characteristics and microwave dielectric properties of A0.75Ti0.75Ta1.5O6 ceramics. The dielectric constant was closely associated with the ionicity of TaO bond, and the Q × f values were correlated with the lattice energy of TaO bond. The τf values were affected by the bond energy of TaO bond and the coefficient of thermal expansion of AO bond.

Bond ionicity, lattice energy, bond energy and the microwave dielectric properties of non-stoichiometric MgZrNb2+xO8+2.5x ceramics

The intrinsic relationship between crystal structure, surface topography, P-V-L theory and dielectric properties of non-stoichiometric MgZrNb2+xO8+2.5x (−0.08 ≤ x ≤ 0.08) ceramics prepared by a normative solid-state reaction route were determined. X-ray diffraction patterns demonstrated that the appropriate addition of excess or insufficient niobium ions could not destroy the crystal phase, but change the lattice parameters and unit cell volume. Meanwhile, the formation of pure phase with monoclinic wolframite structure and P2/c space group was confirmed. The Rietveld refinement was used to obtain structure parameters and investigate the crystal structure of the specimens. SEM showed that insufficient niobium ions didn't show any impact on the dense structure, and the excess niobium ions promoted the growth of grains, but caused the appearance of undesired rough surfaces. The microwave properties as a function of the niobium ions content were found to be closely related to bond ionicity, lattice energy and bond energy calculated according to the P-V-L theory. As a result, when x was taken as −0.04 for the MgZrNb2+xO8+2.5x ceramics sintered at 1320 °C, the best dielectric properties, εr = 25.62, Q × f = 80828.1 GHz, τf = −44.64 ppm/°C, were successfully obtained.

Novel temperature stable NiSnTa2O8 microwave dielectric ceramics with trirutile structure

A novel low-loss and temperature stable NiSnTa2O8 ceramic with trirutile structure was prepared using traditional solid-state method. The structure-performance relationships were investigated by Rietveld refinement, chemical bond theory and far-infrared spectrum. The results show that the relative densities play a dominant role in the change of dielectric constant. Theoretical dielectric constant calculated via bond theory, Clausius-Mossotti equation and fitted result of far-infrared spectrum are close to experimental value. Ta–O bonds with greatest bond ionicity and bond energy have the primary contributions to dielectric polarizabilities and dielectric loss. The optimal microwave dielectric performances of NiSnTa2O8 ceramics were obtained: εr ∼21.04, Q×f ∼31328 GHz and τf = −2.63 ppm/°C at 1425 °C.

Influence of Zn0.5Ti0.5NbO4 on the structure and microwave dielectric properties of ZrTiO4 ceramics

In this work, xZn0.5Ti0.5NbO4-(1-x) ZrTiO4 (0.1 ≤ x ≤ 0.5) ceramics were prepared through the traditional solid-state route. The microstructure, phase evolution and crystallographic variations were investigated in detail. An orthorhombic structure solid solution was formed with the composition of Zr1-xZn0.5xTi1-0.5xNbxO4 (0.1 ≤ x ≤ 0.5). Variations of lattice parameters were responsible for the opposite movement of different XRD peaks as increasing x value. Chemical bond theories include that bond ionicity, bond susceptibility, lattice energy, bond energy and linear thermal expansion coefficient are correlated with dielectric properties. Temperature stable ceramics were obtained when x = 0.2 with excellent dielectric performances: Q × f = 26741 GHz, εr = 41.93 and τf = −2.3 ppm/oC.

Intrinsic dielectric behavior of Mg2TiO4 spinel ceramic

In the work, we focused on the intrinsic dielectric behavior of Mg2TiO4 spinel ceramic by P–V–L theory and infrared spectra analysis. Ti–O bonds have larger bond ionicity values, thus playing an important role in dielectric polarization. The theoretical dielectric constant was predicted by calculating the bond susceptibility of each chemical bond. Furthermore, Ti(1)–O bonds are responsible for the structural stability of Mg2TiO4 ceramic. Based on classical dispersion theory, permittivity and loss corresponding to each infrared active mode were quantified, and then the crucial contribution of low-frequency modes to intrinsic dielectric properties were determined.

Distinct electronic and transport properties between 1T-HfSe2 and 1T-PtSe2

In this study, we employed the first-principles calculation to investigate the structural, electronic and transport properties of 1T-HfSe2 and 1T-PtSe2 transition metal dichalcogenides, and further explain why they share the same 1T (octahedral) layered structure but exhibit very different electronic and transport properties. There are two underlying concepts: the degree of interlayer bond ionicity and the number of 5d valence electrons of transition metal. The high degree of Hf-Se bond ionicity not only gives rise to the indirect energy gap of HfSe2 bulk and thin films, but also results in the weak Se-Se vdW interlayer coupling to further restrict the electron transport only within a HfSe2 layer. On the other hand, the modulation of metallic/semiconducting property of PtSe2 bulk and thin films can be understood by the significant vdW interlayer coupling, which induces charge redistribution of Se atom and allows electrons to transport within a PtSe2 layer as well as cross neighboring layers. Finally, our transport calculation for 1T-HfSe2/1T-PtSe2 bulks and monolayers suggests the great electron transport within Hf-Se/Pt-Se layer but suppresses/allows electron from neighboring layers. The robust two-dimensional characteristic of 1T-HfSe2 and the metal-to-semiconductor transition of 1T-PtSe2 may provide more knowledge for future application in nanoelectronic and optoelectronic devices.

The SiC Single Crystal Growth from Nanomaterial Precursor

Unlike the conventional layer by layer growth, three dimensional growth experiments of SiC single crystal by the Chemical Particle Deposition (CPD)method were carried out both on the polar and nonpolar plane of the SiC seed crystal. The comparison of the morphology of the grown crystals on both samples indicated that the electric field formed by the seed crystal strongly effected the diffusion of the supplied Si and C atoms and their compounds to grow the epitaxial crystal. In spite of the low ionicity of Si-C bonds, this remarkable effect of the electric field on the three dimensional crystal growth mechanism in the CPD method strongly suggested its contribution to the ordering of the stacked layers with its long working range, beyond the deformed boundary layers between the seed surface and the grown crystal.

Combined synthesis methods for producing LaNbO4 ceramics and investigation of microwave dielectric properties based on complex chemical bond theory

Highly reactive LaNbO4 nanopowders with particle sizes of 20–50 nm were synthesized at 700 °C through the citric gel process. Niobium oxalate was used as raw material for the first time, which provided a more convenient and precise way for the whole process. In order to reduce the sintering temperature of the LaNbO4 ceramic without introducing other impurities, LaNbO4 nanopowders were added to densify the ceramic at a lower temperature. When different mass percentage x wt% (x = 0, 0.5, 1, 2, 4) of the nanopowders were added, the sintering temperature of LaNbO4 ceramics prepared through conventional solid-state reaction method was successfully lowered more than 100 °C. The obtained ceramics presented good microwave dielectric properties. Rietveld refinement was used to investigate the crystal structure of the samples. The relationships between bond ionicity and dielectric constant (εr), lattice energy and the Q×f0 value, bond energy and the temperature coefficient of resonant frequency (τf) were calculated and analyzed with P–V-L theory. LaNbO4 ceramics with 1 wt% LaNbO4 nanopowders additive sintered at 1275 °C for 4 h showed excellent microwave dielectric properties: εr = 20.26, Q×f0 = 59740 GHz and τf = 7.44 ppm/°C.

Divalent doping-induced thermoelectric power factor increase in p-type Bi2Te3 via electronic structure tuning

We use first-principles calculations to reveal the effects of divalent Pb, Ca, and Sn doping of Bi2Te3 on the band structure and transport properties, including the Seebeck coefficient, α, and the reduced power factor, α2σ/τ, where σ is the electrical conductivity and τ is the effective relaxation time. Pb and Ca additions exhibit up to 60%–75% higher peak α2σ/τ than that of intrinsic Bi2Te3 with Bi antisite defects. Pb occupancy and Ca occupancy of Bi sites increase σ/τ by activating high-degeneracy low-effective-mass bands near the valence band edge, unlike Bi antisite occupancy of Te sites that eliminates near-edge valence states in intrinsic Bi2Te3. Neither Pb doping nor subatomic-percent Ca doping increases α significantly, due to band averaging. Higher Ca levels increase α and diminish σ, due to the emergence of a corrugated band structure underpinned by high-effective-mass bands, attributable to Ca–Te bond ionicity. Sn doping results in a distortion of the bands with a higher density of states that may be characterized as a resonant state but decreases α2σ by up to 30% due to increases in the charge carrier effective mass and decreases in both spin–orbit coupling and valence band quasidegeneracy. These results, and thermal conductivity calculations for nanostructured Bi2Te3, suggest that Pb or Ca doping can enhance the thermoelectric figure of merit ZT to values up to ZT ∼ 1.7, based on an experimentally determined τ. Our findings suggest that divalent doping can be attractive for realizing large ZT enhancements in pnictogen chalcogenides.

A Correlation between Hardness and Bond Ionicity in Crystals

A Correlation between Hardness and Bond Ionicity in Crystals

Intrinsic dielectric properties of columbite ZnNb2O6 ceramics studied by P–V–L bond theory and Infrared spectroscopy

AbstractIn this study, intrinsic dielectric properties of ZnNb2O6 ceramics were investigated using P–V–L chemical bond theory and far‐infrared reflectivity spectra. The largest bond ionicity and bond susceptibility of Nb–O bonds suggest that the dielectric polarizability is mainly determined by Nb–O bonds. Relative permittivity, calculated via P–V–L bond theory, is close to experimental value determined using TE011 mode. The Nb–O bonds are also crucial for the structural stability. According to far‐infrared reflectivity spectra and complex dielectric function analysis, a relatively consistent result between calculated and measured dielectric properties, fit using classical damped oscillator mode, indicate that the B1u mode at 168.87 cm−1 provides majority contribution to the dielectric properties.

Bond ionicity, lattice energy and structural evolution of Ta substituted 0.15ZnO-0.15Nb2O5-0.55TiO2 dielectric ceramics

In the present study, influences of Ta5+ substitution on Nb site on crystal structural evolution, Raman spectra and microwave dielectric properties of Zn0.15Nb0.3-xTaxTi0.55O2 (0 ≤ x ≤ 0.3) ceramics were investigated. Bond ionicity, lattice energy and octahedral distortion of Zn0.15Nb0.3-xTaxTi0.55O2 structure calculated based on chemical bond theory and Rietveld refinement analysis were used together to understand correlation between crystal structure and microwave dielectric properties. The results showed that rutile Zn0.15Nb0.3-xTaxTi0.55O2 solid solutions could be synthesized in whole composition range of x = 0–0.3. With increase of x value, cell volume and bond length are reduced, which is confirmed by blue shift of Raman vibrational peaks. Dielectric constant (εr) is closely related to average NbO bond ionicity. Quality factor (Q × f) values and temperature coefficient values of resonant frequency (τf) are mainly affected by lattice energy and octahedral distortion respectively. In Zn0.15Nb0.3-xTaxTi0.55O2 ceramics, proper substitution of Ta5+ ion efficiently improves Q × f values and reduces the τf values. Optimal microwave dielectric properties of εr = 75.87, Q × f = 22050 GHz and τf = 234.6 ppm/°C were obtained for Zn0.15Nb0.3-xTaxTi0.55O2 solid solutions at x = 0.2 when sintered at 1100 °C.

Crystal structure and microwave dielectric properties of La2(Zr1−xTix)3(MoO4)9 (0 ≤ x ≤ 0.1) ceramics

AbstractLa2(Zr1−xTix)3(MoO4)9 (0 ≤ x ≤ 0.1) ceramics were prepared by the traditional solid‐state reaction method. XRD analysis showed that La2(Zr1−xTix)3(MoO4)9 (0 ≤ x ≤ 0.1) ceramics belonged to a trigonal system. Based on the chemical bond theory, the consequences of bond energy, bond ionicity, lattice energy, and thermal expansion coefficient of ceramics on microwave dielectric properties were discussed. As Ti4+ addition was increased, the reduction in dielectric constant was ascribed to the fact that the polarizability of Ti4+ is smaller than Zr4+, and the downward trend was related to the bond ionicity. Besides, the tendency of Q·f value depended on the packing fraction and the lattice energy. The improvement in τf value, the increase in bond energy, and the decrease in the coefficient of thermal expansion were all correlated. The far‐infrared spectra implied that the absorptions of structural phonon oscillation were the main reason for the maximum polarization contribution. La2(Zr0.92Ti0.08)3(MoO4)9 ceramics sintered at 750°C for 4 hours exhibited the best dielectric properties (εr = 10.33, Q·f = 80 658 GHz, and τf = 3.48 ppm/°C).

The Correlations between Complex Chemical Bond Theory and Microwave Dielectric Properties of Ca2MgSi2O7 Ceramics

Magnesium melilite (Ca2MgSi2O7) ceramic with tetragonal crystal structure was prepared through the conventional solid-state method. Crystal structure and complex chemical bond theory were used to analyze the microwave properties of Ca2MgSi2O7 ceramics and the correlations between the bond ionicity, lattice energy, bond energy and relative dielectric constant (er), quality factor (Q × f0), temperature coefficient of resonant frequency (τf) were discussed in depth. Optimum microwave dielectric properties were obtained when sintered at 1300°C for 4 h: er= 9.86, the dielectric loss (tanδ) was 1.24 × 10−4 (at 1 MHz), Q × f0= 8016 GHz (resonant frequency f0 = 7.90 GHz), τf= − 42 ppm/°C.

Ba2BiSbO6: A novel microwave dielectric ceramic with monoclinic structure

A novel Ba2BiSbO6 microwave dielectric ceramic was prepared via the traditional solid-state method, and the microwave dielectric properties of Ba2BiSbO6 were investigated for the first time. The analysis of X-ray diffraction patterns indicated that the pure monoclinic Ba2BiSbO6 phase could be formed in the temperature range of 950–1150 °C. Rietveld refinement results further confirmed that Ba2BiSbO6 belonged to a monoclinic system with space group I2/m (12). Dense and homogeneous microstructures of Ba2BiSbO6 ceramics sintered at 1100–1150 °C could be revealed from the scanning electric microscope. The bond ionicity, lattice energy and coefficient of the thermal expansion were initially calculated based on the complex chemical bond theory. The complex dielectric constant and loss were gained from the fitting data of far infrared spectra, which were in good agreement with the measured values. The optimum microwave dielectric properties of εr = 26.61, Q·f = 13,764 GHz (at 5.95 GHz) and τf = −26.6 ppm/°C were achieved in Ba2BiSbO6 ceramic sintered at 1100 °C.

Crystal Chemistry, Raman Spectra, and Bond Characteristics of Trirutile-Type Co0.5Ti0.5TaO4 Microwave Dielectric Ceramics.

A novel trirutile-type Co0.5Ti0.5TaO4 ceramic was reported here for the first time. The correlations between the sintering behavior, crystal structure, chemical bond, and dielectric properties were investigated. Pure Co0.5Ti0.5TaO4 ceramic was synthesized in the temperature range of 1000-1100 °C. A trirutile structure and refined parameters of a = b = 4.71163 Å, c = 9.13586 Å, and Vcell = 202.811 Å3 could be obtained (1075 °C). According to the P-V-L chemical bond theory, majority contributions to the dielectric constant originated from Ta-O bonds, owing to its largest bond ionicity and bond susceptibility values. The experimental dielectric constant is close to the theoretical values calculated via the P-V-L chemical bond theory and Clausius-Mossotti relationship. The Ta-O bonds that present the largest lattice energy are also the main factors influencing the intrinsic loss. The τf value is consistent with the oxygen distortions of the octahedron. More importantly, variations of the densification, average grain size, and grain boundary are crucial factors for development of the microwave dielectric properties. The Raman spectra and group theory were analyzed together, and the results indicated that the A1g mode at 687.45 cm-1, which reflects the stretching vibrations of the O anions, dominates the Raman vibrations. Typical microwave dielectric properties of Co0.5Ti0.5TaO4 ceramics were obtained when sintered at 1075 °C: εr = 40.69, Qf = 17291 GHz, and τf = 114.54 ppm/°C.