NbO6 Octahedra Research Articles

Dielectric polarizability of SiO2 in niobiosilicate glasses

AbstractUnderstanding the mechanisms contributing to dielectric properties of glasses is critical for designing new compositions for microwave frequency applications. In this work, dielectric permittivity was measured using a cavity perturbation technique at 10 GHz for a series of niobiosilicate glasses with the compositions (100‐2x)SiO2‐ xNb2O5‐ xLi2O where x = 32.5, 30, 25, and 15 mol%. Permittivity measurements and glass compositions were used to calculate the polarizability of each cation‐anion unit in the glass network using the Clausius‐Mossotti equation. The SiO2 polarizability in niobiosilicates was calculated to be 6.16 Å3, which is much higher than the SiO2 polarizability in fused silica glass (5.25 Å3), alkali modified silicates (5.37 Å3), and aluminosilicates (5.89 Å3). The increasing trend in SiO2 polarizability is attributed to the disruption in the connectivity of the SiO4 tetrahedral network as it accommodates different network formers. The high SiO2 polarizability of 6.16 Å3 accurately predicts measured dielectric permittivity when Nb2O5 = 25, 30, and 32.5 mol%, but overpredicts measured permittivity when Nb2O5 ≤ 15 mol%, which is attributed to a decrease in SiO2 polarizability as the percentage of corner sharing SiO4 tetrahedra with NbO6 octahedra goes down. This work demonstrates that SiO2 polarizability depends on chemistry and connectivity of the glass, which has important implications in designing glass compositions for microwave frequency applications.

A Polar Tetragonal Tungsten Bronze with Colossal Second-Harmonic Generation.

A polar tetragonal tungsten bronze, Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 (□: vacancies), has been successfully synthesized by a high temperature solid-state reaction. Single crystal and powder X-ray diffraction indicate that the structure of Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 crystallizing in the noncentrosymmetric (NCS) space group, P4bm, consists of 3D framework with highly distorted NbO6 , LiO9 , PbO12 , and (Pb/K)O15 polyhedra. While NCS Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 undergoes a reversible phase transition between polar (P4bm) and nonpolar (P4/mbm) structure at around 460 °C, the material decomposes to centrosymmetric Pb1.45 K3.56 Li3.54 Nb10 O30 (P4/mbm) once heated to 1200 °C. Powder second-harmonic generation (SHG) measurements with 1064nm radiation indicate that Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 exhibits a giant phase-matchable SHG intensity of ≈71.5 times that of KH2 PO4 , which is the strongest intensity in the visible range among all nonlinear optical materials reported to date. The observed colossal SHG should be attributable to the synergistic effect of dipole moments from the well-aligned NbO6 octahedra, the constituting distortive channels with vacancies, and highly polarizable cations.

Photoelectrocatalytic oxygen evolution reaction on visible-light irradiated W-doped alkali niobate-based perovskite

The photoelectrocatalytic oxygen evolution reaction was studied under visible-light irradiated W-doped niobate-based perovskites. Nanostructured K0.5Na0.5(WxNb1−x)O3 materials were prepared by ultrasonic spray pyrolysis as a function of 1 – 3 mol% W6+ cations within the host structure. A careful characterization of solids including morphology, structure, texture, optical, and electrochemical properties was performed. From the characterization analysis it can be concluded the materials are constituted by nanostructured mesoporous hollow spheres and the substitution of Nb by W leads to the distortion of NbO6 octahedra. The photoelectrocatalytic activity was correlated with the structural properties and electrochemical properties of materials. To the best of our knowledge, the present W-doped niobate-based perovskites showed the highest turnover frequency reported in the photoelectrocatalytic OER under similar overpotential conditions. Accordingly, this new class of perovskite-based crystalline materials opens a door for the scaling-up of catalytic and photocatalytic processes related to energy production.

Small Changes, Big Impact: Tungsten Bronzes with Extremely Large Second Harmonic Generation Achieved by the Transition Metal Doping on the B‐Site

AbstractTwo novel transition metal‐doped tungsten bronze oxides, Pb2.15Li0.85Nb4.85Ti0.15O15 (PLNT) and Pb2.15Li0.55Nb4.85W0.15O15 (PLNW), are synthesized by high‐temperature solid‐state reactions. The Rietveld method using the high‐resolution synchrotron radiation indicates that PLNT and PLNW crystallize in the orthorhombic polar noncentrosymmetric space group, Pmn21 (no. 31). As a class of tungsten bronze oxide, PLNT and PLNW retain a unique rigid framework composed of d0 transition metal cation (Ti4+ or W6+)‐doped highly distorted NbO6 octahedra along with the subsequently generated Pb/LiO12 and PbO15 polyhedra. Interestingly, the d0 transition metal‐doped tungsten bronzes, PLNT and PLNW, exhibit extremely large second‐harmonic generation (SHG) responses of 56 and 67 × KH2PO4, respectively. The observed immeasurably strong SHG is mainly attributed to a net polarization originating from the alignment of highly distorted NbO6 octahedra with doped transition metals in the frameworks. It is believed that doping transition metal cations at the B‐site of the tungsten bronze structures should be an innovative strategy to develop novel high‐performance nonlinear optical materials.

Structure-Property Correlation in Sodium Borophosphate Glasses Modified with Niobium Oxide

Bulk glasses of the series (100−x)[0.4Na2O-0.2Nb2O5-0.4P2O5]-xB2O3 with x = 0–48 mol% B2O3 were prepared by slow cooling in air. Their glass transition temperature increases within the range of 0–16 mol% B2O3, but further additions of B2O3 result in its decrease. Their structure was investigated by Raman, 11B, and 31P MAS NMR spectroscopy. The relative number of BO4 units decreases with increasing B2O3 content, while the number of BO3 units increases up to 59 % at x = 48. The upfield shift of a broad resonance peak in the 31P MAS NMR spectra is ascribed to an increasing connectedness of the structural network with increasing B2O3 content. Strong Raman band at 916–929 cm−1 shows on the presence of NbO6 octahedra in the structural network of these glasses. With the B2O3 addition, a decrease in DC conductivity is observed, which is attributed to the decrease in the concentration of Na+ ions.

Large strain response in lanthanide-doped potassium sodium niobate-based piezoceramics

Developing polymorphic phase coexistence near room temperature in potassium sodium niobate-based piezoceramics is a widespread way to improve strain performance. Herein, 0.97(K0.48Na0.52)1-3xLaxNbO3-0.03SrTiO3 piezoceramics were studied by analyzing their grain images, crystal structures, dielectric properties, Raman spectra, ferroelectric properties, strain responses, and piezoelectric properties. A high strain of 0.43% was achieved at x = 0.0050 presenting a diffuse phase transition state, due to various intrinsic factors, including large electrostriction, small piezoelectric effects, and extrinsic microdomain wall reorientation. The large electrostriction was the chief contribution to the strain due to the considerable electrostrictive coefficient (Q33) of 0.039 m4/C2. Furthermore, the considerable Q33 was due to B-site shifting by distorting the slightly compressed NbO6 octahedron arising from La3+ doping. The compressed of the NbO6 octahedra was proven by XRD results and generated distortion according to the obtained Raman spectra.

Thermal, structural and crystallization study of Na2O–P2O5–Nb2O5 glasses

The present investigation deals with the study of the structure, properties and thermal behaviour of Na2O–P2O5–Nb2O5 glasses prepared in the compositional series 40Na2O-(60-x)P2O5-xNb2O5 (x=0–40 ​mol% Nb2O5). The modification of starting 40Na2O–60P2O5 glass by Nb2O5 revealed changes both in the structure and properties. The glass density steeply increases with increasing Nb2O5 content while the molar volume decreases. Parent sodium ultraphosphate glass was covered with a thin layer of partially hydrolysed glass in air. The chemical durability of Nb2O5 containing glasses was high. The replacement of P2O5 by Nb2O5 resulted in a gradual increase in the refraction index as well as in an increase in glass transition temperature and dilatometric softening temperature. DSC measurements demonstrated that glasses crystallize under heating in the temperature range of 330–880°C. The lowest thermal stability was found for the glass containing 40 ​mol% Nb2O5. The Raman and NMR spectra indicated that the structure of 40Na2O–60P2O5 glass is formed mainly by Q2 and Q1 phosphate units interconnected by P–O–P bonds. The replacement of P2O5 by Nb2O5 leads to depolymerization and the partial transformation of Q2 units into Q1 units and finally to isolated Q0 units. Nb2O5 forms in the glass structure NbO6 octahedra.

Local Structure Manipulates Nonlinear Optical Properties in Non-stoichiometric Lithium Niobate

As a typical and commercial nonlinear optical (NLO) material, LiNbO3 usually suffers from non-stoichiometric compositions, but the role of non-stoichiometry in second harmonic generation (SHG) remains unclear. In this study, we prepared non-stoichiometric LiNbO3 with a Li mole percentage amount covering critical ranges from 47 to 50. We found that a 47 mol % Li amount can remarkably weaken the SHG response by 50% without an obvious change of the band gap. A detailed structural analysis indicates that reducing the Li content brings anti-occupied Nb (NbLi4·) together with Li vacancies (VLi′), which decreases local distortions and local polarizations of both NbO6 and LiO6 octahedra and ultimately lead to the decline of the SHG response. Such effects are discussed in terms of the looser and less anisotropic structure character of LiNbO3. The above results are supported by joint synchrotron X-ray/neutron diffraction, atomic pair distribution function (PDF) analysis, and theoretical calculations. These insights are helpful to the future design of functional materials in non-stoichiometric compositions.

Enhanced photocatalytic activity of La and Zr-codoped AgNbO3 for rhodamine B and methylene blue degradation

In this study, AgNbO3, 1%, 5%, and 10% La, Zr-codoped AgNbO3 photocatalyst were synthesized by the solid state method. The synthesized AgNbO3 was codoped with La and Zr to enhance its effectiveness in photocatalytic dye degradation. The as-synthesized materials were also characterized via XRD and SEM analysis to determine their structural and morphological properties. The vibrational bands of the photocatalysts observed in FT-IR spectroscopy confirmed the presence of NbO6 octahedra which can be found in AgNbO3. Furthermore, all the photocatalysts showed high crystallinity and are single-phase materials with an orthorhombic crystal structure. It was observed that the particle size decreased with the increasing concentration of La and Zr codopants. The calculated lattice parameters showed that codoping AgNbO3 with 10% La and Zr had caused its unit cell volume to expand. The UV–Vis absorption spectra showed a slight band gap widening and a decrease in surface plasmonic resonance (SPR) effect with increasing La, Zr-codoping. The photocatalytic RhB and MB degradation results of undoped AgNbO3 and La, Zr-codoped AgNbO3 were compared and they showed that there are improvements in the photocatalytic performance. The highest degradation (98.1%) of RhB was achieved by 5% La, Zr-codoped AgNbO3 while the highest MB degradation (48.3%) was achieved by 1% La, Zr-codoped AgNbO3. Last but not least, La, Zr-codoped AgNbO3 is a promising material for water remediation application as it showed enhanced performance for photocatalytic dye degradation under visible light irradiation.

Raman vibration, bond chemistry and enhanced microwave dielectric properties of Li3Mg2NbO6 ceramics under an oxygen atmosphere

An effective synthesis provides a significant influence on microwave dielectric ceramics for commercial production. In this work, single-phase Li3Mg2NbO6 ceramics were synthesized under an oxygen atmosphere to depress oxygen vacancies. Raman vibration, bond characteristics and crystal structure were analyzed to calculate the inherent features of the Li3Mg2NbO6 ceramics. The Raman spectra and group theory indicated that the mode at 772 cm−1 reflected the symmetric vibrations of Nb–O bonds in NbO6 octahedra and dominated the Raman vibrations. XRD patterns and Rietveld refinement indicated an orthorhombic rock-salt phase under an oxygen assisted sintering process. Well-distributed and homogeneous microstructures were obtained through SEM and grain distribution. From the chemical theory calculation, Nb–O bonds dominated the dielectric properties in consideration of their largest bond ionicity, lattice energy, and bond energy. The Li3Mg2NbO6 ceramics sintered at 1100 °C with an oxygen atmosphere exhibited remarkable performance: εr = 15.3, Q × f = 109,600 GHz and τf = −17.3 ppm/°C, providing enormous potential for communication applications.

First principle understanding of antiferroelectric ordering in La-doped silver niobate

The antiferroelectric Pbcm phase of silver niobate (AgNbO3) has received increasing attention owing to its environmental safety (as it is lead-free), compatibility, and superior energy-storage density compared with its ferroelectric counterparts. We comprehensively investigated the effects of La doping at Ag sites (i.e., Ag0.88La0.12NbO3) on the structural, electronic, and ferroelectric properties of AgNbO3 by using ab initio density functional theory calculations and the Berry phase method for polarization calculations, respectively. The polarization in the [001] direction of pristine AgNbO3 is estimated to be 1.9 μC/cm2, which is significantly enhanced (to 14.86 μC/cm2) for Ag0.88La0.12NbO3. The enhancement in the spontaneous polarization of AgNbO3 after the incorporation of La is attributed to suppressed distortion in the Nb–O–Nb antiparallel ordering and tilting of the NbO6 octahedra. Furthermore, the electron density maps of AgNbO3 and its doped counterpart were calculated to investigate the lattice-distortion and bond-change observations.

93Nb NMR Study of (K, Na)NbO3‐Doped SiO2–Na2O–Al2O3 Glasses

Aluminosilicate glasses containing octahedral NbO6 units are developed for future applications as optical materials. Soda‐aluminosilicate glasses doped with Nb‐based alkaline perovskite crystals (K0.5Na0.5NbO3 [KNN]) at different doping levels (2–30 mol%) are studied by 93Nb nuclear magnetic resonance (NMR) spectroscopy. The Al2O3 concentration in the starting soda‐aluminosilicate glass matrix is varied from 5 to 17 mol% to investigate the influence of the composition of local Nb structures with 4–5, 6, and 7–8 fold‐oxygen coordination on the optical properties of the glasses. The chemical shifts are analyzed by comparison with the 93Nb NMR data of reference crystals, including KNN, to determine the feasibility of producing more NbO6 octahedra in the aluminosilicate glasses, where these octahedra are expected to promote the development of optically active Nb‐glass‐ceramics. The data show that both glass polymerization and KNN content influence the Nb local environment. Therefore, an appropriate amount of Al2O3 combined with an adequate doping level of the KNN perovskite is required to achieve the desired optical features. Contrary to expectations, a higher KNN doping level is achieved with the starting depolymerized soda‐aluminosilicate glass containing 5 mol% Al2O3, leading to perovskite‐like structures of NbO6 octahedra in the resultant glass system.

Carbon-coated monoclinic NbOPO4 with polyanionic framework for rechargeable aqueous lithium-ion batteries beyond 2 V

The choice of anode materials for high-voltage aqueous batteries beyond 1.5 V is still far from satisfactory, due to the narrow electrochemical stability window of aqueous electrolytes. With the inherent advantages of low working voltage and fast ionic diffusion, niobium phosphates with polyanionic framework groups built of NbO6 octahedra and PO4 tetrahedra are very promising anode materials for high-voltage aqueous batteries. Herein, carbon-coated NbOPO4 (NPO/C) materials with monoclinic structure has been prepared by the solid-state reaction & chemical vapor deposition method. Between -1.34 V and 0.6 V vs. standard hydrogen electrode, it could deliver a specific capacity of 116 mAh/g with a high coulombic efficiency of 92.0% at a low current rate of 0.33C in aqueous electrolytes based on methylsulfonylmethane, LiClO4 and H2O (DES). The diffusion-controlled process dominates the electrochemical reaction of NPO/C. Upon the reversible insertion/extraction of lithium ion into/from NPO/C, it maintains the monoclinic structure with a small lattice expansion, which is a characteristic of zero-strain effect. Finally, the full cell based on NPO/C anode, LiMn2O4 cathode and DES electrolytes with a high specific energy 101.6 Wh/kg and a high voltage output beyond 2 V has been validated. It also shows excellent cycling performance with the capacity retention of 94.3% after 1000 times at 2C rate, which suggests that monoclinic β-NbOPO4 can be used as a promising electrode material for aqueous lithium-ion batteries with high voltage output for energy storage in the future.

Temperature-dependent ferroelastic behaviour of antiferroelectric AgNbO3

The macroscopic ferroelastic response of polycrystalline AgNbO3 was investigated under the uniaxial compressive stress of up to -300 MPa from -150 °C to 450 °C, covering weak ferroelectric, antiferroelectric, and paraelectric phase regions. It is found that AgNbO3 exhibits ferroelasticity in the weak ferroelectric (M1), antiferroelectric (M2, M3), as well as the paraelectric regions (O and T), i.e. displaying ferroelastic hysteresis and remnant strain. The changes in mechanical parameters such as coercive stress, back switching strain, and remnant strain values reflect the specific crystal symmetry features at different phase regions together with the influence of temperature on domain wall motion. With stress removed and cooled down to room temperature, X-ray diffraction showed that the sample (at M1 state) exhibited different degrees of remnant domain textures and lattice strains, depending on its phase state when being compressed. Additionally, in situ stress- and temperature-dependent Raman spectroscopy measurements revealed that uniaxial compressive stress could induce changes in NbO6 octahedra and cation displacement, especially for high-temperature phases. Overall, the ferroelasticity, effects of stress on the structure and phase transitions offer opportunities for property engineering of AgNbO3 in capacitor applications.

Observation of Anisotropic Thermal Expansion and the Jahn-Teller Effect in Double Perovskites Sr2-xLaxCoNbO6 Using Neutron Diffraction.

We use temperature dependent neutron powder diffraction (NPD) to investigate the structural changes and magnetic interactions in double perovskite Sr2-xLaxCoNbO6 (x = 0.4 and 0.6). A structural phase transition from tetragonal (I4/m) to monoclinic (P21/n) is observed between x = 0.4 and 0.6 samples. Interestingly, the temperature evolution of the unit cell parameters follows the Grüneisen approximation, and the analysis suggests an isotropic thermal expansion in the case of the x = 0.4 sample, whereas the x = 0.6 sample shows the anisotropy where the thermal expansion along the c-axis significantly deviates from the Grüneisen function. We observe the z-out Jahn-Teller distortion in the CoO6 and consequently z-in distortion in the adjacent NbO6 octahedra. With an increase in the La substitution, a decrease in the degree of octahedral distortion is evident from the significant reduction in the local distortion parameter Δ around the B-site atoms.

Pressure driven phase transitions in honeycomb Fe4Nb2O9: A possible re-entrant multiferroic behavior

A detailed high-pressure investigation is carried out on Fe4Nb2O9 using angle resolved x-ray diffraction and Raman spectroscopy measurements. We find a structural transition from the ambient trigonal phase to a monoclinic phase above 8.8 GPa. The structural transition is assumed to be driven by a large distortion of Nb–O6 octahedra as seen from x-ray diffraction analysis and a large pressure dependence of an Nb–O6 octahedra breathing Raman mode. Anomalous behavior of Raman modes and an increase in the phonon lifetime at the phase transition pressure indicate a possible change in the magnetic property of the sample above 8.8 GPa. A decrease in the diffusive scattering rate of a low-frequency electronic contribution contradicts the results of a decrease in the intensity of a high-frequency electronic response and excludes the phenomenon of an insulator to metal transition. On the contrary, the enhancement of the intensity of the Raman modes up to about 8.8 GPa indicates a large change in ferroelectric polarization of the sample, indicating a possible pressure induced re-entrant multiferroic behavior in Fe4Nb2O9.

Preparation and improved photochemical properties of V-doped Nb2Te4O13

Mixed oxides combining d0-and p-cations with stereoactive lone pair electrons exhibit excellent polarization abilities for potential photoelectric applications. This work reports the photochemical properties of V-doped Te–Nb mixed oxide Nb2Te4O13, which has the typical framework constructed by NbO6 (d0-) octahedra, TeO4, and TeO3 (p-) polyhedra. The samples were prepared using the sol-gel method, which resulted in the final nanoparticles. The luminescence spectra, lifetimes of photoinduced electrons and holes, and conductivity properties of the samples were characterized. Nb2Te4O13 has an indirect transition with a band energy of 2.98 eV V5+-doping shows a narrowing effect on the band gap, enhancing the harvest of the optical absorption in the visible light region. Photocatalysis tests were conducted via the photodegradation of RhB solutions under visible-light irradiation. V5+-doping depresses the intrinsic luminescence efficiency, while it significantly enhances photodegradation. It is suggested that the induced redox couples V5+/4+ and defects are responsible for the improvements of photocatalysis of Nb2Te4O13. This work offers an effective method to modify photochemical properties via redox couples and defects induced by impurity ion doping.

Octahedral Tilting and Modulation Structure in Perovskite‐Related Compound La1/3NbO3

The crystal structure of the A‐site‐deficient perovskite La1/3NbO3 is investigated using X‐ray powder diffraction, transmission electron microscopy (TEM), and computational methods. The NbO6 octahedral tilting is observed and confirmed to be in an a − b 0 c 0 manner in Glazer's notation. An incommensurate modulation structure in La1/3NbO3 is observed. Using scanning TEM (STEM) analyses and Monte Carlo simulations, the modulated structure is shown to originate from a striped arrangement of La and vacancies. The satellite reflections arising from the modulation structure are observed as a diffuse scatter in the electron diffraction patterns. By analyses with STEM and molecular dynamics calculations, it is concluded that the origin of diffuse scattering is the existence of nanoscale twin variants with different crystal orientations. The Nb ions in NbO6 octahedra represent an off‐center shift like a pseudo‐Jahn–Teller distortion. The shift of Nb ions is found to be cooperative with the concentration modulation of La ions.

Optimization of hydrogen-ion storage performance of tungsten trioxide nanowires by niobium doping

The transport and storage of ions within solid state structures is a fundamental limitation for fabricate more advanced electrochemical energy storage, memristor, and electrochromic devices. Crystallographic shear structure can be induced in the tungsten bronze structures composed of corner-sharing WO6 octahedra by the addition of edge-sharing NbO6 octahedra, which might provide more storage sites and more convenient transport channels for external ions such as hydrogen ions and alkali metal ions. Here, we show that Nb2O5·15WO3 nanowires (Nb/W = 0.008) with long length-diameter ratio, smooth surface, and uniform diameter have been successfully synthesized by a simple hydrothermal method. The Nb2O5·15WO3 nanowires do exhibit more advantages over h-WO3 nanowires in electrochemical hydrogen ion storage such as smaller polarization, larger capacity (71 mAh g−1, at 10C, 1C = 100 mA g−1), better cycle performance (remain at 99% of the initial capacity after 200 cycles at 100C) and faster H+ ions diffusion kinetics. It might be the crystallographic shear structure induced by Nb doping that does result in the marked improvement in the hydrogen-ion storage performance of WO3. Therefore, complex niobium tungsten oxide nanowires might offer great promise for the next generation of electrochemical energy and information storage devices.

Low dielectric loss ceramics in the Mg4Nb2O9-ZnAl2O4-TiO2 ternary system

This study used a traditional solid-state reaction method to prepare a series of composite ceramics in the 0.7Mg4Nb2O9-(0.3-x)ZnAl2O4-xTiO2 ternary system. Crystalline phases and microstructure of Mg4Nb2O9-ZnAl2O4-TiO2 dielectric ceramic composites were investigated and correlated with the relevant dielectric properties. It was observed that the addition of Ti4+ substituted Nb5+ in the Mg4Nb2O9 structure, which promoted the decomposition of Mg4Nb2O9 to form the second phase, Mg5Nb4O15, during sintering. The synergistic effect of ZnAl2O4-TiO2 co-doping promoted the Mg4Nb2O9 ceramic densification. The sample (0.7Mg4Nb2O9-(0.3-x)ZnAl2O4-xTiO2) with x = 0.15−0.2 exhibited dielectric constants of 13–14, larger than those of ZnAl2O4, Mg4Nb2O9 and Mg5Nb4O15, due to the NbO6 octahedra distortion resulting from the substitution of Al3+/Ti4+ for Nb5+ in Mg4Nb2O9 and Mg5Nb4O15. The long-range order of the NbO6 octahedra was enhanced by co-doping ZnAl2O4 and TiO2, thereby enhancing the Qxf value. A dielectric constant of 13.1, Qxf value of 366,000 GHz and a τf of −60.8 ppm/°C were obtained from 1300 °C sintered 0.7Mg4Nb2O9-0.15ZnAl2O4-0.15TiO2. These results show that 0.7Mg4Nb2O9-0.15 ZnAl2O4-0.15TiO2 ceramic is a good candidate for microwave electronic device applications.