Nb-O Bond Research Articles

Exploring Phase Transition and Structural Complexity in the Mixed Cation Uranium Oxide CaUNb2O8.

The structures and high-temperature phase transition of CaUNb2O8 were studied in situ using synchrotron X-ray and neutron powder diffraction. Rietveld refinements provided an accurate description of the crystal structures of both the monoclinic fergusonite-type I2/b structure observed at room temperature and the tetragonal scheelite-type I41/a structure found at high temperatures. Bond valence sum analysis showed Nb5+ to be octahedrally coordinated in the monoclinic fergusonite-type structure, akin to other ANbO4 materials. Rietveld analysis of the variable temperature data allowed for the determination of accurate unit cell parameters and atomic coordinates, as well as revealing a reversible phase transition around ∼750 °C. The Nb-O bond distances display anomalous behavior, with a discontinuity in the longer Nb-O(1') distance coinciding with the phase transition suggestive of a reconstructive phase transition. Mode analysis identified the Γ2+ mode as the primary mode that drives the phase transition; this is linearly coupled to the induced spontaneous strain within the monoclinic fergusonite-type structure. Analysis of the temperature dependence of the Nb(z) positional parameter, as well as of the ϵ1-ϵ2 and ϵ6 strain parameters, showed that the phase transition is not strictly second order, with the critical exponent β ≠ 1/2. This study demonstrates the complex structural features of mixed cation metal oxides at elevated temperatures.

Unleashing the impact of Nb-doped, single crystal, cobalt-free P2-type Na0.67Ni0.33Mn0.67O2 on elevating the cycle life of sodium-ion batteries

A synergistic stabilization effect in a Nb-doped P2-type single crystal cobalt-free layered oxide cathode material, offering remarkable cycling stability and high-power performance for Na-ion batteries have unveiled in this study. The introduction of Nb in the transition metal layer not only reduces the electronic band gap but also enhances electronic conductivity and mitigates ionic diffusion energy barriers. The induction of a robust Nb-O bond expedites electron and Na+ transfer, contributing to the stabilization of the host structure is further confirmed through the density functional theory calculations, including electron localization function (ELF) and crystal orbital Hamiltonian population (COHP). To the best of our knowledge, this study is the first to demonstrate a homogeneous distribution of niobium throughout the single crystal, specifically doped at the nickel site within the bulk, without inducing atomic-scale surface reorganization. The presence of single crystals improves various kinetic factors, demonstrating the profound correlation between structural defects and chemical proliferation, thereby reducing the evolution of oxygen gas. The P2-type Nb-doped single crystal cathode (Na0.67Ni0.31Mn0.67Nb0.02O2) exhibits remarkable capacity retention, >95 % after 100 cycles at 0.1 C and >90 % after an extended cycling of 2000 cycles at 1 C. Practical assessments in complete cell setups with a pre-sodiated hard carbon anode further validate the material's viability, showcasing capacity retention of over 93 % after 100 cycles in a coin cell and approximately 89 % in a pouch cell format. This comprehensive study establishes the transformative potential of Nb-doped single crystal cobalt-free P2-type layered oxide cathode materials, marking a significant advancement in sodium-ion battery technology.

Metal–insulator transition and G-type antiferromagnetism in CaNbO[formula omitted] perovskite: Insight from DFT+U study

In this work, we have investigated the ground state electronic and magnetic properties of CaNbO3 perovskite using the density functional theory, along with the Hubbard corrections. The study of structural, electronic, and magnetic properties was carried out using semi-empirical parameters. The structural properties reveal that Nb-O bond lengths are unequal in all directions, and ∠O-Nb-O bond angles are not equal to 90°, which makes NbO6 octahedra distorted. The electronic behavior of the compound, considering the strong correlation effect, shows a metal–insulator transition. The reason for this transition is found to be Jahn–Teller distortion. The stable magnetic state of the compound is obtained to be a G-type antiferromagnetic, using various U values.

Bioinspired Herbicides-Ionic Liquids or Liquid Cocrystals?

This research provides information about combinations of several amino acids, including l-proline (Pro), l-arginine (Arg), and l-histidine (His), with phenoxyacetic acid herbicides (MCPA and 2,4-D). Five amino acid ionic liquids (AAILs), one amino acid higher-melting salt (AAHMS), and two amino acid liquid cocrystals (AALCs) were obtained in high yields (>90%). The ionization of the six new structures was confirmed by NMR, IR, and molecular modeling. X-ray crystallography was used to definitively confirm the binding location of the mobile hydrogen. Furthermore, we propose a computational method for estimating the energy of specific hydrogen bond(s) in AAIL crystals based on the NBO and QTAIM hydrogen bond parameters obtained by model calculations. An in-depth analysis of the structures allowed to answer the question posed in the title, ionic liquids or liquid cocrystals? AAILs based on arginine and histidine were obtained. In contrast, combining proline with MCPA and 2,4-D led to AALCs. Finally, the compounds were analyzed to measure their herbicidal activity. These studies proved that the novel form of MCPA or 2,4-D improved its ability to control weeds compared to commercial formulations containing the same active ingredients.

Mechanistic Insights into S-Depalmitolyse Activity of Cln5 Protein Linked to Neurodegeneration and Batten Disease: A QM/MM Study.

Ceroid lipofuscinosis neuronal protein 5 (Cln5) is encoded by the CLN5 gene. The genetic variants of this gene are associated with the CLN5 form of Batten disease. Recently, the first crystal structure of Cln5 was reported. Cln5 shows cysteine palmitoyl thioesterase S-depalmitoylation activity, which was explored via fluorescent emission spectroscopy utilizing the fluorescent probe DDP-5. In this work, the mechanism of the reaction between Cln5 and DDP-5 was studied computationally by applying a QM/MM methodology at the ωB97X-D/6-31G(d,p):AMBER level. The results of our study clearly demonstrate the critical role of the catalytic triad Cys280-His166-Glu183 in S-depalmitoylation activity. This is evidenced through a comparison of the pathways catalyzed by the Cys280-His166-Glu183 triad and those with only Cys280 involved. The computed reaction barriers are in agreement with the catalytic efficiency. The calculated Gibb's free-energy profile suggests that S-depalmitoylation is a rate-limiting step compared to the preceding S-palmitoylation, with barriers of 26.1 and 25.3 kcal/mol, respectively. The energetics were complemented by monitoring the fluctuations in the electron density distribution through NBO charges and bond strength alterations via local mode stretching force constants during the catalytic pathways. This comprehensive protocol led to a more holistic picture of the reaction mechanism at the atomic level. It forms the foundation for future studies on the effects of gene mutations on both the S-palmitoylation and S-depalmitoylation steps, providing valuable data for the further development of enzyme replacement therapy, which is currently the only FDA-approved therapy for childhood neurodegenerative diseases, including Batten disease.

Electronic structure and possible phase transition mechanism of different phases for perovskite structured AgNbO3

Electronic structure, crystal structural evolution and elastic properties of AgNbO3 for phases Pmc21, Pbcm, Cmcm, P4/mbm and Pm3m have been investigated by first-principles calculations. The differences of electronic structure among phases are caused by the various intensity of Nb-4d, O-2p and Ag-4d, 5 s orbitals hybridization. The Nb-O bond and the torsion angle of NbO6 octahedron in different phases are investigated, and the lattice distortion can be attributed to the Jahn-teller effect. The crystal structural evolution among different phases is correlated with the phase transition closely. The elastic constant matrix of each phase satisfies the condition of mechanical stability. AgNbO3 is the ductile material and has the largest rigidity and the strongest volumetric strain resistance in the cubic phase.

Tautomerism in pyridinyl methyl β-diketones in the liquid and the solid state; a combined computational and experimental study

Three 1-(n-pyridinyl)butane-1,3-diones (nPM) (n = 2, 3, and 4) molecules were synthesized and their molecular structure, tautomerism, conformational stability, and intramolecular hydrogen bonding (IHB) of cis-enol forms, was investigated by a combination of computational and experimental methods. Density functional theory (DFT) calculations were used and the Atoms in Molecules (AIM) analysis indicated an intramolecular hydrogen bond strength as 17.9–20.3 kcal/mol using the B3LYP/6–311++G(d,p) level of theory. The calculated structures, NBO, and intramolecular hydrogen bond strengths for the stable cis-enol forms of nPM molecules were compared with each other and with those of acetylacetone (AA) and benzoylacetone (BA). The single crystal X-ray structure of 4PM confirmed that the tautomer with the OH group next to the pyridine ring best describes the structure in the solid-state. The vibrational bands of the title molecules were assigned to the corresponding normal modes, and the equilibrium between the two cis enol tautomers was estimated by means of IR, Raman and UV/Vis spectroscopy.

Orbital hybridization manipulated by doped Cu+ in NbC for boosting hydrogen evolution

Surface adsorption behavior and electron transfer determine the catalytic activity in hydrogen evolution reaction (HER). However, the inevitable surface oxidation of NbC hinders electron transfer and hydrogen conversion. The surface adsorption behavior and electron transfer of NbC are optimized by doping Cu+ as well as modifying the orbital hybridizations. Benefiting from the introduced electron interaction of Cu+, the lower p-d hybridization between Nb and O reduces the Nb-O bond and affords the Cu-NbC thinner surface oxidation layer. The reduced surface oxidation degree favors electron transfer. Further, the electron orbital hybridization modulates the electron states and optimizes the hydrogen adsorption behavior of Cu-NbC for efficient hydrogen conversion. As the electrochemical results, the obtained Cu-NbC demands 271 mV to drive 10 mA cm−2, which is 50 times higher than that of NbC (only 0.2 mA cm−2 at the same overpotential). Hence, we believe the orbital hybridization manipulation strategy renders a valuable solution for designing efficient HER catalysts.

QM/MM study of N501 involved intermolecular interaction between SARS-CoV-2 receptor binding domain and antibody of human origin

Intermolecular interaction between key residue N501 of the epitope on SARS-CoV-2 RBD and screening antibody B38 was studied using the QM/MM and QM approach. The QM/MM optimized geometry shows that angle X-H---Y is 165° for O-H---O between mAb light chain S30 and RBD N501. High level MP2 calculations indicated the interaction between RBD N501 and S30 of B38 Fab light chain provide a relatively strong attractive force of − 3.32 kcal/mol, whereas the hydrogen bond between RBD Q498 and S30 was quantified as 0.10 kcal/mol. The decrease in ESP partial charge on hydrogen atom of hydroxyl group on S30 drops from 0.38 a.u. to 0.31 a.u., exhibiting the sharing of 0.07 a.u. from the lone pair electron oxygen of N501 due to hydrogen bond formation. The NBO occupancy of hydrogen atom also decreases from 25.79 % to 22.93 % in the hydroxyl H-O NBO bond of S30. However, the minor change of NBO hybridization of hydroxyl oxygen of S30 from sp3.00 to sp3.05 implies the rigidity of hydrogen bond tetrahedral geometry in the relative dynamic protein complex. The O-H---O angle is 165° which is close but not exactly linear. The structural requirement for sp3 hybridization of oxygen for hydroxyl group on S30 and dimension of protein likely prevent O-H---O from adopting linear geometry. The hydrogen bond strengths were also calculated using a variety of DFT methods, and the result of − 3.33 kcal/mol from the M06L method is the closest to that of the MP2 calculation. Results of this work may aid in the COVID-19 vaccine and drug screening.

Tuning ferroelectric photovoltaic performance in R3c-CuNbO3 through compressive strain engineering: a first-principles study.

Most ferroelectric oxides exhibit relatively wide bandgaps, which pose limitations on their suitability for photovoltaics application. CuNbO3 possesses potential ferroelectric properties with an R3c polar structure that facilitate the separation of charge carriers under illumination, promoting the generation of photovoltaic effects. The optical and ferroelectric properties of R3c-CuNbO3, as well as the effect of strain on the properties are investigated by first-principles calculation in this paper. The calculated results indicate that R3c-CuNbO3 possesses a moderate band gap to absorb visible light. The interaction of Cu-O and Nb-O bonds is considered to have a crucial role in the photovoltaic properties of CuNbO3, contributing to the efficient absorption of visible light. The bandgap of CuNbO3 becomes smaller and the density of states near the conduction and valence bands becomes relatively uniform in distribution under compressive conditions, which improves the photoelectric conversion efficiency to 29.9% under conditions of bulk absorption saturation. The ferroelectric properties of CuNbO3 are driven by the Nb-O bond interactions, which are not significantly weakened by the compressive strain. CuNbO3 is expected to be an excellent ferroelectric photovoltaic material by modulation of compressive strain due to the stronger visible light absorption and excellent ferroelectric behavior.

Heterobimetallic 21,23-dimetallaporphyrin: activation of metal-metal interactions within the porphyrinoid macrocycle.

Two core-modified porphyrins containing metal atoms, namely platinum(II) or platinum(IV) and rhodium(III), in place of two NH units, have been obtained by a post-synthetic modification of the 21,23-ditelluraporphyrin. The products of the tellurium-to-metal exchange, 21-platina-23-rhodaporphyrins, incorporate rhodacyclopentadiene and platinacyclopentadiene units with the metal atoms facing each other. The two molecules exhibit different degrees of metal-metal interaction depending on the oxidation state of platinum, with the NBO bond order being 0.04 for platinum(IV) and 0.15 for platinum(II). Consistently, the Quantum Theory of Atoms in Molecules analysis revealed the presence of the bond determinant, the bond critical point, in the platinum(II) species, in contrast to the platinum(IV) congener. The two porphyrinoids are interconvertible in redox reactions. They both exhibit fluxional behaviour in solution, studied by 1H NMR, involving alteration in the metal ion coordination sphere accompanied by the macrocyclic skeleton conformation change.

Crystal structure and microwave dielectric characteristics of ixiolite ceramics with molybdenum ion modification and tri-layered structure

The ZnTi1−xMoxNb2O8 (0.000 ≤ x ≤ 0.060) microwave dielectric ceramics were prepared through traditional solid-state reaction. Mo4+ substitution for Ti4+ can markedly improve the microwave dielectric characteristics of ZnTiNb2O8 system. In the microstructures investigation, a suitable degree of Mo4+ substitution was shown to be useful for creating uniform grain size and minimizing dielectric loss. Raman spectroscopy and complex chemical bond theory analyses demonstrate that the bond ionicity of NbO bond is the intrinsic component influencing the εr value. The lattice energy of NbO bond, bond valence of Cation-O bond, and the FWHM of Raman spectra were the primary determinants of Q×f value. The τf value is mostly determined by variations in NbO bond energy and the bond valence of Cation-O bond. When x = 0.010, ZnTi0.990Mo0.010Nb2O8 ceramics exhibit excellent microwave dielectric characteristics: εr = 33.91, Q×f = 64,136 GHz, and τf = − 50.57 ppm/°C. Furthermore, in order to improve the temperature coefficient of resonant frequency (τf values close to zero) for the ZnTi0.990Mo0.010Nb2O8 ceramics, the TiO2 and laminated co-firing technique were used to adjust the comprehensive microwave dielectric characteristics of the ZnTiNb2O8 system. When the mass fraction of the TiO2 layer reaches 0.03 wt% after sintering at 1150 °C, the tri-layer structural ZTMN-0.03TO-ZTMN ceramics obtain outstanding microwave dielectric characteristics: εr = 41.52, Q×f = 50,827 GHz, and τf = − 2.59 ppm/°C, and these characteristics make them potential for usage in 5G communications.

Utilizing fast ion conductor for single-crystal Ni-rich cathodes to achieve dual-functional modification of conductor network constructing and near-surface doping

Ni-rich transition metal oxides are regarded as appealing candidate cathodes for next-generation lithium batteries because of their high discharge capacity. Single-crystal is an effective means to restrain the formation of microcracks to improve the cyclic property of LiNi0.8Co0.15Al0.05O2 (NCA) materials. Nonetheless, critical structural and interfacial instabilities make single-crystal NCA cathodes hard to achieve ideal electrochemical performance at high voltage. Herein, the fast ion conductor Li5La3Nb2O12 with higher conductivity and wider electrochemical window was selected as the material to enhance the high voltage performances for single-crystal NCA. Interestingly, density functional theory calculations found that La and Nb have different distribution characteristics in NCA. Therefore, we take advantage of oxidation reaction and element diffusion during high temperature sintering to achieve dual-functional modification of La-rich conductor network constructing and near-surface Nb doping for single-crystal NCA material. Impressively, the modified electrodes exhibit excellent cycling performances with a retention of 82.9% after 500 cycles at 2.7–4.3 V and even outstanding stability under extended cutoff voltage as well as improved rate capabilities. Detailed study reveals that the doped Nb with stronger Nb-O bond can suppress the migration of transition metal ion, thus impeding the distortion of material structure. Meanwhile, surface ion-conductor network layer provides fast Li+ diffusion channels as well as protecting interface layer from electrolyte decomposition products. The facile approach of Li5La3Nb2O12 modification proposed in this manuscript is suitable to solve interfacial instability and structure stability for cathodes but not only for single-crystal NCA, which provides application hopes for high energy lithium-ion batteries.

Niobium speciation in minerals revealed byL2,3-edges XANES spectroscopy

AbstractThe systematic mineralogy of niobium (Nb) is complex, with more than one hundred species dominated by multicomponent oxides of similar chemistry. The determination of Nb speciation in solids (i.e., the distribution between the phases and the crystal-chemical environment of Nb) is thus a challenge in geological contexts. Here, we present the first Nb L2,3-edges X-ray absorption near-edge structure (XANES) measurements on various Nb minerals and synthetic oxides with geological relevance. The interpretation of Nb L2,3-edges XANES spectra in the light of crystal-field theory shows the sensitivity of spectra to local site symmetry and electronic environment around Nb atoms. Crystal-field multiplet simulations give estimates of the 10Dq crystal-field parameter values for Nb5+, which range from 2.8 to 3.9 eV depending on Nb coordination and Nb—O distances. Rather than a 10Dq vs. R–5 relationship (where R represents the average Nb-O bond distance) expected in a point-charge model, we find a R–3 dependence with the crystal-field splitting for reference materials with octahedrally coordinated Nb. Complementary ligand-field multiplet simulations provide evidence of charge transfer between Nb and O. The contribution of the ionic and covalent characters to the Nb-O bonds is equivalent, unlike more ionic 3d metal–O bonds. This systematic characterization of the L2,3-edges XANES spectral properties of Nb provides information on the mechanisms by which Nb5+ substitutes for Fe3+, Ti4+, or Ce4+ in oxides common in geological contexts. Whereas the substitution of Nb5+ for Ce4+ does not modify the local structure of the cation site in cerianite, the substitution of Nb5+ for Ti4+ in rutile and anatase results in an increase of the cation-ligand distance and a decrease in the symmetry of the cation site. Conversely, the substitution of Nb5+ for Fe3+ in hematite and goethite results in a smaller cation site distortion. Our study demonstrates the usefulness of L2,3-edges XANES spectroscopy to determine Nb speciation in minerals to understand the processes of enrichment of this critical metal.

Significantly fastened redox kinetics in single crystal layered oxide cathode by gradient doping

Nickel-rich layered cathode is a forefront candidate for lithium-ion batteries; however, structural degradation such as intragranular cracking and phase transition in polycrystals focus researchers’ attention towards single-crystals. Nevertheless, the sluggish ionic transport and redox kinetics in single-crystals hinder their rate capability and electrochemical performance in harsh conditions. Herein, a synergy of gradient Nb doping on single-crystal LiNi0.8Co0.1Mn0.1O2 stabilizes the core by strong Nb-O bond and induces Li/Ni antisite migration forming a disordered buffer layer on the surface. The disordered structure maintains the structural integrity by suppressing the phase transition and triggering the Li+ transport. Thus, the modified single-crystal (SNCM@Nb-2) delivers remarkable capacity retention of 92.54% after 100 cycles at 1 C between 2.7 and 4.3 V@ 25 °C (84.26% for SNCM, respectively) and 86.7% at an elevated temperature of 55 °C (SNCM 76.69%). The fast redox kinetics results in remarkably enhanced rate capability and ultra-stable high voltage performance at 4.5 V with SNCM@Nb-2 delivers an initial discharge capacity of 285.2 mAh g−1 with retention of 79.8% (SNCM with 192.1 mAh g−1, 64.1%). The findings of this study provide advancement for the inadequate kinetic properties of single-crystal Ni-rich cathodes under harsh operational conditions.

Elaboration and characterization of ferroelectric materials Sr2(1-x)Na(1+x)GdxNb5O15 synthesized by solid process

The solid solutions Sr2(1-x)Na(1+x)GdxNb5O15 (SNGN) with substitution rates of Sr2+ by Gd3+ varying between 0 and 1 were prepared and have undergone several heat treatments. The XRD spectra show that the powders calcined at 650 °C for 6 h exhibit a mixed structure consisting of orthorhombic-quadratic phases. On the other hand, calcination at 1200 °C for 12 h ensures complete crystallization of the SNGN compounds in the TTB structure of quadratic symmetry. The SEM images show grains of regular and spherical shapes. The Raman spectra of the solid SNGN solution calcined at 1200 °C (x = 0; 0.2; 0.4 and 0.6), show two broad bands attributed to the transverse deformation modes of the O-Nb-O bond and of elongation modes of the Nb-O bond of octahedra (NbO6). IR spectroscopy highlights the existence of absorption bands corresponding to the elongation and deformation vibrations of some bonds. The temperature measurements of the dielectric permittivity εr of the ceramic (SNN; x = 0) show for all the frequencies used, the presence of a narrow peak during the heating and the cooling phases with a maximum located respectively at the Curie temperature Tc ∼ 235.07 °C and Tc ∼ 224.84 °C. The dielectric losses tan (δ) values are very weak in the ferroelectric phase (T < Tc), then an increase, which reaches 2.3 for the lowest frequency, occurs in the paraelectric phase. The maximum permittivity ε'rmax undergoes a decrease as the frequency increases; this diffusive behavior can be associated to the domains walls in the micro-domains of the ferroelectric phase.

Quantum and dynamic investigations of Complex iron- alkaloid-extract Cytisine derivatives of Retama monosperma (L.) Boiss. Seeds as eco-friendly inhibitors for Mild steel corrosion in 1M HCl

• The adsorption of Cytisine derivatives on the iron surface was modeled using DFT calculations as well as the dynamic Monte Carlo simulations. • DFT calculations were used to study the interaction of these inhibitors with iron Fe n (n=1-5). • MC and MD simulations were adopted to simulate the adsorption of Cytisine derivatives on the iron Fe (111) surface in vacuum as well as in acidic solution. • The radial distribution function (RDF), mean square displacement (MSD) and fractional free volume (FFV) were also studied using MD simulation. In this study, the adsorption of Cytisine derivatives as an eco-friendly inhibitor for mild steel corrosion [ 1 , 2 ] on the iron surface was modeled using density functional theory (DFT) calculations and the Monte Carlo (MC) as well as Molecular dynamics (MD) simulations to elucidate the interactions involved. Firstly, the interaction of these inhibitors with iron clusters Fe n (n=1-5) was studied by DFT as a representative example of a corrosion inhibition process. Moreover, structures, binding energies, partial atomic charge NBO and natural bond orbital analysis of the resulting complexes support the existence of considerable binding between these inhibitors and Fe n (n=1-5) clusters. Then, the dynamic Monte Carlo (MC) simulation was also adopted to simulate the adsorption of Cytisine derivatives on the iron Fe (111) surface in vacuum as well as in acidic solution containing 491 H 2 O, 4 hydronium ion H 3 O + and 5 chlorine ion Cl − . Besides, to advance the understanding of interactions between these molecules and iron surface, the radial distribution function (RDF), mean square displacement (MSD) and fractional free volume (FFV) were also studied in this paper. Our results of both the Fe n (n=1-4) clusters and the Fe (111) surface made it possible to show noticeable competitiveness between the three protonated inhibitors with a slight superiority for the n-CytN9H+ molecule. Furthermore, based on the diffusion coefficient we found that the three protonated inhibitors have a good capacity to slow down the movement of corrosive particles and consequently to inhibit the metallic substrate against corrosive electrolyte.

Ge-doped Li3+xMg2Nb1-xGexO6 ceramics with enhanced low loss and high temperature stability properties

New high-performance materials have attracted much attention due to ever-increasing demands for advanced communication technologies. In present work, Ge-doped Li3+xMg2Nb1-xGexO6 (0 ≤ x ≤ 0.08) ceramics are prepared via solid-state reaction route. Microstructural analysis and crystal structure refinement reveal that moderate substitution can promote grain growth and modify crystal structure, thus enhancing microwave dielectric properties of composites. In that sense, special attention is paid to the behavior of dielectric constant εr, quality factor Q×f, and frequency temperature coefficient τf of final products. In these systems, εr parameter depends on the density, miscellaneous phases, and polarizability; Q×f value is shown to be influenced by Nb-O bond energy, grain size, and bulk density; finally, τf characteristic refers to Nb-O bond valence and NbO6 octahedral distortion. Among above ceramics, Li3.02Mg2Nb0.98Ge0.02O6 composite sintered at 1250 °C exhibits outstanding microwave absorption performance with εr = 15.32, Q×f = 969 88 GHz, and τf = −8.25 ppm/°C.

Structural dependence of microwave dielectric performance of wolframite structured Mg1-xCaxZrNb2O8 ceramics: Crystal structure, microstructure evolution, Raman analysis and chemical bond theory

Wolframite-structured Mg1-xCaxZrNb2O8 (0−0.1) ceramics were synthesized through a solid-state procedure. Crystal refinement indicates a pure Mg1-xCaxZrNb2O8 ceramic with a wolframite structure. Ca2+ substitution led to the redshift of the Ag mode at approximately 896 cm−1. Additionally, Ca2+ substitution could promote grain growth and contribute to microstructure evolution from a polyhedral shape to a rod shape. According to chemical bond theory, an appropriate Ca2+ concentration can increase NbO bond iconicity and NbO bond lattice energy, which contributed to the great improvement in the dielectric constant and Q × f value. Additionally, the τf value was affected by the bond valence and thermal expansion coefficient of the MgO bond. The Mg1-xCaxZrNb2O8 (x = 0.04) ceramics exhibited great improvement in the Q × f value: εr = 25.21, Q × f = 116,000 GHz (@7.17 GHz) and τf = −24.4 ppm/°C, which provides enormous potential for future millimeter-wave applications.

Phonon anharmonic investigation on the different structural phase transition processes of cubic KNbO3 and KTaO3

KTaO3 and KNbO3, two typical perovskite ferroelectric crystal materials with identical cubic structure at high temperature, undergoes totally different phase transition path. The underlying mechanism is explored by comparing their phonon anharmonic behavior based on first-principle calculation. The results of Grüneisen parameters indicate larger phonon anharmonicity of KNbO3 than that of KTaO3, which leads to structural unstable of KNbO3. Different with previous studies of soft mode theory, apparent phonon anharmonicity of both crystals are stemmed not from the center, but the corner and boundary of the first Brillouin zone. In order to essentially understand the difference in the anharmonic behavior of phonons, the chemical bond strength of these two perovskite materials was analyzed by ELF. The results show that due to the stronger covalent properties of Ta-O bond, it will cause weak phonon anharmonicity, and ultimately lead to high phonon relaxation time and low Grüneisen parameters of KTaO3. The weakly ionic Nb-O bond give rise to the obvious phonon anharmonic behavior and lattice instability of KNbO3, resulting in a series of temperature-dependent phase transitions.