A study on mixed conducting property of LixV2O5 (x = 0.4–1.4)

The mixed conducting nature of the lithium vanadate Li_xV₂O₅ (x = 0.4 – 1.4) prepared by solid state reaction has been analyzed by Wagner's polarization method. The increase of electronic conductivity with increase in lithium content has been observed. The low diffusion constant has been observed for lithium ionic motion in high lithium content samples. The conductivity spectra analysis shows the high columbic repulsive forces between mobile lithium ions in high lithium content samples. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Amorphous vanadium pentoxide

Structure and semiconducting properties of amorphous vanadium pentoxide obtained by splat cooling have been studied. Long-range order appears suddenly when the oxide is heated to about 200°C. The short-range order of the glass seems to be almost identical to that encountered in orthorhombic V₂O₅. ESR spectra are typical of localized electrons, they show that the electronic mobility is lower in the amorphous state. The hopping frequency of the unpaired 3d electron from a V⁴⁺ to a V⁵⁺ ion increases when long-range order is established. This phenomenon is related to the electrical resistivity which is about one order of magnitude higher in the glass.     

Charge transport in Li2B4O7 in single crystal and glassy states

The electrophysical properties of Li₂B₄O₇ single crystals and glasses are studied in the temperature range 290–1100 K and the frequency range 1–1000 kHz. The strong frequency dispersion of ionic conductivity at temperatures close to room temperature is explained by the low-frequency hoppings of lithium ions from the basic to the interstitial positions. The dielectric relaxation accompanying the transport of lithium ions is analyzed within the framework of the Debye-Skanavi model. The minimum hopping length of Li⁺ ions and the height of the potential barrier between their closest positions are determined. The parameters obtained are compared with the known structural data.     

The application of small polaron theory to transition metal oxide glasses

A review of the experimental data on a range of transition metal oxide glasses shows a strong relationship between activation energy and site spacing for glasses based on V₂O₅. The behaviour confirms that transport in amorphous V₂O₅ and vanadate glasses is by adiabatic hopping with an appreciable polaron bandwidth, while non-adiabatic hopping between isolated centres is apropriate for glasses based on most other transition metal oxides. The behaviour of vanadium is linked to the retention of a well-defined VO^5? structural unit in the glass network and to optimisation of the packing of this unit within the glass.     

Spectroscopy,oxidation-reduction behaviour and DC conductivity of a vanadium-containing barium aluminoborate glass

The vanadium(IV)-vanadium(V) equilibrium in a 37.5BaO, 5.0Al₂O₃, 57.5B₂O3 mol% + X mol% V₂O₅ (where X = 0.25?32.5) glass system has been studied as functions of temperature, partial pressure of oxygen and total vanadium concentration of the melt. The vanadium(V)/vanadium(IV) ratio in the melt increased with increasing partial pressure of oxygen, lowering temperature of melting, and with increasing total vanadium content of the melt. With X ? 10, the vanadium(V)/vanadium(IV) ratio became almost independent of the total vanadium content of the melt.With this knowledge of oxidation-reduction behaviour, a series of glasses containing 2.8?32.5 mol% V₂O₅ (at about 4 mol% intervals) and having a constant vanadium(IV)/vanadium(V) ratio (0.17) were prepared. Density, electronic absorption spectrum (both d-d and charge transfer transitions), and ESR of these glasses were measured. Optical and ESR spectra of these glasses indicated the vanadium(IV) to be present as vanadyl ion, VO²⁺; g| decreased monotonically with increasing vanadium content of these glasses, whereas gβ remained unchanged. The charge transfer transition energy due to vanadium(V) decreased, and the extinction coefficient increased by orders of magnitude with increasing vanadium content of the glass; the most striking changes occurred at X ≈ 10 mol%. DC conductivity of these glasses was measured at different temperatures; a plot of log (?/T) versus 1/T produced straight lines. The slope of these lines remained almost constant (39 ± 1 kcal/mol) for the glasses containing up to about 10 mol% V₂O₅; with further increase of V₂O₅ the slope decreased sharply.It has been concluded that the abrupt changes in properties like partial molar volume of V₂O₅, charge transfer spectrum of vanadium(V), activation energy of polaron hopping — all of which occurred around X ≈ 10 mol% — is due to a major change in the nature of vanadate groups rather than vanadium(IV) in these glasses.     

Majority charge carrier reversal and its effect on thermal and electron transport in xV2O5–(1 − x) As2O3 glasses

DC resistivity, thermopower and optical absorption of xV₂O₅–(1 ? x) As₂O₃ (0.58 ≤ x ≤ 0.93) glasses have been studied as a function of composition. The transport mechanism in these glasses has been identified to be a combination of hopping of small polarons between V⁴⁺ and V⁵⁺ sites and small bipolarons between As³⁺ and As⁵⁺ sites respectively. Electrical conductivity is found to be more of a function of vanadium content than arsenic concentration in the glasses, indicating that the contribution of bipolarons to the conductivity is negligible. Thermopower has also been found to be sensitive to the composition of the glasses. At low vanadium concentrations, the thermopower is negative, which exhibits a sign reversal as vanadium concentration is increased (at x = 0.7). An important feature of these glasses is that the thermopower is not a function of [V⁵⁺]/[V⁴⁺] ratio, as is normally observed in vanadate glasses, and such a phenomenon suggests that the arsenic ions (bipolarons) in these glasses contribute to the thermal transport phenomena in a significant way.     

DC conduvtivity of V2O5TeO2 glasses

The dc conductivity of semiconducting vanadium tellurite glasses of compositions in the range 50 to 80 mol% V₂O₅ has been measured in the temperature region 77 to 400 K. Measurements have been made on annealed samples at different annealing temperatures. Annealing the samples at temperature of about 250°C causes the appearance of a complex crystalline phase resulting in an increase of conductivity. Results are reported for amorphous samples of different compositions. The conductivity of tellurite glasses is slightly higher than the corresponding composition of phosphate glasses, but the general trend of the increase of conductivity and decrease of high temperature activation energy with increasing V₂O₅ content is similar in the two systems. The data have been analysed in the light of existing models of polaronic hopping conduction. A definite conclusion about the mechanics of conduction (adiabatic or nonadiabatic) is difficult in the absence of a precise knowledge of the characteristic phonon frequency v₀. Adiabatic hopping is indicated for v₀～10¹¹ Hz, however this value leads to unreasonably low value for the Debye temperature θ_D, and higher values for v₀～10¹³ hz satifiies the conditions for nonadiabatic hopping which appears to be the likely mechanism of conduction in V₂O₅TeO₂ glasses. The low temperature data (< 100 K) can be fitted to Mott's variable range hopping, which when combined with ac conductivity data gives reasonable values of α, but a high value for the disorder energy.     

Vanadyl ion stability in silica gels

Silica gels doped with vanadium were prepared from tetra ethyl orthosilicate and an aqueous solution of NH₄VO₃, at a final sol pH of 1.5–2.5. Absorption and electron paramagnetic resonance (EPR) spectral studies establish that under ambient conditions the incorporated pentavalent vanadium is stabilized as tetravalent vanadyl ion in the gel monoliths. Dried gels were very stable under ambient conditions, however, transformation of V⁴⁺ to V⁵⁺ was noted during gel densification.     

Growth and ionic conductivity of Li3 + x P1 − x GexO4 (x = 0.34) single crystals

Li_{3 + x} P_{1 ? x} Ge_xO₄ crystals (x = 0.34) with dimensions of about 3 × 3 × 5 mm³ were grown for the first time from flux. The conductivities of the crystals measured along three directions have close values and are equal to σ ≈ 1.8 × 10^?6 and 3.7 × 10^?2 Sm/cm at the temperatures 40 and 400°C, respectively (similar to the case of pure lithium phosphate, somewhat lower values of electric conductivity were measure along the b axis). The activation energy of conductivity is equal to 0.54 eV. A considerable increase in the conductivity of the solid solution in comparison with the conductivity of pure lithium phosphate is explained by the specific features of the lithium sublattice in the crystal structure of the λ-Li₃PO₄ type.     

On the Bindings in VOm Phases

The phenomenological plural correlations model permits to attribute to the intermediate phases of the mixture VO_M bonding types (bindings) which indicate a low energy of the empirically found structures. In the phases various electron subsystems exist which contain lattice-like spatial correlations, and when the corresponding cells are in energetically low commensurabilities (in harmonies) to the crystal and to one another then the phase becomes stable. A fundamental assumption of a binding analysis is the electron count being here V^1,4,8O^0,6,2. As a consequence the phases V₈O, V₁₆O₃, and V₇O₄ consist of a tetra-gonally deformed V(B1) partial structure with interstices partly filled by O atoms. The composition of a phase determines the electron concentrations in the subsystems, and these influence the harmony of the correlations in the binding (BF_U2 for V₁₆O₃, e.g.). In VO, V₁₃O₁₆, V₂O₃, and V₃O₅ an essentially complete close packed partial structure of O accepts V into its octahedral interstices. Once more harmonies of electron correlations determine favourable bindings (FF'2 for VO e.g.). The Magnéli phases V₄O₇ up to V₈O₁₅ being shear homeotypes of VO₂ · h may be considered as homeodesmic to TiO₂ · r, which is stabilized by a CF_U2 binding. From the binding it may be concluded how much Hund insertion is present in a phase. Hund insertion is for instance responsible for the transformation VO₂ · h → r. The phases V₆O₁₃, V₄O₉ and V₃O₇ are homeotypic to V₂O₅. This last phase permits presumably a UH_T3 binding. The results of the binding analysis of VO_M shed some new light on the interpretation of properties of the VO_M phases.     

Vanadium contribution in different glasses in view of the ligand field theory

Spectrophotometric and magnetic studies were carried out on different glasses containing known amounts of V₂O₅ in order to throw some light on the valence states and coordination of vanadium in such glasses (in view of the ligand field theory).The results obtained were found to be in agreement with those of aqueous solutions, e.g. [V(H₂O)₆]³⁺, [VO(H₂O)₅]²⁺, and VO²⁺ which showed a slight difference in band positions due to the distortion as well as the difference between the ligand field strength of glass and water. Generally it may be concluded that vanadium is present in borate and silicate glasses in three possible valencies, V⁵⁺, V⁴⁺ and V³⁺, while in phosphate glasses it is generally present in two valencies V⁴⁺ and V³⁺.     

The effect of pressure on fast ion conductivity in glasses

We have measured the effect of pressure and temperature on the ionic conductivity of glasses in the system B₂O₃/Li₂O(LiX)₂ (X = F, Cl, Br, I), where X_B2O3 ? 0.7 and X/O ratio was varied while maintaining a constant Li⁺ content of X_(LiX)2 + Li₂O ? 0.3. All glasses exhibited a very small pressure coefficient of conductivity, i.e. ΔV ? 1 cm³ mol^?1, which decreased very slightly as the Cl/O ratio increased or as Cl was replaced by Br at constant X/O ratio. The results were compared with the ELectrostatic Strain Energy theory; the results of this comparison lead to the conclusion that for Li⁺ conduction the electrostatic term is dominant in determining the activation energy for conduction.     

The role of vanadium valence states and coordination on electrical conduction in lithium iodide borate glasses mixed with small concentration of silver iodide

LiI–AgI–B₂O₃ glasses mixed with different concentrations of V₂O₅ (ranging from 0 to 1.0 mol%) were prepared. Electrical and dielectric properties over wide ranges of frequency (10^?2–10⁷ Hz) and temperature (173–523 K) have been studied. Additionally spectroscopic properties viz., optical absorption and ESR spectra have been investigated. The optical absorption and ESR studies have revealed that vanadium ions do exist in both V⁴⁺ and V⁵⁺ states and the redox ratio is the highest in the glasses containing 0.8 mol% of V₂O₅. The results of conductivity measurements have indicated that there is a mixed conduction (both ionic and electronic). The ionic conduction seems to be dominant over polaron hopping only in the glasses containing V₂O₅ more than 0.8 mol% of V₂O₅. The impedance spectra have also indicated that the conduction is predominantly polaronic in nature. The frequency and temperature dependence of the electrical moduli as well as dielectric loss parameters have exhibited relaxation character attributed to the vanadyl complexes. The relaxation effects have been analyzed by the graphical method and from this analysis it has been established that there is a spreading of relaxation times. The results have been further discussed quantitatively in the light of different valance states of vanadium ions with the aid of the data on spectroscopic properties.     

Crystal structure of Li2MgSiO4

The crystal structure of Li₂MgSiO₄ was established by single-crystal X-ray diffraction analysis. The crystals are monoclinic, a = 4.9924(7) Å, b = 10.681(2) Å, c = 6.2889(5) Å, β = 90.46(1)°, Z = 4, sp. gr. P2₁/n, V = 335.54 ų, R = 0.062. In a Li₂MgSiO₄ crystal, four types of independent T(1–4) tetrahedra share vertices to form a three-dimensional framework. Three of these tetrahedra are occupied simultaneously by Li and Mg cations, which corresponds to the crystallochemical formula (Li_0.98Mg_0.02)(Li_0.80Mg_0.20) · (Li_0.22Mg_0.78)SiO₄. In slightly distorted SiO₄ tetrahedra denoted as T(1), the average Si-O distance is 1.635(2) Å. The distortions of other tetrahedra and the average (Li _x Mg_{1 ? x} )-O distances increase with an increase in lithium content. These distances in the T(2), T(3), and T(4) tetrahedra are 1.955(2), 1.971(4), and 2.019(6) Å, respectively. The structure of the new compound is compared with the crystal structures of other Li₂ M ²⁺SiO₄ compounds and the luminescence spectra of Cr⁴⁺: Li₂MgSiO₄.     

Thin layers deposited from V2O5 gels: I. A conductivity study

The conductivity ? of V₂O₅ layers deposited from gels of various V⁴⁺ content C was measured over a very large temperature range. The observed variations agree with Mott predictions on polaron hopping. Indeed below ≈ 50 K we reached a new conductivity regime with a well-defined activation energy. The fit of the experimental data to the Schnakenberg model was excellent and thus we could determine the polaron energy W_p, the disorder energy W_D and the mean optical phonon frequency v₀. All these values decrease with increasing C. In contrast whatever C the coupling constant γ was ≈ 10. The variation of W_D with C agrees quantitatively with the values calculated by means of Miller and Abrahams' theory and this led us to the conclusion that in those amorphous V₂O₅ layers disorder is mainly a potential disorder related to non-stoichiometry.     

The cation distributions in the spinel phases of the mixed vanadium oxides LiZnx V2-xO4 and LiMgx V2-xO4

Cation distributions and values of the oxygen parameter in the spinel phases of mixed vanadium oxides with the general formula Li_xMeV_2–xO₄, where Me is Zn or Mg, respectively, were studied by X-ray diffraction measurements. The distribution of cations on the sublattices in the zinc series can be expressed by the formula Li_1–x–δ Zn_x V_δ [Li_x+δ V_2–x-δ] and for the magnesium series by the formula Li_1–δ V_δ [Li_δ Mg_x V_2–x-δ]. For the zinc series a general uniform increase of the oxygen parameter was determined. The substitution of magnesium ions leads to an anomal course of this parameter, which is possible to explain by substitution of magnesium ions on intersticial sites. The results are discussed using the values of the Madelung energy. The recently observed tetragonal deformation of the spinel lattice for the bulk composition Li Mg_0.375 V_1.625 O₄ is explained by a cooperative Jahn Teller effect of V⁴⁺ ions.     

Neutron and X-ray scattering studies of Li2O–TeO2–V2O5 glasses

Neutron and X-ray scattering studies of (Li₂O)_x–(TeO₂)₂–V₂O₅ glasses with x = 0, 1 and 2, obtained by melting and subsequent cooling in air to room temperature, have been performed. Reciprocal space data have been Fourier transformed into real space yielding the radial distribution functions. Nearest and next neighbor peaks have been analyzed using a least-squares fitting method which suggests the presence of both TeO₃ and TeO₄ coordination polyhedra, the fivefold coordination of lithium and mixture of 6, 5 and 4 coordination numbers of vanadium. These results are discussed in relation to the electronic and ionic conductivity properties.     

Some features of EPR and optical spectra of vanadium in aluminophosphate glasses

The EPR and optical spectra of vanadium in glasses of ternary Al₂O₃P₂O₅SiO₂ and Al₂O₃P₂O₅B₂O₃ systems have been measured. The results were compared with earlier data for vanadium in binary phosphate, aluminophosphate and silicaphosphate glasses and with results of de-Biasi for V⁴⁺ in crystalline powder α-crystobalite AlPO₄. The superpositions of two hyperfine spectra (ASB-I and ASB-II) were found for many glasses of ternary systems. Both spectra can be attributed to VO²⁺ ions. The intensity ratio of the ASB-II spectrum to ASB-I depends on glass composition but is great (> 7) for all the glasses. Only the ASB-II spectrum was observed in glasses with low concentration of Al₂O₃. The spectral parameters of ASB-II spectrum are g_| = 1.916–1.921; g_⊥ 1.980–1.988; A_| = (188?190) × 10^?4cm^?1 and A_⊥ = (74–77) × 10^?4cm^?1. Three intense bands at 370, 455 and 700 ans 720 nm observed in these glasses can be attributed to V³⁺ ions. The excellent agreement of the parameters of the EPR spectrum of V⁴⁺ ions in crystalline α-crystobalite AIPO₄ and ASB-II spectra in the glasses under study suggest the identical electron structure of the paramagnetic species. This species is believed to be characterized by optical bands at 680 and 790 nm which have been observed by de Biasi. The orbital mixing coefficients indicate strong tetragonal distortion of vanadyl complexes responsible for the ASB-II spectrum. It is assumed that VO²⁺ ions responsible for this spectrum act as modifiers fitting into the relatively small holes of the three-dimensional networks of phosphate glasses containing no cations of large radii. The microscopic basicity of oxygens in such holes must be about 0.48.     

EPR study of crystalline and vitreous forms of the Li2OAl2O3SiO2 system

The epr spectra of V⁴⁺ and radiation centres have been studied in β-eucryptite (LiAlSiO₄), β-, γ-spodumene (LiAlSi₂O₆) and in glasses prepared by the fusion of the single crystals. It is shown that the electronic structures of the vitreous state in the Li₂OAl₂O₃SiO₂ system and that of the crystalline forms differ considerably. The change of the electronic structure on crystallization is not direct, but is realized through the intermediate state whose electronic structure differs from that of glasses and crystals.     

Iron redox equilibrium, structure and properties of zinc iron phosphate glasses

Iron redox equilibrium, structure and properties were investigated for the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses melted at different temperatures. The structure and valence states of the iron ions in these glasses were investigated using Mössbauer spectroscopy, Raman spectroscopy and differential thermal analysis. Mössbauer spectroscopy indicated that the concentration of Fe²⁺ ions increased in the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glass with increasing melting temperature. The Fe²⁺/(Fe²⁺ + Fe³⁺) ratio increased from 0.18 to 0.38 as the melting temperature increased from 1100 to 1300 °C. The measured isomer shifts showed that both Fe²⁺ and Fe³⁺ ions are in octahedral coordination. It was shown that the dc conductivity strongly depended on Fe²⁺/(Fe²⁺ + Fe³⁺) ratio in glasses. The dc conductivity increases with the increasing Fe²⁺ ion content in these glasses. The conductivity arises from the polaron hopping between Fe²⁺ and Fe³⁺ ions which suggests that the conduction is electronic in nature in zinc iron phosphate glasses.