Study of EPR, optical properties and electrical conductivity of vanadyl doped Bi2O3.PbO.B2O3 glasses

Heavy metal based oxide glasses having composition xBi(2)O(3).(0.30 - x)PbO.0.70B(2)O(3) have been prepared (0.00 < or = x < or = 0.15, mol%) containing 2.0mol% of V(2)O(5) by normal melt-quenching technique. Electron paramagnetic resonance (EPR), optical spectra and dc conductivity of these glasses have been studied. Spin Hamiltonian parameters (SHP) of VO(2+) ions, dipolar hyperfine parameter, P and Fermi contact interaction parameter, K, molecular orbital coefficients (alpha(2) and gamma(2)) and optical band gap have been calculated. It is observed that in these glasses, the tetragonal nature of V(4+)O(6) complex increases with Bi(2)O(3) content. Increase in Bi(2)O(3):PbO ratio results in the contraction of 3d(xy) orbit of the unpaired electron in the vanadium ion, and the SHP are dependent on the theoretical optical basicity, Lambda(th). In present glasses, the conductivity (activation energy) first decreases (increases) with increase in mol% of Bi(2)O(3) content upto x = 0.08 and then shows a maxima (minima) at x = 0.10 and then starts decreasing (increasing) upto x < or = 0.15 with mol% of Bi(2)O(3) content.     

Synthesis and optical properties of V2O5 nanorods

A two-step method was proposed in synthesizing V2O5 nanorods on planar substrates, i.e., depositing a V2O3 thin film at approximately 220 degrees C (by heating a pure sheet of vanadium in a rough vacuum) and then heating it in air at approximately 400 degrees C. The V2O5 nanorods produced by this technique are single crystalline and could emit intense visible light at room temperature, possibly due to some defects such as oxygen vacancies which got involved during growth. This study provides a simple and low-substrate-temperature route in fabricating V2O5 nanorods on planar substrates, which might be also applicable to other metal oxides.     

Non-isothermal crystallization kinetics of V2O5-MoO3-Bi2O3 glasses

Characteristic temperatures, such as T _g (glass transition), T _x (crystallization temperature) and T _l (liquidus temperature) of glasses from the V₂O₅-MoO₃-Bi₂O₃ system were determined by means of differential thermal analysis (DTA). The higher content of MoO₃ improved the thermal stability of the glasses as well as the glass forming ability. The non-isothermal crystallization was investigated and following energies of the crystal growth were obtained: glass #1 (80V₂O₅·20Bi₂O₃) E _G=280 kJ mol^-1, glass #2 (40V₂O₅·30MoO₃·30Bi₂O₃) E _G=422 kJ mol^-1 and glass #3 (80MoO₃·10V₂O₅·10Bi₂O₃) E _G=305 kJ mol^-1. The crystallization mechanism of glass #1 (n=3) is bulk, of glass #3 (n=1) is surface. Bulk and surface crystallization was supposed in glass #2. The presence of high content of a vanadium oxide acts as a nucleation agent and facilitates bulk crystallization. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of gamma irradiation on optical and structural properties of PbO–Bi2O3–B2O3 glasses

Optical and structural properties of xPbO·2xBi₂O₃(1−3x)B₂O₃ glasses of different composition have been studied using UV-VIS and FTIR spectroscopic techniques. Effects of gamma radiations on glass network and structural units have been studied by irradiating glass samples with a ⁶⁰Co radioisotope to the overall dose of 2.5 kGy. It is shown that irradiation causes compaction of the borate network by breaking the bonds between trigonal elements, which leads to a decrease in the optical band gap energy. Changes in the atomic structure before and after the irradiation are observed and explained.     

Structure,chemical stability and electrical properties of BaCe0.9Y0.1O3−δ modified with V2O5

Wet vacuum impregnation method was applied in order to evaluate the possibility of the formation of the material in BaCe_0.9Y_0.1O_3?δ–V₂O₅ system. Single-phase BaCe_0.9Y_0.1O_3?δ samples, synthesised by solid-state reaction method, were impregnated with the solution of vanadium(V) oxide precursor. Multi-step, multi-cycle impregnation procedure was applied to enhance the impregnation efficiency. Partial decomposition of Y-doped BaCeO₃ in contact with the solution of the precursor, resulting in the formation of vanadium containing phases (CeVO₄ and BaV₂O₆) on the materials surface, was observed. However, the presence of vanadium was also confirmed for the inner parts of the materials. The synthesised materials were submitted for exposition test to evaluate their chemical stability towards CO₂/H₂O. All BaCe_0.9Y_0.1O₃-based materials modified by impregnation revealed higher chemical stability in comparison with single-phase un-modified BaCe_0.9Y_0.1O_3?δ, since the amount of barium carbonate formed during the exposition was significantly lower. The total electrical conductivity of the received multi-phase materials was generally slightly lower than for the reference BaCe_0.9Y_0.1O_3?δ sample, since the presence of the additional phases had a blocking effect on materials conductivity. The values of BaCeO₃ lattice parameters and the Seebeck coefficient did not show the modification of the defects structure of Y-doped BaCeO₃ during applied synthesis procedure.

     

Thermal,optical and electrical properties of MnO2-doped mixed sodium potassium phosphate glasses

Journal of Thermal Analysis and Calorimetry - Glasses in the system (1???x)(0.5NaPO3–0.5KPO3)–xMnO2, with 0?≤?x?≤?50&nbsp;mol%,...     

Thermal properties and crystallization of BaO–MoO3–P2O5 glasses

Journal of Thermal Analysis and Calorimetry - The thermal behavior and crystallization of barium molybdate-phosphate glasses were studied in two compositional series, namely A:...     

Mixed alkali effect and optical properties of Ni2+ doped 20ZnO+xLi2O+(30-x)Na2O+50B2O3 glasses

Optical and physical properties of Ni2+ doped 20ZnO+xLi2O+(30-x)Na2O+50B2O3 (5≤x≤25) glasses are carried out at room temperature. Powder XRD pattern of all the glass samples confirms the amorphous nature. Several physical parameters are evaluated for all the glasses with respect to the composition. The optical absorption spectra confirm the site symmetry of the Ni2+ doped glasses are near octahedral. Crystal field and inter-electronic repulsion parameters are also evaluated. It is interesting to observe that the optical band gap and Urbach energies exhibit the mixed alkali effect. The FT-IR spectral investigations of Ni2+ doped glasses exhibit characteristic vibrations of BO3 and BO4 units.     

Reactivity of T-Nb2O5 or H-Nb2O5 towards V2O5. Synthesis in the solid state and properties of V4Nb18O55

The IR spectrum of V₄Nb₁₈O₅₅ has been compared with the IR spectra of selected niobates of known structures to show structural relations between these compounds. This comparison shows that V₄Nb₁₈O₅₅ has crystal structure related to T-Nb₂O₅, W16Nb18O94 and Ba₂NaNb₅O₁₅. On the other hand, reaction between V₂O₅ and H-Nb₂O₅ yields a solid solution of V₂O₅ in VNb₉O₂₅. It has been proposed two models of synthesized solid solution with formulas V_1+xNb_9-xO₂₅ or V_1+xNb₉O_25+5x/2.Independently of Nb₂O₅ polymorph, used for synthesis, the metastable compound VNbO5 cannot be synthesized in the solid state below 650°C      

Structural properties of CeRh3B2

Time-of-flight neutron powder diffraction data have been obtained for the intermetallic compound CeRh₃B₂ at four different temperatures and analyzed using the Rietveld profile refinement technique. The CeCo₃B₂ structure is confirmed. The thermal expansion is highly anisotropic and anisotropic effects are also found in the atomic thermal vibrations. An unusually small metallic radius of 1.55 Å is observed for cerium in this compound.     

Electron microscopy of V2O5 microcrystals in the V2O5-NH3-H2O colloid system

Summary The formation of V₂O₅ fibrous microcrystals in the V₂O₅-NH₃-H₂O system has been investigated by electron microscopy. The morphology of these microcrystals is dependent on the concentration of the particular components in the colloidal system and on the time of aging. In some colloid systems the orientation of V₂O₅ fibrous crystals with their long axes parallel to one another is also possible. The microcrystal growth in these systems can be attributed to very effective conditions for the recrystallization of vanadium pentoxide crystals in equilibrium with ammonium vanadate in solution.With 3 figures     

Untersuchungen zum chemischen Transport der Vanadiumoxide V2O5, V3O7 und V6O13

Chemical transport of the vanadium oxides V₂O₅, V₃O₇, and V₆O₁₃ The suitability of water and some halogenating transport agents (NH₄Cl, NH₄Br, I₂) for the chemical transport (temperature gradient 850/750 K) of V₂O₅, V₃O₇, and V₆O₁₃ has been investigated. Transport rates for V₂O₅ and V₆O₁₃ could be measured and reproduced. The best transport agent for V₂O₅ is NH₄Cl or H₂O. For V₃O₇ a combination of the transport agents I₂/H₂O give the best results and for V₆O₁₃ the combination of NH₄Br/H₂O was most appropriate.     

Phase diagram of V2O5-MoO3-Ag2O

Phase equilibria in the V₂O₅-Ag₂O system were investigated at a constant pressure of oxygen (0.2 atm) and the phase diagram found under these conditions was compared with the results of the authors who investigated the same system in vacuum and at an oxygen pressure of 1 atm. On the basis of all these results, an attempt was made to construct the hypothetical diagram of V₂O₅-Ag₂O-O₂.     

Vanadium oxides on aluminum oxide supports. 2. Structure, vibrational properties, and reducibility of V2O5 clusters on alpha-Al2O3(0001)

The structure, stability, and vibrational properties of isolated V2O5 clusters on the Al2O3(0001) surface have been studied by density functional theory and statistical thermodynamics. The most stable structure does not possess vanadyl oxygen atoms. The positions of the oxygen atoms are in registry with those of the alumina support, and both vanadium atoms occupy octahedral sites. Another structure with one vanadyl oxygen atom is only 0.12 eV less stable. Infrared spectra are calculated for the two structures. The highest frequency at 922 cm(-1) belongs to a V-O stretch in the V-O-Al interface bonds, which supports the assignment of such a mode to the band observed around 941 cm(-1) for vanadia particles on alumina. Removal of a bridging oxygen atom from the most stable cluster at the V-O-Al interface bond costs 2.79 eV. Removal of a (vanadyl) oxygen atom from a thin vanadia film on alpha-Al2O3 costs 1.3 eV more, but removal from a V2O5(001) single-crystal surface costs 0.9 eV less. Similar to the V2O5(001) surface, the facile reduction is due to substantial structure relaxations that involve formation of an additional V-O-V bond and yield a pair of V(IV)(d1) sites instead of a V(III)(d2)/V(V)(d0) pair.     

Structural and optical properties of barium titanate modified bismuth borate glasses

Glass samples with composition (70B₂O₃–29Bi₂O₃–1Dy₂O₃) modified with Barium titanate (BT), where BT is added in different successive weight percents, have been synthesized by conventional melt quenching technique. X-ray diffraction studies were performed in order to confirm the amorphous nature of the samples. The density of the samples has been found to decrease with an increase in the BT content, whereas an opposite trend has been observed in the molar volume. The analysis of FTIR and Raman spectra of the samples depicts that the glass network is built up of mainly BiO₆, BiO₃, BO₃ and BO₄ units. Its detailed analysis also revealed that the glass structure depends upon the amount of BT in the glass matrix and hence it acts as a modifier in the glass network. Introduction of BT into the glass matrix leads to the conversion of BO₃ trigonal units into BO₄ tetrahedral units, which results in a decrease in the degree of disorder in the glass network and makes the glass system more stable. The values of Urbach energy obtained for the prepared samples also confirmed the decrease in disorder in the glass network. The optical absorption measurements carried out for well-polished samples show a decrease in optical band gap energy with an increase in BT content whereas the molar refractivity shows the reverse trend. The Hydrogenic excitonic model applied to the studied glasses suggested that the present glass system favors direct transitions. The metallization criterion of the presently studied samples suggests that the prepared glasses may be potential candidates for nonlinear optical applications.     

Oxidative dehydrogenation of propane over V2O5/MoO3/Al2O3 and V2O5/Cr2O3/Al2O3: structural characterization and catalytic function

The structure and catalytic properties of binary dispersed oxide structures prepared by sequential deposition of VO(x) and MoO(x) or VO(x) and CrO(x) on Al(2)O(3) were examined using Raman and UV-visible spectroscopies, the dynamics of stoichiometric reduction in H(2), and the oxidative dehydrogenation of propane. VO(x) domains on Al(2)O(3) modified by an equivalent MoO(x) monolayer led to dispersed binary structures at all surface densities. MoO(x) layers led to higher reactivity for VO(x) domains present at low VO(x) surface densities by replacing V-O-Al structures with more reactive V-O-Mo species. At higher surface densities, V-O-V structures in prevalent polyvanadates were replaced with less reactive V-O-Mo, leading to lower reducibility and oxidative dehydrogenation rates. Raman, reduction, and UV-visible data indicate that polyvanadates predominant on Al(2)O(3) convert to dispersed binary oxide structures when MoO(x) is deposited before or after VO(x) deposition; these structures are less reducible and show higher UV-visible absorption energies than polyvanadate structures on Al(2)O(3). The deposition sequence in binary Mo-V catalysts did not lead to significant differences in structure or catalytic rates, suggesting that the two active oxide components become intimately mixed. The deposition of CrO(x) on Al(2)O(3) led to more reactive VO(x) domains than those deposited on pure Al(2)O(3) at similar VO(x) surface densities. At all surface densities, the replacement of V-O-Al or V-O-V structures with V-O-Cr increased the reducibility and catalytic reactivity of VO(x) domains; it also led to higher propene selectivities via the selective inhibition of secondary C(3)H(6) combustion pathways, prevalent in VO(x)-Al(2)O(3), and of C(3)H(8) combustion routes that lead to low alkene selectivities on CrO(x)-Al(2)O(3). VO(x) and CrO(x) mix significantly during synthesis or thermal treatment to form CrVO(4) domains. The deposition sequence, however, influences catalytic selectivities and reduction rates, suggesting the retention of some of the component deposited last as unmixed domains exposed at catalyst surfaces. These findings suggest that the reduction and catalytic properties of active VO(x) domains can be modified significantly by the formation of binary dispersed structures. VO(x)-CrO(x) structures, in particular, lead to higher oxidative dehydrogenation rates and selectivities than do VO(x) domains present at similar surface densities on pure Al(2)O(3) supports.     

Structural correlations in BaO–PbO–B2O3 glasses as inferred from FTIR spectra

Infrared spectra of xBaO·(30-x)PbO·70B₂O₃, xBaO·(40-x)PbO·60B₂O₃ and xBaO·(50-x)PbO·50B₂O₃ glasses have been quantitatively analyzed. The fraction of four coordinated boron atoms varies linearly, for each group, between the values of the corresponding binary borate glasses. The data could be used to calculate and follow the composition dependence of the concentration of structural units in all glasses. The results show a linear increase in the ratio of PbO forming BO₄ units to the total content of PbO, with increasing B₂O₃ in binary PbO–B₂O₃ glasses. Similar behavior has been observed for the ratio of BaO forming BO₄ units to the total content of BaO in binary BaO–B₂O₃ glasses. The ratio of PbO forming PbO₄ units to the total PbO content, and that of BaO forming asymmetric BO₃ units to the total BaO content, shows a reversed dependence. The linear change in fraction of four coordinated boron atoms and in density and molar volume suggests that the studied glasses can be treated as mixtures of binary PbO–B₂O₃ and BaO–B₂O₃ matrices.     

Effect of Bi2O3 addition on electron paramagnetic resonance, optical absorption, and conductivity in vanadyl-doped Li2O-K2O-Bi2O3-B2O3 glasses

Glasses with composition 15Li(2)O-15K(2)O-xBi(2)O(3)-(65 - x)-B(2)O(3)/5V(2)O(5) (3 ≤ x ≤ 15) have been prepared by the conventional melt quench technique. The electron paramagnetic resonance spectra of VO(2+) in these glasses have been recorded in the X-band frequency (≈9.3 GHz) at room temperature. The spin Hamiltonian parameters and covalency rates were evaluated. It was found that the V(4+) ions exist as vanadyl (VO(2+)) ions and are in an octahedral coordination with a tetragonal compression. The covalency rates (1 - α(2)) and (1 - γ(2)) indicate moderate covalency for the σ- and π-bonds. It was observed that the spin-Hamiltonian parameters depend slightly on the relative concentration of Bi(2)O(3). The optical properties of this glass system are studied from the optical absorption spectra recorded in the wavelength range 200-800 nm. The fundamental absorption edge has been identified from the optical absorption spectra. The values of optical band gap for indirect allowed transitions have been determined using available theories. The direct current electrical conductivity, σ, has been measured in the temperature range 373-573 K. The conductivity decreases with the increase in Bi(2)O(3) concentration. This has been discussed in terms of the decrease in the number of mobile ions and their mobility. An attempt is made to correlate the EPR, optical, and electrical results and to find the effect of Bi(2)O(3) content on these parameters.     

Structural,thermal, and optical properties of Pr3+/Yb3+ co-doped oxyhalide tellurite glasses and its nano-crystalline parts

Nano-crystalized glass-ceramics and ceramics co-doped with Pr³⁺/Yb³⁺ have been successfully prepared by heat treatment of the precursor oxyhalide glass sample synthesized by melt-quench method. Thermal measurement of glass sample shows its glass transition temperature (T_g), and onset crystallization temperature (T_c). Formation of nano-crystals was verified by X-ray diffraction patterns and transmission electron microscopy (TEM) images. Values of optical band gap (E_opt) were found to be 2.85 eV and 1.54 eV for direct and indirect transitions, respectively. Up and downconversion emission spectra recorded with 976 and 532 nm laser wavelengths show emission from Pr³⁺ ions. Emission of Pr³⁺ enhances approximately thirty times when Yb³⁺ ion is added with the matrix. The Stark splitting and the intensity of different emission bands increase effectively when we approach to ceramic via glass-ceramic sample. Intense upconversion emission observed by naked eyes has been quantified in terms of standard chromaticity diagram (CIE). Power dependence study shows that the upconversion of NIR radiation to visible takes place mainly via photon avalanche (PA) process. Upconversion mechanisms have been discussed in terms of excited state absorption (ESA) and energy transfer (ET) processes.     

Structural, energetic, electronic, bonding, and vibrational properties of Ga3O, Ga3O2, Ga3O3, Ga2O3, and GaO3 clusters

The results of density functional theory based calculations on Ga3O, Ga3O2, Ga3O3, Ga2O3, and GaO3 clusters are reported here. A preference for planar arrangement of the constituent atoms maximizing the ionic interactions is found in the ground state of the clusters considered. The sequential oxidation of the metal-excess clusters increases the binding energy, but the sequential removal of a metal atom from the oxygen-excess clusters decreases the binding energy. The increase in the oxygen to metal ratio in these clusters is accompanied by increase in both electron affinity and ionization potential. The ionization induced structural distortions in the neutral clusters are relatively small, except those for Ga3O2. In anionic (cationic) clusters, the added (ionized) electron is shared by the Ga atoms, except in the case of GaO3. The vibrational frequencies and charge density analysis reveal the importance of the ionic Ga-O bond in stabilizing the gallium oxide clusters considered in this study.