Nb Bond Research Articles

Unlocking the secrets of pressure-driven physical properties of NaNbO3 perovskite-oxide: A computational insight

The current investigation presents the various pressure-induced properties such as structural, elastic, mechanical, optical, and electronic features of NaNbO3 across a pressure range of 0.0–100 GPa. Utilizing the CASTEP tool alongside GGA-PBE based on DFT, comprehensive calculations are conducted. The material displays an indirect energy bandgap, with its magnitude increasing from 1.633 to 2.223 eV under rising pressure. Analysis reveals a covalent character in O–Nb bonds, while O–O and Na–O interactions exhibit an ionic nature. As pressure rises, both lattice parameters and volume exhibit a tendency to decrease. Interestingly, the material exhibits anisotropic behaviour and maintains mechanical stability, transitioning from brittleness at 0.0 GPa to ductility between 10 and 100 GPa. Additionally, it demonstrates superhard characteristics at 0.0–20 GPa and progresses to an ultrahard state from 30 to 100 GPa. Optical properties display a trend of sharpening peaks with a slight shift towards higher energy in the curves. Notably, the material's indirect bandgap renders it suitable for prolonged photocatalysis applications.

Flotation performance and adsorption mechanism of α-hydroxyoctyl phosphinic acid to pyrochlore

The flotation performance and adsorption mechanism of α-hydroxyoctyl phosphinic acid (HPA) on pyrochlore were investigated through micro-flotation experiments, zeta-potential tests, contact angle measurements and XPS analysis. The flotation results demonstrated that, in comparison to N-hexadecylpropane-1,3-diamine (NHDA), HPA exhibited a comparable collecting ability for pyrochlore but displayed superior selectivity against feldspar and quartz. Adsorption experiments revealed that the pyrochlore exhibited a higher adsorption capacity for HPA compared to feldspar and quartz. The contact angle experiments indicated that the treatment with HPA significantly increased the contact angle for pyrochlore and calcite, while only minimal changes were observed for feldspar and quartz. Zeta potential and XPS data provided evidence for the chemical adsorption of HPA onto pyrochlore surfaces. Detailed XPS results showed that P-OH groups in HPA formed P(III)-O-M (M = Ca and Nb) bonds with Ca and Nb species on the pyrochlore surface. Additionally, the P-H bond in the HPA molecule could undergo a Redox reaction with highly oxidized Nb, resulting in the formation of a P(V)-Nb(IV) bond. This P(V)-Nb(IV) bond enhances the adsorption of hydrophobic HPA onto the mineral, making it easier to recover pyrochlore through flotation.

High-electrophilic (SiO)2Nb(OH)(=O) sites confined in silanol defects over Nb-Beta zeolite for efficient cyclic alkene epoxidation reactions

Dealuminated Beta zeolite has a large amount of silanol defects on its interface, which provides an ideal place for embedding metal species and creating metal active sites in a confined microenvironment. The confined metal sites as well as their surroundings are closely related to the reactant activation and transient state achievement. Hence, unraveling the confined metal sites is of great significance for the catalytic reaction process. Herein, niobium species were incorporated into the silanol defects over dealuminated Beta zeolite with a facile dry impregnation method, co-grinding the niobium precursor with dealuminated Beta zeolite support. The successful incorporation of niobium into the silanol defects for 30Nb-Beta zeolite was verified by DRIFT, 1H MAS NMR, UV–Vis and UV-Raman characterizations. XAS characterization and DFT calculations further disclosed that the confined Nb species existed as (SiO)2Nb(OH)(=O), containing two Si−O−Nb bonds, one Nb=O bond as well as one Nb−OH bond. The synthesized 30Nb-Beta zeolite catalyst displayed a superior cyclohexene conversion of 51.1%, cyclohexene oxide selectivity of 83.1% as well as TOF value of 188.2 h−1 ascribed to the inherent electrophilicity of Nb(V) for confined (SiO)2Nb(OH)(=O) species as well as the low oxygen transfer energy barrier for NbV−OOH species. Furthermore, the prepared 30Nb-Beta zeolite can be effectively employed to other cyclic alkene epoxidation reactions.

Study of the effect of rare earth elements X(Pm, Ce, Pr, La) on the modification of NbN/Fe interface using first-principles calculations

This study presents an investigation of the basic properties of bulk phase NbN and bulk phase Fe as well as the impact of doping with rare earth elements (Pm, Ce, Pr, La) on the degree of bonding at the NbN/Fe interface using first-principles calculations. For each low-index surface of the two bulk phases, calculations were performed and analyzed. Based on the analyses, the selected NbN (211) and Fe (110) surfaces were then combined as the most suitable interface. The analysis reveals a reduced N–Nb bond length at the interface, with an attachment work of −0.21292 J/m2 for the NbN(211)/Fe(110) interface. Furthermore, the interfacial energy is 0.89769 J/m2, and the combination of the two leads to a tighter bonding at the interface. The four rare earth elements are doped into the interface, and the calculation results of the rare earth element atom substitution for Nb are as follows: γ-Ce is −5.02865 J/m2, γ-Pr is −3.91418 J/m2, γ-Pm is −4.65489 J/m2, γ-La is −4.95645 J/m2; the rare earth element atom substitutions for Fe are as follows: γ-Ce is −5.12654 J/m2, γ-Pr is −6.59452 J/m2, γ-Pm is −4.98143 J/m2, and γ-La is −5.01213 J/m2. The addition of the four rare earth elements can bring the interface closer. Furthermore, substituting rare earth elements for Fe atoms considerably enhances the interface. Notably, the lowest value of doping formation energy is −0.12086 J/m2, which is observed when Pr replaces Fe, and the lowest value of doping solvation energy is −3.08754 eV. Among the four rare-earth elements, replacing Fe atoms with Pr demonstrates the most significant enhancement of interfacial bonding.

Atomic-level insights from density functional theory and ab initio molecular dynamics calculations for oxidation mechanism of transition metal doping Nb4AlC3 (0 0 0 1) surface

Poor oxidation resistance limits the further application of Nb4AlC3 MAX phase, a high-temperature structural candidate material. Transition metal doping can significantly address this problem. In this paper, we investigated the oxidation mechanism of the Nb4AlC3 (0 0 0 1) surface with Ti doping by combining density functional theory (DFT) and ab initio molecular dynamics (AIMD). DFT results indicate that the Nb surface has a lower surface energy, making it more easily exposed and reactive towards O2 molecules. O atoms are expected to preferentially occupy the HCP and FCC hollow sites on the surface. The Ti-doped surface has a stronger adsorption capacity for O atoms. O atoms gain electrons, while Nb/Ti atoms lose electrons. The formation of O–Nb/Ti bonds is facilitated by the hybridization of O-p and Nb/Ti-d orbitals. And O–Ti bonds are more stable than O–Nb bonds. AIMD simulation results show that O2 molecules on the clean and doped surfaces gradually dissociate into O atoms over time. These O atoms then combine with surface atoms to form Nb oxides, Ti oxides, and Nb–Ti oxides. With the passage of time, the concentration of Ti oxides increases while that of Nb oxides decreases. And the oxidation rate at 1073K is higher. The O–Nb/Ti bond population gradually increases over time, indicating that the bonding between O atoms and the surface is slowly strengthened. This study provides fundamental insights into the oxidation mechanism of transition metal-doped 413 MAX phase Nb4AlC3 surfaces.

Coexistence of relaxor ferroelectricity and magnetism in multi-element substituted Aurivillius phases Pb1-2xBi1.5+2xNd0.5Nb2-xMnxO9

We synthesized the novel double-layer Aurivillius phases Pb1-2xBi1.5+2xNd0.5Nb2-xMnxO9 using the fluxes-salt approach and systematically studied its crystal structure, morphology, dielectric, ferroelectric, magnetic and optical properties. The single-phase samples for x = 0, 0.1, and 0.3 were identified by X-ray diffraction (XRD). The refinement results demonstrated that all samples adopt an A21am orthorhombic structure and the cell volume becomes smaller with increasing x values. The perovskite B-site is occupied by Nb5+/Mn3+ ions, suggesting the local distributions of Mn3+ ions as Mn–O–Mn and Mn–O–Nb bonds based on the splitting of IR spectra. The grain morphology was observed as an anisotropic plate-like that became larger with increasing x. The broader dielectric peak with a strong frequency dependence reflected the relaxor-ferroelectric behavior and the higher Tc signifies an increased structural distortion. The non-linear fashion with open ferroelectric hysteresis loops demonstrated that ferroelectric ordering occurs at room temperature. The weak ferromagnetic ordering also existed with the introduction of Mn3+ ions. Further investigation into potential multiferroic properties in Aurivillius materials will be greatly aided by these findings.

Особенности дефектной структуры и фотолюминесценция номинально чистых кристаллов LiNbO3, выращенных из шихты, содержащей бор

Features of the defect structure of nominally pure LiNbO3:В crystals were investigated by X-ray diffraction analysis and photoluminescence. Crystals were grown by the Czochralski from a mixture of congruent composition charge containing 0.08 and 0.12 wt. % boron. At this, the concentration of boron in crystals is at the level of trace amounts of metallic impurities and is ~ 10–4 wt%. MeO6 oxygen-octahedral clusters are responsible for the ferroelectric and nonlinear optical properties of the crystal. It has been found that in LiNbO3:B crystals, the lengths of O–O, Me–O, and Me–Me (Me–Li, Nb) bonds in clusters, the arrangement of Me cations, vacancies, and NbLi point defects along the polar axis differ significantly from those for nominally pure congruent crystal. NbLi defects and transition metals are deep electron traps responsible for the photorefraction effect. The photoluminescence spectra showed that the concentration of defects and metals in the investigated LiNbO3:B crystals is lower than in the congruent crystal. these differences can be due to both a change in the properties of the boron-containing melt and the localization of trace amounts of boron in the O4 tetrahedral gaps of the LiNbO3 crystal structure. In the first case, reactive boron binds cations of niobium and transition metals in the melt into stable complexes.

Band gap anomaly in single-layer Nb<sub>2</sub>SiTe<sub>4</sub>-based compounds

Two-dimensional (2D) niobium silicon telluride (Nb<sub>2</sub>SiTe<sub>4</sub>) with good stability, a narrow band gap of 0.39 eV, high carrier mobility and superior photoresponsivity, is highly desired for applications in mid-infrared (MIR) detections, ambipolar transistors. Intensive investigations on its ferroelasticity, anisotropic carrier transport, anisotropic thermoelectric property, etc., have been reported recently. Motivated by the above prominent properties and promising applications, we systematically study the electronic properties of single-layer (SL) <i>A</i><sub>2</sub><i>BX</i><sub>4</sub> analogues (<i>A</i> = V, Nb, Ta; <i>B</i> = Si, Ge, Sn; <i>X</i> = S, Se, Te) and find a band-gap anomaly with respect to anion change, which differs from conventional 2D metal chalcogenide. In conventional binary chalcogenide, when cations are kept fixed, the bandgap tends to decrease as the atomic number of anions in the same group increases. However, in SL <i>A</i><sub>2</sub><i>BX</i><sub>4</sub>, as atomic number of anions increases, its bandgaps tend to increase, with cations kept fixed. In order to find the underlying mechanism of such an abnormal bandgap, using first-principles calculations, we thoroughly investigate the electronic structures of Nb<sub>2</sub>Si<i>X</i><sub>4</sub> (<i>X</i> = S, Se, Te) surving as an example. It is found that the valance band maximum (VBM) and conduction band minimum (CBM) are mainly derived from the bonding and antibonding coupling between Nb 4d states. The bandwidth of Nb 4d states determines the relative value of the band gap in Nb<sub>2</sub>Si<i>X</i><sub>4</sub>. We demonstrate that the band gap is largely influenced by the competition effect between Nb—Nb and Nb—<i>X</i> interactions in Nb<sub>2</sub>Si<i>X</i><sub>4</sub>. As the anion atomic number increases, the Nb—Nb bond length increases, yielding an increased bandwidth of Nb 4d state and a smaller bandgap of Nb<sub>2</sub>Si<i>X</i><sub>4</sub>. Meanwhile, as Nb—<i>X</i> bond length increases, the bandwidth of Nb 4d however decreases, yielding a larger bandgap. The interaction between Nb and <i>X</i> should be dominant and responsible for the overall bandgap increase of Nb<sub>2</sub>Si<i>X</i><sub>4</sub> compared with the Nb—Nb interaction.

Sr2Nb6O13F8·4H2O and Sr3Nb2O2F12·2H2O: A Variant of Three-Dimensional Tungsten Bronze and a Polar Molecular Oxide Fluoride.

Crystals of two strontium niobium oxyfluorides, Sr2Nb6O13F8·4H2O and Sr3Nb2O2F12·2H2O, have been grown in phase pure forms via hydrothermal reactions using SrCO3, Nb2O5, and an aqueous HF solution. Single-crystal X-ray diffraction suggests that Sr2Nb6O13F8·4H2O, crystallizing in the orthorhombic centrosymmetric space group, Pbam (No. 55), reveals a new variant of the three-dimensional tungsten bronze structure with three-, four-, and five-membered rings that are composed of corner-sharing NbO2(O/F)2F2, NbO4(O/F)F, NbO3(O/F)3, and SrO3F6 groups. Sr3Nb2O2F12·2H2O with the noncentrosymmetric polar space group, Cmc21 (No. 36), however, reveals a molecular structure consisting of Nb(O/F)2F5 pentagonal bipyramids and two unique Sr2+ cations interacting with F, O/F, and water molecules. Band gaps calculated by the Kubelka-Munk function based on the ultraviolet-visible diffuse-reflectance spectra of Sr2Nb6O13F8·4H2O and Sr3Nb2O2F12·2H2O are estimated to be ca. 3.22 and 4.11 eV, respectively, in which the values are related to the contents of electronegative F atoms and the Nb-O(F)-Nb bond angles influenced by structural distortion. An interesting phase transition reaction from Sr3Nb2O2F12·2H2O to thermodynamically more stable Sr2Nb6O13F8·4H2O occurs under a hydrothermal condition.

Insights into the physical properties of a new 211 MAX phase Nb2CuC

A systematic density functional theory study with two functionals –generalized gradient approximation (GGA) and local density approximation (LDA)–is carried out to explore the structural, electronic, elastic, thermal, vibrational and optical properties of a new 211 MAX phase Nb2CuC. To facilitate comparison we also study Nb2AlC, the precursor of Nb2CuC. The calculated band structures reveal the metallic conductivity of both compounds. The replacement of Al with Cu modifies the band profiles of Nb2CuC and consequently leads to its improved physical properties. Considering the position of the Fermi level on the total density of states (DOS), the new compound Nb2CuC is structurally less stable than Nb2AlC. The total DOS at the Fermi level obtained with GGA is slightly larger than those obtained with LDA. The Nb–C and Nb–A (A = Cu/Al) are covalent bonds, and Nb–Nb bonds lead to antibonding states in both MAX phases. The charge transfer among constituent atoms indicates some ionic character in the chemical bonds of Nb2CuC and Nb2AlC. Both MAX phases are mechanically and dynamically stable. The Nb2CuC is ductile and consequently damage tolerant, whereas Nb2AlC is brittle. However, Nb2CuC is relatively soft and machinable. In most cases Nb2CuC is more elastically anisotropic than Nb2AlC, and Nb2CuC is expected to be a promising thermal barrier coating material. We propose that Nb2CuC is a better coating material for preventing solar heating than Nb2AlC, and Nb2CuC is expected to be superconductive because its Fermi surface has a nesting nature.

A hexaniobate expanded by six [Hg(cyclam)]2+ complexes via Hg–O bonds yields a positively charged polyoxoniobate cluster

Abstract The room temperature reaction of Hg(NO3)2 · H2O, cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) and K8{Nb6O19} · 16 H2O in a mixture of H2O and DMSO led to crystallization of the novel compound {[Hg(cyclam)]6Nb6O19}(NO3)4 · 14 H2O, which is the first mercury containing polyoxoniobate. The structure consists of a {Nb6O19}8− cluster core which is expanded by six [Hg(cyclam)]2+ complexes via Hg–μ 2-O–Nb bond formation. The title compound contains a positively charged polyoxoniobate cluster. The crystal water molecules form small aggregates by O–H · · · O hydrogen bonding which are joined into larger aggregates by N–O · · · H–O hydrogen bonding integrating the nitrate anions.

Crystal structures of new layered perovskite-type oxyfluorides, CsANb2O6F (A = Sr and Ca) and comparison with pyrochlore-type CsNb2O5F

New layered perovskite-type oxyfluorides, CsSrNb2O6F and CsCaNb2O6F, were prepared via solid state reactions and their crystal/electronic structures were compared to those of a compositional analogue, CsNb2O5F which has a 3-dimensional pyrochlore-type structure. The CsSrNb2O6F and CsCaNb2O6F phases are derived from the Dion-Jacobson family of a general formula A[A′n−1BnX3n+1] with n = 2. Rietveld refinements based on synchrotron X-ray diffraction data revealed that CsSrNb2O6F belongs to the tetragonal system with a = 3.86716(1) Å, c = 11.45257(2) Å (P4/mmm, Z = 1), while CsCaNb2O6F adopts a 2ap × 2ap × 2cp superstructure with orthorhombic unit cell parameters, a = 7.64813(1) Å, b = 7.67516(1) Å, c = 22.43663(3) Å (Bmmb, Z = 8). The anion distributions in the lattices were confirmed by applying Bond Valence Sum (BVS) method; the F- ions occupy the central sites of the corner-sharing double perovskite block in the layered structures, but no ordering of anions was found in the pyrochlore structure. Through UV–Vis diffuse reflectance measurement, the band gap energy was estimated to 3.43 eV for CsSrNb2O6F, 3.65 eV for CsCaNb2O6F and 4.00 eV for CsNb2O5F. The band gap energy showed a good correlation with the Nb-O(F)-Nb bond angle(θ) in the oxyfluorides. As the Nb-O(F)-Nb bond angle deviates from 180°, the value of Eg increase. The good linearity observed in the Egvs. cos(180 - θ) plot suggests that Eg can be finely controlled by adjusting of the Nb-O(F)-Nb bond angle in the lattice.

Monoclinic structures of niobium trisulfide

Two new polymorphs of niobium trisulfide are established by single crystal x-ray diffraction. NbS3-iv crystallizes in the monoclinic space group P21/c with lattice parameters a = 6.7515(5) Å, b = 4.9736(4) Å, c = 18.1315(13) Å, and β = 90.116(2)°. Its structure is based on chains of [NbS6] trigonal prisms containing Nb–Nb pairs with a bond length of 3.0448(8) Å; this pairing causes the chains to corrugate slightly along their axis, a feature also present in triclinic NbS3-i that leads to semiconductor properties. The stacking arrangement of chains is different in these polymorphs, however, with NbS3-i having an ABCDE repeating sequence of chain bilayers and NbS3-iv having an AB repeating sequence. HRTEM studies show the presence of topotactically-oriented intergrown zones and numerous dislocations, which result in mosaic structuring. A second new polymorph, NbS3-v, crystallizes in the monoclinic space group P21/m with lattice parameters a = 4.950(5) Å, b = 3.358(4) Å, c = 9.079(10) Å, β = 97.35(2)°. In contrast to NbS3-iv, NbS3-v maintains fixed a Nb–Nb bond distance of 3.358(4) Å along the chains, and it has an ABCDE repeating sequence of chain bilayers similar to NbS3-i. High resolution scanning transmission electron microscopy (HR-STEM) imaging of an exfoliated NbS3-v nanoribbon shows the continuous [NbS6] chains oriented along the b-axis. These results provide the first firmly established structural data for monoclinic NbS3. In addition, SEM images show the formation of NbS3 rings and cylinders, and a combination of powder x-ray diffraction and Raman spectroscopy provides a way to distinguish between NbS3 polymorphs.

A high pressure La K-edge X-ray absorption fine structure spectroscopy investigation of La1/3NbO3

ABSTRACTLa K-edge X-ray absorption spectroscopy has been used to elucidate the changes in the local electronic and lattice structure that occur in the A-site deficient double perovskite LaNbO up to 6 GPa. The pressure evolution of the oxygen dodecahedrum around the A-site has been examined. XANES (X-ray absorption near edge structure) data show modifications ascribed to the increase of bands overlapping as a consequence of the bond distance contraction, which has been directly probed by EXAFS (extended x-ray absorption fine structure) spectra. The La–O Debye Waller factors (DWFs) tend to increase whereas the La–Nb bond DWFs show only a tendency to decrease indicating the robustness of the crystal lattice structure, even in presence of the oxygen disordering. This permits the system to reverse back to its original conditions in this pressure range as evident from the measurements upon pressure release. The present results have been interpreted in the light of charge transfer related to the two-step reduction mechanism acting at the Nb site (with niobium ions passing from Nb to Nb) which also results in the elongation of the Nb–O bond distances in the octahedra, in agreement with the Nb K-edge results reported earlier.

Anion Photoelectron Spectroscopy and Theoretical Investigation on Nb2Sin–/0 (n = 2–12) Clusters

We measured the photoelectron spectra of Nb2Sin– (n = 2–12) anions and investigated the geometric structures and electronic properties of Nb2Sin– anions and their neutral counterparts with ab initio calculations. The most stable structures of Nb2Sin–/0 (n = 2–12) clusters can be regarded as a central axis of Nb2 surrounded by the Si atoms. The most stable isomers of Nb2Sin– anions are in spin doublet states, while those of the neutral clusters are in spin singlet states. The results showed that the two Nb atoms in Nb2Sin–/0 clusters incline to form a strong Nb–Nb bond and also prefer to occupy the high coordination sites to form more Nb–Si bonds. The most stable isomers of anionic and neutral Nb2Si3 are D3h-symmetric trigonal bipyramid structures, and that of Nb2Si6– has C2h symmetry with the six Si atoms forming a chair-shaped structure. The ground state structure of the Nb2Si12– anion is a C6v-symmetric capped hexagonal antiprism in which one Nb atom is encapsulated inside the Si12 cage and the second N...

CH Bond Activation of Unsaturated Hydrocarbons by a Niobium Methyl Cyclopropyl Precursor. Cyclopropyl Ring Opening and Alkyne Coupling Reaction

The transient intermediate η2-cyclopropene/bicyclobutane niobium complex [TpMe2Nb(η2-c-C3H4)(MeCCMe)] A, generated by an intramolecular β-H abstraction of methane from the methyl cyclopropyl complex [TpMe2NbMe(c-C3H5)(MeCCMe)] (1), is able to cleave the CH bond of a variety of unsaturated hydrocarbons RH in a selective manner to give the corresponding hydrocarbyl complexes [TpMe2NbR(c-C3H5)(MeCCMe)] (R = 2-furyl, 2-thienyl, 1-alkynyl, 1-cyclopentenyl, 1-ferrocenyl (Fc), pentafluorophenyl). The activation of the C–H bond occurs stereospecifically via a 1,3-CH addition across the Nb(η2-cyclopropene) bond of A. Full characterization of several of these complexes includes multinuclear NMR spectroscopy, X-ray diffraction, UV/vis spectroscopy, and electrochemical data. A charge transfer between the ferrocenyl moiety and the niobium center is responsible for the characteristic purple color of the bimetallic complex [TpMe2NbFc(c-C3H5)(MeCCMe)]. The reactivity of these complexes with benzene follows qualitatively ...

Structure–property relationships in barium borophosphate glasses modified with niobium oxide

Phosphate glasses containing a high amount of niobium oxide exhibit non-linear optical properties which make them attractive for various optical devices. This contribution is devoted to the investigation of glasses of the series (100−x)[0.5BaO–0.1B2O3–0.4P2O5]–xNb2O5 with x=0–45 and 80[0.5BaO–yB2O3–(0.5−y)P2O5]–20Nb2O5 with y=0–0.25. The basic characteristic parameters of the prepared glasses were determined. The index of refraction of these glasses increases substantially with increasing Nb2O5 content from 1.620 to 2.007 and the glass transition temperature increases within the range of 551–704°C. The glass structure was investigated by 31P, 11B and 93Nb NMR, and Raman spectroscopies. Raman spectra revealed that at low Nb2O5 content a strong vibrational band of isolated NbO6 octahedra dominates the spectra. New bands appear in the Raman spectra in the glasses with 30mol% of Nb2O5 that were assigned to vibrations of Nb–O–Nb bonds interconnecting NbO6 octahedra. 11B MAS NMR spectra reveal the dominant role of BO4 units in the glass structure. 11B MAS NMR spectra revealed that with increasing Nb2O5 content new resonances of BO4 units appear in the spectra due to the replacement of B–O–P bonds by B–O–Nb and B–O–B bonds. With an increasing Nb2O5 content, part of the BO4 units is also converted to BO3 units. 93Nb MAS NMR spectra primarily reflect changes in the length of Nb–O bonds in the NbO6 octahedra manifested in a decreasing asymmetry of the octahedra with increasing Nb2O5 content and the formation of niobate clusters. Niobo-borophosphate glasses reveal very high chemical durability as well as high glass transition temperatures due to the replacement of weaker P–O bonds with stronger Nb–O bonds.

Microwave dielectric properties of Li2SrTa2(1−x)Nb2xO7 ceramics investigated by Raman spectroscopy

We investigated the crystal structure and microwave dielectric properties of the Li2SrTa2(1−x)Nb2xO7 (x=0−1.0) compounds using Raman spectroscopy. The Raman spectra clearly exhibit the orthorhombic structure (Cmcm space group) of these compounds. The Raman mode at 800cm−1 corresponds to the B–O3 (B=Nb and Ta) bond and shifted towards the higher frequency with increase of Nb content. The increase in separation between the two modes at 589 and 596cm−1 gives an indication of change in the difference between B–O1 and B–O2 bond lengths. The microwave dielectric properties of these compounds were found to be strongly dependent on the BO6 octahedra. The peak width of Ag (800cm−1) mode and quality factor (Q×f) is inversely proportional to each other and the shift of this mode is correlated to dielectric constant (εr). Further analysis of the Raman modes at 800, 596 and 589cm−1 (due to the BO6 octahedra) indicates the enhancement of octahedral distortion with increase in Nb content, which led to increase of temperature coefficient of resonant frequency (τƒ).

Structural characterization and photoluminescence behavior of pure and doped potassium strontium niobates ceramics with tetragonal tungsten–bronze structure

KSr2Nb5O15 and K1.2Sr1.6Ln0.2Nb5O15 (Ln=La, Pr, and Nd) were synthesized by the Pechini method and calcined at 1373K for 10h under air atmosphere. Rietveld refinements indicated the presence of structural defects in both KSN and KSN-Ln, especially distortions in O–Nb–O and Nb–O–Nb bonds. Modifications in bond distances of [NbO6] clusters were also confirmed by Raman spectroscopy. Ultraviolet-visible absorption spectra and photoluminescence profiles were influenced by lanthanide atoms in the band structure of modified KSN materials, creating new defects and intermediary energy levels within the band gap.

Prediction of ordered omega phase formation by coupled replacive–displacive processes in Zr3Al2Nb and Zr4AlNb alloy: a first-principles study

We report ab initio study of the formation of chemically ordered athermal ω phase (trigonal symmetry, space group , non-ideal ω, ω′) in Zr3Al2Nb and (trigonal symmetry, space group P3m1, ideal ω) in Zr4AlNb alloy by coupled replacive–displacive transformation mode. The phase stability of body-centred cubic (bcc), non-ideal ω (ω′) and ideal ω structures of Zr3Al2Nb and Zr4AlNb was examined by first-principles calculations with projector augmented wave potentials and generalized gradient approximations. Position of Nb atom in the bcc lattice was decided by examining several atomic configurations, and it was also shown that the stability of each configuration is directly related to the number of Zr–Al bonds. Among these configurations, the most stable bcc Zr3Al2Nb and Zr4AlNb configurations showed instability for atomic displacements leading to ω-type structure in our ground-state energy calculations. In order to estimate strengths of Zr–Al, Nb–Al and Zr–Nb bonds, the heats of the formation for several virtual compounds were calculated and we find that the strength of Zr–Al bonds is higher than Nb–Al and Zr–Nb bonds. It was also confirmed that the formation of the ω phase in Zr3Al2Nb and Zr4AlNb is a combined displacive–replacive transformation. Moreover, our mechanical stability analysis predicts that both ω′-Zr3Al2Nb and B82-Zr2Al are mechanically stable phases with very poor ductility. In contrast, less pronounced directional bonding in ω-Zr4AlNb indicates this alloy to be ductile.