Niobium(V) oxide doped with Mg<Superscript>2+</Superscript> and Gd<Superscript>3+</Superscript> cations: Synthesis and structural studies

Methods for direct doping of niobium pentoxide with photovoltaically inactive Mg²⁺ and Gd³⁺ cations were developed for subsequent use in the synthesis of a stock for growing single crystals of lithium niobate with improved optical characteristics. The Raman spectra of doped pentoxides Nb₂O₅: Mg and Nb₂O₅: Gd revealed their island structures.     

Effect of the method used to prepare solid precursors Nb2O5:Mg on the characteristics of LiNbO3:Mg crystals produced on their basis

We investigated how the type of an extraction system affects the characteristics of precursors Nb₂O₅:Mg produced at the stage of extraction processing of niobium-containing HF-H₂SO₄ and HF-HCl solutions with an extractant comprising a mixture of dimethylamides of carboxylic acids of the C₁₀–C₁₃ fraction and 1-octanol in Escaid diluent. We also studied the optical properties of homogeneously doped crystals LiNbO₃:Mg grown from a batch synthesized with the use of pentoxide Nb₂O₅:Mg obtained from various extraction systems. On Z-cut LiNbO₃:Mg crystal wafers, such mechanical characteristics as Young’s modulus and microgravity were measured.     

Synthesis of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>〈B〉 solid precursors and LiNbO<Subscript>3</Subscript>〈B〉 batches and their phase compositions

A process has been developed for preparing boron-doped niobium pentoxides Nb₂O₅〈B〉 to be used as precursors in the sysnthesis of nithium biobate batches LiNbO₃〈B〉 having tailored dopant concentrations. Solutions of various origins were used to isolate Nb₂O₅〈B〉. A method has been advanced to account for boron loss as volatile compounds upon the heat treatment of niobium hydroxide in order to determine the boron amount to be added to niobium hydroxide in the form of H₃BO₃. The boron concentration in LiNbO₃〈B〉 during lithium niobate synthesis is shown to be independent of the origin of the Nb₂O₅〈B〉 precursor with the same as-batch boron concentration. The phase compositions of Nb₂O₅〈B〉 and LiNbO₃〈B〉 have been characterized by X-ray powder diffraction and IR spectroscopy and boron concentrations have been determined for the synthesis of single-phase lithium niobate batches for use in the production of optically uniform single crystals and pore-free piezoelectric ceramics.     

Phase formation in the V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-MoO<Subscript>3</Subscript> system

The solid-phase interaction in the V₂O₅-Nb₂O₅-MoO₃ system has been investigated, and the formation of a solid solution bounded by the compositions MoNb₂V₄O_{18 ? δ}, Mo₂NbV₅O_{21 ? δ}, Mo₂Nb₃V₃O_{21 ? δ}, and Mo₄Nb₉V₉O_{57 ? δ} has been found (δ is nonstoichiometry). In the V₂O₅?Nb₂O₅ system, the formation of three compounds is verified, namely, VNbO₅ (tetragonal structure), VNb₉O₂₅, and V₂Nb₂₃O_62.5. The first two compounds are isostructural and form a continuous solid solution with tetragonal symmetry. A new compound of the composition Mo₃NbVO_{14 ? δ} has been synthesized. This compound is isostructural to the Mo₃Nb₂O₁₄ compound described in the literature and forms a tetragonal solid solution with it. The phase equilibria in the V₂O₅-Nb₂O₅-MoO₃ system in the subsolidus region have been determined.     

Preparation of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> and N co-doped TiO<Subscript>2</Subscript> photocatalysts and their enhanced photocatalytic activities under visible light

Niobium pentaoxide (Nb₂O₅) and nitrogen co-doped TiO₂ photocatalysts were prepared by mechanical alloying with Nb₂O₅, TiO₂ and urea as raw materials. The pure TiO₂ powders of Degussa P25 were chosen as precursor. Chemical and physical properties of the Nb₂O₅ and N co-doped TiO₂ photocatalysts were investigated, including XRD patterns, XPS spectra, DRS spectra, FT-IR spectra and N₂ adsorption-desorption isotherms. Experiments on photodegradation of methylene blue (MB) and sulfosalicylic acid (SSA) under visible light were carried out to evaluate the photoactivities of the prepared samples, and the chemical oxygen demand (COD) analysis was also conducted as a comparison.     

Composition and Homogeneity of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>〈В〉 Solid Precursors and LiNbO<Subscript>3</Subscript>〈В〉 Batches

Nb₂O₅〈В〉 solid precursors and LiNbO₃〈В〉 batches prepared on their basis, which can be used for preparing optical-quality lithium niobate single crystals and pore-free piezoelectric ceramics, have been studied by laser ablation inductively coupled mass spectrometry (LA-ICP-MS). The compositions of powdery samples pelletized without binder have been determined. The calculated mean-square deviations Sr of laser ablation ICP-MS have been used to show a homogeneous distribution of the boron dopant over Nb₂O₅〈В〉 precursors and LiNbO₃〈В〉 batches.     

The reactivity of SbVO<Subscript>5</Subscript> with T-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> in solid state in air

The reactivity of SbVO₅, a compound known since a short time, with T-Nb₂O₅ in solid state in air has been investigated over the whole component concentration range of a system built by these two reacting substances. The investigation results have shown that an equimolar mixture of SbVO₅ and T-Nb₂O₅ reacts with a subsequent formation of a hitherto unknown compound of the formula Nb₂VSbO₁₀. This compound has been characterized by the methods XRD, DTA/TG, and SEM. Its orthorhombic unit cell parameters have been calculated, and its stability in air up to 880 ± 10 °C has been proved. At this temperature, the compound melts incongruently with an accompanying deposition of solid Nb₉VO₂₅, i.e., of a compound that crystallizes in the binary oxide system V₂O₅–Nb₂O₅.     

Specific heat on Sr<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>7</Subscript> and Sr<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>7</Subscript>

Summary Specific heats on the single crystals of Sr₂Nb₂O₇, Sr₂Ta₂O₇ and (Sr_1-xBa_x)₂Nb₂O₇ were measured in a wide temperature range of 2-600 K. Heat anomalies of a λ-type were observed at the incommensurate phase transition of T_INC (=495 K) on Sr₂Nb₂O₇ and at the super-lattice phase transition of T_SL (=443 K) on Sr₂Ta₂O₇; the transition enthalpies and the transition entropies were estimated. Furthermore, a small heat anomaly was observed at the low temperature ferroelectric phase transition of T_LOW (=95 K) on Sr₂Nb₂O₇. The transition temperature T_LOW decreases with increasing Ba content x and it vanishes for samples of x>2%.     

On potassium hexaniobates released in the course of preparation of concentrated niobium(V) alkaline solution

Solutions containing 500 g L^?1 Nb₂O₅ were obtained by sintering Nb₂O₅ with potash followed by leaching with water. The potassium niobate released from niobium(V) alkaline solutions was examined with an application of methods of physical and chemical analysis. Hydrates K₈Nb₆O₁₉·4H₂O and K_7.5[H_0.5Nb₆O₁₉]·14H₂O were first obtained.     

Structure and real composition of undoped and Cr- and Ni-doped Sr<Subscript>0.61</Subscript>Ba<Subscript>0.39</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> single crystals

Strontium barium niobate crystals with congruent melting composition Sr_0.61Ba_0.39Nb₂O₆ (SBN-61), both nominally pure and doped with Cr³⁺ и Ni³⁺ ions, have been investigated by neutron diffraction. Different strontium and barium contents as well as their different distribution over the Sr1, of Sr2 and Ba2 crystallographic sites of SBN-61 structure, caused by introduction of dopants, have been revealed. Coordination polyhedra of cations have been established based on the analysis of cation–anion internuclear distances together with the calculation of bond-valence sums for cations, which are equal to their formal charge. It was found that the Nb1 and Nb2 atoms are located in distorted octahedra with quadfurcated (the Nb1O₆ polyhedron) or bifurcated (the Nb2O₆ polyhedron) vertices, and the Sr1 atoms are located in a cuboctahedron with bifurcated vertices in the base plane. Different polyhedra have been revealed for the Sr2 and Ba2 atoms: Sr2 atoms are coordinated by 15 oxygen atoms to form a highly distorted five-capped pentagonal prism, whereas Ba2 atoms are located in a highly distorted three-capped trigonal prism with a coordination number 9. Comparison of interatomic and internuclear distances, determined by X-ray and neutron diffraction analyses, respectively, allowed to reveal a highly pronounced shift of electron density in Nb1 and Sr2 polyhedra, responsible for the covalent bond and properties of crystals. Location of Cr³⁺ и Ni³⁺ dopant ions in the SBN-61 structure as well as their formal charges has been discussed.     

Electrospun Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>/Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript> composite nanofibers for enhanced high-rate lithium ion batteries

Li₄Ti₅O₁₂/Li₂TiO₃ composite nanofibers with the mean diameter of ca. 60 nm have been synthesized via facile electrospinning. When the molar ratio of Li to Ti is 4.8:5, the Li₄Ti₅O₁₂/Li₂TiO₃ composite nanofibers exhibit initial discharge capacity of 216.07 mAh g^?1 at 0.1 C, rate capability of 151 mAh g^?1 after being cycled at 20 C, and cycling stability of 122.93 mAh g^?1 after 1000 cycles at 20 C. Compared with pure Li₄Ti₅O₁₂ nanofibers and Li₂TiO₃ nanofibers, Li₄Ti₅O₁₂/Li₂TiO₃ composite nanofibers show better performance when used as anode materials for lithium ion batteries. The enhanced electrochemical performances are explained by the incorporation of appropriate Li₂TiO₃ which could strengthen the structure stability of the hosted materials and has fast Li⁺-conductor characteristics, and the nanostructure of nanofibers which could offer high specific area between the active materials and electrolyte and shorten diffusion paths for ionic transport and electronic conduction. Our new findings provide an effective synthetic way to produce high-performance Li₄Ti₅O₁₂ anodes for lithium rechargeable batteries.     

One-pot synthesis of Nb-modified Al<Subscript>2</Subscript>O<Subscript>3</Subscript> support for NiMo hydrodesulfurization catalysts

The effect of Nb as a support modifier on the NiMo₆/Al₂O₃–Nb₂O₅(x) (x?=?0, 1, 4, and 8?wt% Nb) catalysts was studied. The supports were prepared by one-pot coprecipitation from soluble precursors. The XRF analysis of the catalysts showed that the contents of Mo and Ni increased slightly with the presence of Nb. Micropore area and pore volume augmented importantly with Nb content, resulting in pore diameters between 5.3 and 9.3?nm. XPS analysis showed that the presence of Nb decreases the active metal–support interaction, improving the Mo and Ni sulfidation degree. The Raman spectra of sulfided catalysts suggested an increase in the number of layers of MoS₂ in the presence of Nb. Generally, the thiophene HDS activity at normal pressure of sulfided NiMo₆/Al₂O₃–Nb₂O₅(8) was greater than that of the sulfided catalysts with x?=?0, 1, and 4?wt% Nb, which can be attributed to the Nb promotion that would have an effect on the type of active site (Brønsted or Lewis acidic sites), since the number of sites by CO chemisorption for sulfided NiMo₆/Al₂O₃–Nb₂O₅(x) did not show correlation with the catalytic activity. The high-pressure HDS activity of dibenzothiophene was also greater in the presence of Nb, and the hydrogenation route was preferred for the Nb-promoted solid, while the unpromoted one showed a larger yield of direct desulfurization products.

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Comparative investigation of LiNbO3 crystals Raman spectra in the temperature range 100–400 K

We have investigated Raman spectra of congruent and stoichiometric LiNbO₃ crystals in the temperature range 100–450 K. Slope gradient is greater for the temperature dependence of band width associated with Nb⁵⁺ ions vibrations than that associated with Li⁺ ions vibrations in a lithium niobate crystal structure. This fact indicates that the anharmonicity of Nb⁵⁺ ions vibrations along the polar axis is greater compared to Li⁺ ions vibrations. It is likely that O^2– ions contribute to this anharmonicity. The O^2– ions vibrations are characterized by an anharmonic potential in the LiNbO₃ crystal structure. The O^2– ions vibrations according to ab initio calculations strongly interact with vibrations of Nb⁵⁺ ions. We have found that the temperature dependence of the fundamental bands intensity is nonmonotonic and the “extra bands” intensity is strictly linear.     

Composition investigation of lithium niobate crystals and its influence on the optical damage resistance

Optical methods of investigation of the defect structures and the composition of lithium niobate (LiNbO₃) single crystalsis discussed. The intrinsic defects concentrations in lithium niobate crystals (lithium vacancies (V_Li)⁻ and (Nb_Li)⁴⁺ defects, Nb on Li site in the valence state 4+), as a function of the Li/Nb ratio, are also reported. The optical damage resistance of various lithium niobate samples was investigated as a function of the composition. A remarkable increase in the optical damage resistance was found in MgO-doped almost stoichiometric lithium niobate crystals.     

Synthesis of a new PET/layered niobate hybrid material

A new organic/inorganic hybrid material was prepared by the polymerization of bis(hydroxyethyl) terephthalate (BHET) with a layered niobate compound, H₄Nb₆O₁₇, modified by 11-aminoundecanoic acid (AUA). The hybrid polymer films of BHET with H₄Nb₆O₁₇/AUA exhibited favorable characteristics, particularly of being optically clear, indicating the exfoliation and homogeneous dispersion of H₄Nb₆O₁₇ into the PET/niobate hybrid.     

Phase equilibria in the K<Subscript>2</Subscript>O-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> system

Phase equilibria in the potassium oxide-niobium oxide system were studied by oscillation phase-analysis and thermal analysis on 35 samples with compositions lying the range from 24.9 to 66.4 mol % Nb₂O₅. The melting temperatures and melting types of the compounds of the system were refined. The composition of crystallizing phases was shown to depend on the thermal history of the sample.     

Effect of protonation and deprotonation on surface charge density of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>

The protonation and deprotonation of the Nb₂O₅ surface has been followed in order to understand the reactions of surface of this catalyst. The simultaneous potentiometric and conductometric titrations had been carried by using 50 mL of water suspension of Nb₂O₅ 40 g L⁻¹. The oxide was entirely deprotonated when adding 0.4 mL NaOH 1 mol L⁻¹, and later titrated with 0.1 mol L⁻¹. The titration had supplied K ₁ and K ₂ and the obtained values were 3.24 × 10⁻³ and 4.17 × 10⁻⁸, respectively. The zero point charge was pH_pcz = 4.94. The thermodynamic studies were carried out by using 50 mL of a 40 g/L Nb₂O₅ aqueous suspension with the pH adjusted to pH_PZC value. The suspension was titrated with 0.5 mol/L of HNO₃ or NaOH for protonation or deprotonation studies, respectively, in an isoperibol calorimeter CSC ISC-4300. Thus, the obtained thermodynamic values of the protonation and deprotonation of Nb₂O₅ were Δ_dp G = −37.60 kJ/mol, Δ_dp H = −23.72 kJ/mol and Δ_dpS = 47 J/(mol K).     

Impurity and radiation defects in LiB<Subscript>3</Subscript>O<Subscript>5</Subscript> crystals. The nature of centers in LiB<Subscript>3</Subscript>O<Subscript>5</Subscript> crystals,which are responsible for coloration during the long-term operation of optical elements

An attempt is made to explain the causes of coloration of LiB₃O₅ crystals after their long-term operation as laser elements. By EPR and optical spectroscopy the impurity and radiation centers are studied in as-grown LiB₃O₅ crystals and in the crystals whose color appeared after the long-term operation as laser elements. In a number of as-grown crystals a copper impurity is detected. EPR spectral parameters and the structural positions of Cu²⁺ ions are found. Defect formation features in electron irradiated as-grown LiB₃O₅ crystals and in the most colored regions of crystals of spent laser elements are analyzed. It is shown that in both growth crystals and crystals after long-term operation as laser elements the same set of radiation defects is observed: oxygen O^– in the interstitial position, an O^– hole center in the crystal structure, and the B²⁺ electron center due to the removal of an oxygen atom near the lithium vacancy. The only distinction is that the concentration of these radiation defects in crystals long used as laser elements is higher than that in growth ones by an order of magnitude. The results obtained enable the conclusion that the cause of coloration of LiB₃O₅ crystals is photo-induced diffusion of lithium atoms and their capture by cation vacancies in the dark part of the crystal, which provides the formation and accumulation of lithium vacancies in the region where the laser beam passes.     

Etude des propriétés cristallographiques,diélectriques et d'optique non linéaire de quelques nouvelles phases de composition BaxLi5−2xNb5(1−y)Ta5yO15 et de structure bronzes oxygénés de tungstène quadratiques

Two series of phases with tetragonal bronze-like structure and composition Ba_xLi_5?2xT₅O₁₅ (T = Nb, Ta) have been isolated in the systems BaNb₂O₆LiNbO₃ and BaTa₂O₆LiTaO₃. All these phases show ferroelectric-paraelectric transitions. The Curie temperature increases with the lithium content. The value of T_C for Ba_2.03Li_0.94Nb₅O₁₅ is the highest ever observed for this type of structure: the obtained phases are potentially good materials for the harmonic generation of the 0.53-μm radiation. The optical yield of the niobate Ba_2.14Li_0.71Nb₅O₁₅ is about 2.5 times that of Ba₂NaNb₅O₁₅ and 250 times that of the K.D.P. The crystallographic and dielectric data of the system Ba_2.14Li_0.71Nb₅O₁₅Ba_2.14Li_0.71Ta₅O₁₅ characterize three domains, which are respectively antiferroelectric, ferroelectric, and paraelectric. The Curie temperature and the optical yield decrease with increasing tantalum content.