Synthesis of nanopowders of pentoxides Ta2y Nb2(1−y)O5

The processes of production of high purity nanopowders of niobium and tantalum pentoxide Ta_2y Nb_2(1–y)O₅ with a low content of fluorine and Nb₂O₅ in low-temperature polymorph were studied. Ceramic samples were prepared from a charge of solid solutions LiTa _y Nb_1–y O₃ and Li _x Na_1–x Ta _y Nb_1–y O₃ synthesized using coprecipitated pentoxide Ta_2y Nb_2(1–y)O₅. Therewith for solid solutions LixNa_1–x Ta _y Nb_1–y O₃ significantly larger values of high dielectric constant and ionic conductivity were achieved compared to the solid solutions obtained by using a mechanical mixture of Ta₂O₅ and Nb₂O₅. This converts solid solutions LixNa_1–x Ta _y Nb_1–y O₃ from acoustoelectronic and piezoelectric type of materials into the capacitor and ion-conductive type of solid materials.     

A study of the formation of LiNbO3 in the system Li2CO3Nb2O5

The formation process of LiNbO₃ in the system Li₂CO₃Nb₂O₅ was discussed from the results of non-isothermal or isothermal TG experiments and X-ray analysis. The mixture Li₂CO₃ and Nb₂O₅ in mole ratios of 1:3, 1:1 or 3:1 was heated at a rate of 5°C min^?1 or at various temperatures fixed in the range 475 to 677°C. If the system has a composition of Li₂CO₃ + 3Nb₂O₅ or 3Li₂CO₃ + Nb₂O₅, the reaction between Li₂CO₃ and Nb₂O₅ proceeds with CO₂ evolution to form LiNbO₃ at ca. 300–600°C, but Nb₂O₅ or Li₂CO₃ remains unreacted. A composition of Li₂CO₃ + Nb₂O₅ gives LiNbO₃ at 300–700°C. The diffusion of Li₂O through the layer of LiNbO₃ is rate-controlling with an activation energy of 51 kcal mol^?1. The reaction between LiNbO₃ and Nb₂O₅ gives LiNb₃O₈ at 600–700°C. At 700–800°C, a slight formation of Li₃NbO₄ occurs by the reaction between LiNbO₃ and Li₂O at the outer surface of LiNbO₃ and the Li₂O component of Li₃NbO₄ diffuses toward the boundary of the LiNb₃O₈ layer through the LiNbO₃ layer. The single phase of LiNbO₃ is formed above 850°C.     

Optical absorption and EPR spectroscopic studies of (30 − x)Li2O–xK2O–10CdO–59B2O3–1Fe2O3: An evidence for mixed alkali effect

Optical absorption and EPR spectroscopic studies were carried on (30 ? x)Li₂O–xK₂O–10CdO–59B₂O₃–1Fe₂O₃ (x = 0–30) glass system to understand the effect of progressive doping of Li⁺ ion with K⁺ ion. Optical absorption results show typical spectra of Fe³⁺ ions and the various optical parameters such as, optical band gap, Urbach energy, oxide ion polarizability, optical basicity and interaction parameter were evaluated from the experimental data. The observed optical band gap and Urbach energy values show large deviation from the linearity where as the other parameters show small deviation from the linearity with the progressive substitution of Li⁺ ions with K⁺ ions. The observed EPR spectra are representative of Fe³⁺ ion in octahedral and axial fields in the glass network. The number of paramagnetic centers and paramagnetic susceptibility values were evaluated at different resonance lines for all the specimens and these parameters show non-additive nature with the progressive substitution of Li⁺ ions with K⁺ ions in the glass network. This is first ever observation of mixed alkali effect (MAE) in EPR and optical parameters of mixed alkali borate glasses.     

Dielectric behavior and impedance analysis of lead-free CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 ceramics

Pure (Na_0.50K_0.50)_0.95(Li_0.05Sb_0.05Nb_0.95)O₃ (NKNLS) and CuO doped NKNLS perovskite structured ferroelectric ceramics were prepared by the solid-state reaction method. x wt% of CuO (x = 0.2–0.8 wt%) was added in the NKNLS ceramics. X-ray diffraction patterns indicate that single phase was formed for pure NKNLS while a small amount of second phase (K₆Li₄Nb₁₀O₃₀ ∼ 3%) was present in Cu²⁺ doped NKNLS ceramics. Dielectric anomalies around the temperatures of 120 °C and 350 °C have been identified as the ferroelectric–paraelectric transition (orthorhombic to tetragonal and tetragonal to cubic) temperatures for pure NKNLS compound. The electrical behavior of the ceramics was studied by impedance study in the high temperature range. Impedance analysis has shown the grain and grain boundary contribution using an equivalent circuit model. The impedance response in pure and Cu²⁺ doped NKNLS ceramics could be resolved into two contributions, associated with the bulk (∼grains) and the grain boundaries. From the conductivity studies, it is found that activation energies are strongly frequency dependent. The activation energy obtained from dielectric relaxation data may be attributed to oxygen ion vacancies.     

Effect of sintering on the ionic conductivity of garnet-related structure Li5La3Nb2O12 and In- and K-doped Li5La3Nb2O12

Garnet-structure related metal oxides with the nominal chemical composition of Li₅La₃Nb₂O₁₂, In-substituted Li_5.5La₃Nb_1.75In_0.25O₁₂ and K-substituted Li_5.5La_2.75K_0.25Nb₂O₁₂ were prepared by solid-state reactions at 900, 950, and 1000 °C using appropriate amounts of corresponding metal oxides, nitrates and carbonates. The powder XRD data reveal that the In- and K-doped compounds are isostructural with the parent compound Li₅La₃Nb₂O₁₂. The variation in the cubic lattice parameter was found to change with the size of the dopant ions, for example, substitution of larger In³⁺(r_CN6: 0.79 Å) for smaller Nb⁵⁺ (r_CN6: 0.64 Å) shows an increase in the lattice parameter from 12.8005(9) to 12.826(1) Å at 1000 °C. Samples prepared at higher temperatures (950, 1000 °C) show mainly bulk lithium ion conductivity in contrast to those synthesized at lower temperatures (900 °C). The activation energies for the ionic conductivities are comparable for all samples. Partial substitution of K⁺ for La³⁺ and In³⁺ for Nb⁵⁺ in Li₅La₃Nb₂O₁₂ exhibits slightly higher ionic conductivity than that of the parent compound over the investigated temperature regime 25-300 °C. Among the compounds investigated, the In-substituted Li_5.5La₃Nb_1.75In_0.25O₁₂ exhibits the highest bulk lithium ion conductivity of 1.8×10⁻⁴ S/cm at 50 °C with an activation energy of 0.51 eV. The diffusivity (“component diffusion coefficient”) obtained from the AC conductivity and powder XRD data falls in the range 10⁻¹⁰-10⁻⁷ cm²/s over the temperature regime 50-200 °C, which is extraordinarily high and comparable with liquids. Substitution of Al, Co, and Ni for Nb in Li₅La₃Nb₂O₁₂ was found to be unsuccessful under the investigated conditions.     

Non-stoichiometric 1:2 ordered perovskites in the Ba(Li1/4Nb3/4)O3-Ba(Li2/5W3/5)O3 system

Although both end members in the (1−x)Ba(Li_1/4Nb_3/4)O₃-xBa(Li_2/5W_3/5)O₃ (BLNW) system adopt a hexagonal perovskite structure, B-site ordered cubic perovskites are formed for the majority of their solid solutions (0.238?x?0.833). Within this range, single-phase 1:2 order (, , ) is stabilized for 0.238?x?0.385. In contrast to all known A(B_1/3^IB_2/3^II)O₃ perovskites, the 1:2 ordered BLNW solid solutions do not include any composition with a 1:2 cation distribution and the structure exhibits extensive non-stoichiometry. Structure refinements support a model where Li and W occupy different positions and Nb is distributed on both sites, i.e. Ba[(Li_3/4+y/2Nb_1/4−y/2)_1/3(Nb_1−yW_y)_2/3]O₃ (y=0.21-0.35, where y=0.9x). The stabilization of the non-stoichiometric order arises from the large charge/size site differences; the loss of 1:2 order for W-rich compositions is related to local charge imbalances on the A-site sub-lattice. The range of single-phase 1:1 order is confined to x=0.833, (Ba(Li_3/4Nb_1/4)_1/2(W)_1/2)O₃), where the site charge/size difference is maximized and the on-site mismatches are minimized. The microwave dielectric loss properties of the ordered BLNW solid solutions are significantly inferior as compared to their stoichiometric counterparts.     

Na(V3−xNbx)Nb6O14 – ein neues Oxoniobat mit [Nb6O12]- und [M2O9]-Clustern

Na(V_3?xNb_x)Nb₆O₁₄ — A Novel Oxoniobate with [Nb₆O₁₂] and [M₂O₉] Clusters Goldcolored single crystals and black powders of Na(V_3?xNb_x)Nb₆O₁₄ have been prepared by heating a pellet containing a mixture of NaNbO₃, NbO₂, NbO, VO₂ and NaF or Na₂B₄O₇ (as mineralizers) at 900°C in a sealed gold capsule. The analytically determined Nb : V ratio is 5 : 1 and means that x is about 1.5. The compound crystallizes in P6₃/m with a = 603.4(1), c = 1807.9(5) pm and Z = 3. The crystal structure can be described in terms of common close packing of sheets of O and Na atoms together with Nb₆ octahedra. Characteristic building groups of the new structure type are [Nb₆O₁₂] clusters, [M₂O₉] clusters and NbO₅ bipyramids. V atoms are distributed only on the positions of the Nb atoms within the trigonal bipyramids or the [M₂O₉] clusters. The [Nb₆O₁₂] clusters show characteristicaly short distances d_Nb-Nb = 279.4 and 281.3 pm, respectively. In the [M₂O₉] units, which are built from two MO₆ octahedra that share a common face, V or Nb atoms form M–M dumbbells with d_M–M = 255.9 pm. The electronic structure is discussed using Extended Hückel calculations.     

Al2O3/K2O-containing non-stoichiometric lithium disilicate-based glasses

The crystallisation kinetics of experimental glasses in 3 different systems: (A) Li₂O–SiO₂, (B) Li₂O–Al₂O₃–SiO₂ and (C) Li₂O–K₂O–Al₂O₃–SiO₂ were studied under non-isothermal conditions. The DTA results revealed a stronger tendency to crystallisation of binary compositions in comparison to the ternary and quaternary compositions comprising Al₂O₃ and K₂O which present the lower crystallisation, i.e. the crystallisation propensity follows the trend A > B > C. The devitrification process in the Li₂O–SiO₂ and Li₂O–Al₂O₃–SiO₂ systems began earlier and the rate was higher in comparison to that of glasses in the quaternary Li₂O–K₂O–Al₂O₃–SiO₂ system. Thus, addition of Al₂O₃ and K₂O to glasses of Li₂O–SiO₂ system was demonstrated to promote glass stability against crystallisation. However, the activation energy for crystallisation was shown to depend also on the SiO₂/Li₂O ratio with the binary system showing a decreasing trend with increasing SiO₂/Li₂O ratio, while the opposite tendency was being observed for compositions with added Al₂O₃ and K₂O.     

Sur quelques Nouvelles Phases Oxyfluorées de Structure “Bronzes Oxygénés de Tungstène Quadratiques”

On Some New Oxide Fluoride Phases of Tetragonal Tungsten Bronze Structure Six new oxide fluorides of tetragonal tungsten bronze type structure have been obtained by partial substitution of oxygen by fluorine in the ABCNb₅O₁₅ compounds (A = Ca, Sr, Ba; B = Ca, Sr, Ba; C = Na, K): CaK₂Nb₅O₁₄F, SrK₂Nb₅O₁₄F, SrKNaNb₅O₁₄F, BaK₂Nb₅O₁₄F, BaKNaNb₅O₁₄F and BaNa₂Nb₅O₁₄F. An investigation on Sr_2?xK_1+xNb₅O_15?xF_x and Ba_2?xNa_1+xNb₅O_15?xF_x solid solutions characterizes ferroelectric behaviour. Replacement of oxygen by fluorine decreases the Curie temperature, but for a small oxygenfluorine substitution rate an increase of the dielectric constant is observed.     

Ionic conductivity and crystal chemistry of ramsdellite type compounds,Li2+x(LixMg1−xSn3)O8, 0 ≤ x ≤ 0.5 and Li2Mg1−xFe2xSn3−xO8, 0 ≤ x ≤ 1

Solid solution phases Li_2+x(Li_xMg_1−xSn₃)O₈: 0 ≤ x ≤ 0.5 and Li₂Mg_1−xFe_2xSn_3−xO₈: 0 ≤ x ≤ 1, both with ramsdellite type structure, have been synthesized by solid state reaction at 1773 and 1523 K, respectively. The relationship of the ramsdellite structure to the recently illustrated, tetragonal-packed structure is given. The Li_2+x(Li_xMg_1−xSn₃)O₈ solid solutions exhibit conductivities 4 × 10⁻⁶–5 × 10⁻⁴ (Ω cm)⁻¹ at 573 K and corresponding activation energies, 0.93−0.74 eV. The highest conductivity was observed for Li_2.3(Li_0.3Mg_0.7Sn₃)O₈, x = 0.3. In the solid solution series Li₂Mg_1−xFe_2xSn_3−xO₈, the highest conductivity was exhibited by Li₂Fe₂Sn₂O₈, 2 × 10⁻⁵ (Ω cm)⁻¹ at 573 K.     

Microporous Framework (Nb,Fe)-Silicate with Much Potential to Remove Rare-Earth Elements from Waters

Nanoparticles of a new small-pore metal silicate formulated as Na_2.9(Nb_1.55Fe_0.45)Si₂O₁₀ ⋅ xH₂O and exhibiting the structure of previously reported Rb₂(Nb₂O₄)(Si₂O₆) ⋅ H₂O have been synthesized under mild hydrothermal conditions. Replacement of the bulky Rb⁺ by smaller Na⁺ ions was accomplished by stabilizing the framework structure via partial occupancy of the Nb⁵⁺ sites by Fe³⁺ ions. Exploratory ion-exchange assays evidence the considerable potential of this new silicate to remove rare-earth elements from aqueous solutions.     

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.     

Electrode materials based on lanthanum ferrite cobaltite for molten carbonate fuel cells

New cathode and anode materials for fuel cells with an electrolyte based on alkali carbonate melts have been studied. The regions of the orthorhombic and rhombohedral phases in the LaFe_1–yCo_yO₃ + x/2Li₂O system in air and in contact with molten (Li_0.68K_0.32)₂CO₃ electrolyte were investigated. The electric conductivity was analyzed in the range 300–1020 K. The electrocatalytic activity in oxygen reduction was analyzed for the new cathode materials. A method for introducing the Al₂O₃ additive in the anode material was suggested. The polarization characteristics of the porous gas-diffusion electrodes were determined.     

Magnetische Eigenschaften von Ti3−xMxO5-Phasen (M = V3+, Cr3+, Nb4+)

Magnetic Properties of Ti_3?xM_xO₅ Phases (M = V³⁺, Cr³⁺, Nb⁴⁺) The magnetic properties of Ti_3?xV_xO₅, Ti_3?xCr_xO₅, and Ti_3?xNb_xO₅ phases are reported. In the case of V³⁺ and Cr³⁺ the magnetic leaping-temperature decreases, however Nb⁴⁺ shift the phase-transition towards higher temperatures. All samples show a “memory-effect” in magnetic properties, i. e. the results of heating- and cooling-cycles are higher susceptibilities of the α-phase of Ti₃O₅. Endowed Ti₃O₅ phases show for the α- and β-Ti_3?xM_xO₅ til the leap Curie-Weiss characteristic in 1/X vs. temperature measurements. Exception is β-Ti_3?xNb_xO₅, its susceptibility is independend of the temperature up to x ? 0.3.     

Thermochemische Untersuchungen im System V/Nb/O. II Zum chemischen Transportverhalten im Bereich V2O5/Nb2O5/VO2/NbO2

Thermochemical Investigations in the System V/Nb/O. II. Chemical Transport in the Region V₂O₅/Nb₂O₅/VO₂/NbO₂ Transport experiments were used to support the phase relationships of the V₂O₅/Nb₂O₅/VO₂/NbO₂ system, which were established by annealing experiments of powder mixtures. The phase relations were studied in the NbO₂-rich region of the system by means of X-ray and ESMA methods. The NbO₂-rich section is characterized by the following two phase and three phase regions: Two phase region: V₃Nb₉O₂₉/rutile mixed crystal V_1?xNb_xO₂ Two phase region: BI-mixed crystal/V_xNb_1?xO₂ Three phase region: V₃Nb₉O₂₉/solubility limit LG1 (V_1?xNb_xO₂)/BI-mixed crystal Three phase region: solubility limit LG1 (V_1?xNb_xO₂)/BI-mixed crystal/solubility limit LG2 (V_xNb_1?xO₂). The composition of the solubility limits LG1 and LG2 was ascertained by means of ESMA-investigation: LG1: 57.5 ± 5 mol% NbO₂/43.5 ± 5 mol% VO₂ LG2: 22.5 ± 5 mol% NbO₂/78.5 ± 5 mol% VO_2?     

Crystal chemistry of the (Ta6Si4O26)6− and (Ta14Si4O47)8− frameworks

The range of chemical flexibilities of the hexagonal frameworks (Ta₆Si₄O₂₆)^6? and (Ta₁₄Si₄O₄₇)^8? have been partially explored. This has been done with high-temperature preparations as in general ionic mobilities in these frameworks are too low to permit low-temperature ion exchange. Ionic site potential calculations indicate that preferential site-occupancy factors as well as geometric constraints are responsible for the absence of ionic motion. New phases K_6?xNa_xTa₆Si₄O₂₆ (x ? 4), K_8?xNa_xTa₁₄Si₄O₄₇ (x ? 5), and impure Ba_3?xNa_2xTa₆Si₄O₂₆ have been prepared. Introduction of up to 2 moles of Li⁺ and 1 mole of Mg²⁺ ions per formula unit into sites of the framework not normally occupied has been demonstrated as well as the possibility of partially substituting Zr⁴⁺ for Ta⁵⁺ ions. Substitutions designed to introduce large tunnel vacancies in the presence of only monovalent K⁺ or Na⁺ ions (P for Si, W for Ta and F for O) generally proved unsuccessful. Competitive phases also frustrated attempts to substitute either the larger Rb⁺ or the smaller Li⁺ ions into the large-tunnel sites. A large area of solid solution was discovered in the BaONa₂OTa₂O₅ phase diagram; it has a (TaO₃)-framework with the structure of tetragonal potassium tungsten bronze.     

Synthesis and characterization of new borate-vanadate mixed glasses

Summary It has been found that a series of M_xO_y-V₂O₅-B₂O₃glasses (M_xO_y=Li₂O, Na₂O, K₂O and MgO) containing 10 mol%<span style='font-size:12.0pt;font-family: Symbol;mso-bidi-font-family:Symbol'>a-Fe₂O₃exhibited glass-forming regions that shifted with the content of network modifier (NWM) compared to B₂O₃and V₂O₅glasses. The M?ssbauer spectra of a series of M_xO_y-V₂O₅-B₂O₃glasses showed increased quadrupole splitting (D) with increasing NWM content. This suggests that the coordination numbers of the V⁴⁺and V⁵⁺are fixed and that the formation number of non-bridging oxygens (NBO) is considered to increase with increasing NWM content, and with increasing formation number of NBO, the Fe³⁺ion site changes from VO₄to BO₄tetrahedra. Consequently, the quadrupole splitting increases with increasing NWM content.     

Phase relations in the system V/Nb/O. V. Investigation of mixed crystals V1-xNbxO2

Mixed crystals V_1-xNb_xO₂ exist over the whole area of the quasibinary line VO₂-NbO₂. The existence of Nb_{5+ beside V3+ and V4+ on the V-rich side and V3+ beside Nb5+ and Nb4+ on the Nb-rich side of the mixed crystals is demonstrated by XANES-measurements. The compound VNbO}4_(V0.5_Nb0.5_O2_{) is described as a double oxide with vanadium only as V3+ and niobium only as Nb5+. At this point the electric resistivity of the solid solution shows a maximum.} Received: 11 May 1998 / Revised: 4 August 1998 / Accepted: 10 August 1998     

Layered manganese oxide with O2 structure,Li(2/3)+x(Ni1/3Mn2/3)O2 as cathode for Li-ion batteries

Using LiI as the reducing agent, the compound O2-Li_(2/3)+x(Ni_1/3Mn_2/3)O₂, x∼1/3 (O2(Li+x)) has been prepared from the O2-Li_2/3(Ni_1/3Mn_2/3)O₂ (O2(Li)). Cyclic voltammetry and voltage-capacity profiles of the O2(Li+x) phase are qualitatively different from that of O2(Li) phase. The first extraction capacity of O2(Li+x) at C/10 rate is 190 mAh/g corresponding to the removal of 2/3 mole of Li from the compound. At C/5 rate it delivers a reversible capacity of 158 mAh/g at 25 °C and 184 mAh/g at 50 °C (vs Li metal; voltage window 2.5–4.6 V). In Li-ion cells, with MCMB anode and O2(Li+x) as cathode, a discharge capacity of 140 mAh/g was obtained at C/5 rate in the voltage window 2.5–4.5 V (25 °C). The charge–discharge cycling performance and the cyclic voltammograms reveal that O2(Li) and O2(Li+x) do not convert to the spinel structure.     

Capturing the Heptaniobate {Nb7O22}9– Anion by Covalent Bond Formation: Synthesis,Crystal Structure,and Selected Properties of {[Fe(cyclam)]3Nb7O22}· ≈ 19 H2O

The new compound {[Fe(cyclam)]₃Nb₇O₂₂} · ≈ 19 H₂O ( I ) was synthesized at room temperature reacting aqueous solutions of K₈{Nb₆O₁₉} · 16 H₂O, Fe(NO₃)₃ · 9 H₂O and cyclam (1,4,8,11-tetraazacyclotetradecane). In the crystal structure the heptaniobate {Nb₇O₂₂}^9– anion is observed which is expanded by three [Fe(cyclam)]³⁺ complexes, thus suppressing further condensations into larger aggregates. The complexes are solely bound to the three terminal O^2– anions of the NbO₆ octahedron which expands the hexaniobate {Nb₆O₁₉}^8– anion to form the heptaniobate cluster. The two O atoms in the FeN₄O₂ octahedron are in cis position leading to a severe distortion of the cyclam ligand with one of the CH₂–CH₂–CH₂–N–CH₂–CH₂ fragments being rotated by about 90°. The {[Fe(cyclam)]₃Nb₇O₂₂} units are arranged to form channels which host the crystal water molecules. The crystal water molecules can be removed by thermal treatment. Storing the sample on air the pristine sample is recovered.