Characterization and Conduction Mechanism of Highly Conductive Vanadate Glass

This paper reviews recent studies of highly conductive barium iron vanadate glass with a composition of 20 BaO ∙ 10 Fe2O3 ∙ 70 V2O5 (in mol %). Isothermal annealing of the vanadate glass for several ten minutes at a given temperature, higher than glass transition temperature or crystallization temperature, caused an increase in σ. Substitution of CuI (3d10), ZnII (3d10) and CuII (3d9) for FeIII (3d5) was investigated to elucidate the effect of electron configuration on the conductivity (σ). A marked decrease in the activation energy of conduction (Ea) was also observed after the annealing. Values of Ea were correlated to the energy gap between the donor level and the conduction band (CB) in the n-type semiconductor model. Isothermal annealing of ZnII-substituted vanadate glass (20 BaO ∙ 5 ZnO ∙ 5 Fe2O3 ∙ 70 V2O5) at 450 °C for 30 min showed an increase in σ from 2.5 × 10–6 to 2.1 × 10–1 S cm–1, which was one order of magnitude larger than that of non-substituted vanadate glass (3.4 × 10–2 S cm–1). Under the same annealing condition, σ’s of 2.0 × 10–1 and 3.2 × 10–1 S cm–1 were observed for 20 BaO ∙ 5 Cu2O ∙ 5 Fe2O3 ∙ 70 V2O5 and 20 BaO ∙ 5 CuO ∙ 5 Fe2O3 ∙ 70 V2O5 glasses, respectively. These results demonstrate an increase in the carrier (electron) density in the CB, primarily composed of anti-bonding 4s-orbitals.


INTRODUCTION
ILICATE (SiO2-based) glass is generally known as an "insulator" having the resistivity (ρ) of the order of "terra" Ω cm.However, silicate glass, synthesized by using recycling fly ash (coal ash) containing ≥ 12 wt % Fe2O3 after isothermal annealing at 1100 °C for 1 h, showed ρ of "mega" Ω cm. [1,2]This iron-containing silicate glass had decrease in ρ or increase in conductivity (σ).A change in the property of fly-ash recycled glass was ascribed to the precipitation of magnetite (Fe3O4) particles in the matrix.Formation of Fe3O4 was due to the partial reduction of Fe2O3 caused by several weight percent of carbon atoms, which were originally presented in the fly ash. [1,2]he presence of Fe3O4 particles was confirmed from the Mössbauer spectra of fly-ash recycled glass. [1,2]The fraction of magnetite particles increased after a prolonged annealing at 1100 o C. [1,2] Fe3O4 are generally known as semiconductor and may have created a "pathway" of the electric conduction along the crystalline particles.Mössbauer spectra showed a decrease in δ of Fe III (doublet) from 0.32 -0.34 mm s -1 to 0.14 -0.15 mm s -1 after the annealing.This indicates an increased covalency of the distorted Fe III O4 tetrahedra occupying the network forming (NWF) sites in the glass matrix.In the as-quenched glass, another doublet of Fe II (δ = 1.02 -1.09 mm s -1 ) was also observed, which was assigned to octahedral species occupying network modifying (NWM) sites.After the annealing, Fe II occupied tetrahedral NWF sites, as revealed from the values of δ (0.81 -0.86 mm s -1 ).This suggests distorted Fe II O6 octahedra were transformed to distorted FeO II 4 tetrahedra during the annealing.
In the fly-ash recycled glass, increase in σ from the order of 10 -8 S cm -1 to 10 -6 S cm -1 was observed, which was ascribed to the promotion of electron hopping from Fe II to Fe III in the glass matrix.In other words, a "pathway" of the electron hopping was created in the glass-ceramics.Although the fly-ash recycled glass or glass-ceramic has S attracted much interest, it is noted that the value of ρ was at least of the order of "mega" Ω cm.This indicates limited industrial applications of glass-ceramic having conductivity (σ) ranging from "insulator" to "semiconductor".This leads to interest in as-quenched vanadate (V2O5-based) glass that originally has resistivity of "mega" Ω cm.
Small polaron hopping theory has been usually applied to describe the conduction mechanism of vanadate glass. [3]Isothermal annealing of alkali vanadate glass, 25 K2O • 10 Fe2O3 • 65 V2O5 (composition in mol %), was carried out at 380 °C for 10 min.This temperature was higher than the glass transition temperature (Tg) (217 °C) and also than the crystallization temperatures of 284 °C (Tc(1)) and 344 °C (Tc(2)).Isothermal annealing at 380 °C caused a substantial decrease in ρ from 1.6 × 10 7 Ω cm (σ = 6.3 × 10 -8 S cm -1 ) to 2.3 × 10 3 Ω cm (σ = 4.3 × 10 -4 S cm -1 ). [4,5]Nishida et al. suggested that the lowering of ρ by four orders of magnitude was due to an increased probability of the small polaron hopping from V IV (or V III ) to V V .The decrease in quadrupole splitting (Δ) of Mössbauer spectra measured at room temperature (RT) confirmed a decrease in the distortion of pseudo-1D network structure of 25 K2O • 10 Fe2O3 • 65 V2O5 glass. [4,5]Decrease of the distortion of network structure or "structural relaxation" became large after the annealing of several ten minutes.Prolonged isothermal annealing of 25 K2O • 10 Fe2O3 • 65 V2O5 glass for more than 150 min resulted in a gradual decrease of σ in conjunction with the precipitation of "insulating" KV3O8 particles. [4,5]A Kissinger plot of Tc in the differential thermal analysis (DTA) of x K2O • 10 Fe2O3 • (90 -x) V2O5 glasses (x = 20 -35) showed that the crystallization was triggered by the cleavage of Fe III -O bonds forming distorted FeO4 tetrahedra with the Fe III -O bond energy of 2.6 eV.These experimental results suggested that the crystallization of glass (precipitation of crystalline particles or phase in the glassy phase) was not related to the increase in σ, but to the structural relaxation.[8] It was also noted that the "pathway" of conduction was restricted to the pseudo-1D direction.
20 BaO • 10 Fe2O3 • x WO3 • (70 -x) V2O5 glasses (x = 10 -50) were prepared in order to enhance the waterand chemical-durability of 20 BaO • 10 Fe2O3 • 70 V2O5 glass. [16]XRD study of these glasses showed precipitation of crystalline particles like FeVO4, BaFe2O4, BaFe12O19 and α-Fe2O3, depending on the composition and the annealing temperature [17] These crystalline particles were not in volved with the marked increase of σ values, recorded in the highly conductive vanadate glass.A Kissinger plot in the DTA study for 20 BaO • 10 Fe2O3 • x WO3 • (70 -x) V2O5 glasses revealed that the crystallization was triggered by the cleavage of Fe III -O bonds having bond energy of 2.6 eV. [17]This was also reported for 25 K2O • 10 Fe2O3 • 65 V2O5 glass. [4]össbauer spectroscopy has become a very powerful tool for studying the local structure of oxide glass, since Mössbauer nuclides like 57 Fe and 119 Sn could be easily incorporated into oxide glasses as a probe.[1,2,[4][5][6][7][8][9][10][11][12][13][15][16][17][18][19][20][21][22][23][24][25] We could determine whether Fe III atoms occupied NWF or NWM sites in oxide glasses like silicates, borate, borosilicate, aluminates, tellurite and gallate glasses by plotting the Tg values (ca.180 -770 o C) against the Δ of Fe III (ca.] A slope of the straight line, obtained from the Tg vs. Δ plot, was 680 K (mm s -1 ) -1 when Fe III atoms occupied tetrahedral NWF sites.[18,19] However, the slope was 260 K (mm s -1 ) -1 when Fe III atoms occupied octahedral NWF sites, as in tungstate glass.[21] Significantly, the slope was only 35 K (mm s -1 ) -1 when Fe III atoms occupied NWM sites, as in phosphate glasses.[18,19] A calibration plot of Tg vs. Δ was very useful when we characterized unknown glass samples because the structural role of Fe III atoms could be easily known.For example, a Tg vs. Δ plot for 20 BaO • 10 Fe2O3 • x WO3 • (70 -x) V2O5 glasses (x = 10 -50) yielded a slope of 680 K (mm s -1 ) -1 , [16] reflecting that Fe III atoms occupied only tetrahedral NWF sites.It means that they substituted V IV or V V atoms, [18,19] not octahedral W VI sites observed in tungstate glass.[21] Isomer shift (δ) provides chemical information like oxidation number, coordination number and electronic structure of the Mössbauer atoms.Magnetic information is available from the internal magnetic field (Hint).The linewidth (Γ ) offers useful information on the uniformity of the structural units in the materials of interest, i.e., whether or not they are homogeneously distributed in the material or whether the material is crystalline or glassy.[18] Magnitude of Γ also gives information on the degree of crystallization of glass, i.e., estimation of the crystalline phase and particles precipitated in the glass matrix.
The present paper reviews highly conductive 20 BaO • x MmOn • (10 -x) Fe2O3 • 70 V2O5 glasses (M = Cu, Zn; x = 5 & 10), in which substitution of Cu II (3d 9 ), Cu I (3d 10 ) and Zn II (3d 10 ) for Fe III (3d 5 ) was investigated focusing on the jump of the conductivity.These highly conductive vanadate glasses were prepared by heat treatment at 450 °C, a given temperature sufficiently higher than the Tg and Tc.Conduction mechanism for ZnO-substituted vanadate glass was discussed in conjunction with CuO-and Cu2O-substituted vanadate glasses. [22]PERIMENTAL 20 BaO • x MmOn • (10 -x) Fe2O3 • 70 V2O5 glasses (M = Cu, Zn; x = 0, 5, 10) were prepared by a conventional meltquenching method with BaCO3, CuO, Cu2O, ZnO, Fe2O3 and V2O5 of guaranteed reagent grade.Each reagent mixture placed in a platinum crucible or an alumina crucible was melted at 1100 °C for 2 h in the air using an electric muffle furnace.Homogeneous glass samples of almost black color were prepared by quenching the melt with ice-cold water or with air.Annealing of as-quenched glass sample was also carried out in an electric furnace.DTA study was conducted at a heating rate of 10 K min -1 from RT to 600 °C under N2 atmosphere.Powder of α-Al2O3 was used as a reference of the temperature.Electrical conductivity (σ) was estimated by measuring the resistivity (ρ) of rectangular sample by a conventional DC-four probe method.A linear relationship was obtained by plotting the voltage against the electric current (e.g.0 -10 mA) that depended on the magnitude of ρ and the sample size.Mössbauer measurement was made by a constant acceleration method with a source of 57 Co(Rh).A foil of α-Fe was used as a reference of δ and also for calibrating the velocity scale of the spectrometer.A lowering of Tc from 373 °C to 344 °C was also seen in these glasses.In the case of 20 BaO  1. [22] It is noteworthy that 30 min-annealing caused an increase of σ from 3.9 × 10 -6 to 3.2 × 10 -1 S cm -1 (ρ = 3.These σ values are one order of magnitude higher than that of 20 BaO • 10 Fe2O3 • 70 V2O5 glass (○), which had an increase in σ from 1.9 × 10 -6 to 3.4 × 10 -2 S cm -1 (ρ = 29 Ω cm) under the same annealing condition (450 °C, 60 min).Figure 1 indicates that σ of CuO-and Cu2O-containing vanadate glasses finally converged to 2.8 × 10 -1 S cm -1 after the annealing at 450 °C.It is likely that Cu2O and CuO had equilibrium in the air.The gradual increase in σ from 2.0 × 10 -1 to 2.8 × 10 -1 S cm -1 observed for 20 BaO • 5 Cu2O • 5 Fe2O3 • 70 V2O5 glass (□) was ascribed to slow oxidation of Cu2O to CuO during the heat treatment in the air.As mentioned above, the increase of σ was observed in K2O-Fe2O3-V2O5, [4,5] BaO-Fe2O3-V2O5 [10][11][12][13] and BaO-Fe2O3-WO3-V2O5 glasses [16,17] after isothermal annealing at temperatures higher than Tg or Tc.It is noteworthy that σ values of CuO-and Cu2O-containing vanadate glasses were one order of magnitude higher than that of 20 BaO • 10 Fe2O3 • 70 V2O5 glass, in spite that they had essentially the same 3D-network structure as 20 BaO • 10 Fe2O3 • 70 V2O5 glass.Figure 1 evidently demonstrated that the electron configurations of 3d 9 (Cu II ) and 3d 10 (Cu I ) were involved in the high conductivity.

RESULTS AND DISCUSSION
Similar result was recently observed in 20 BaO • 5 ZnO • 5 Fe2O3 • 70 V2O5 glass (Figure 2), which shows that RT-conductivity (σ) increased from 2.5 × 10 -6 S cm -1 to 2.1 × 10 -1 S cm -1 after isothermal annealing at 450 °C for 30 min (•).The latter conductivity was larger than that of 20 BaO • 10 Fe2O3 • 70 V2O5 glass (3.4 × 10 -2 S cm -1 ).This result further provides the evidence that substitution of ZnO for Fe2O3 is quite effective in improving the RT-conductivity of BaO-Fe2O3-V2O5 glass.It appears that 3d 10 -configuration of Zn II contributes to the increase in the conductivity.] A marked decrease of Δ was observed in 20 BaO • 10 Fe2O3 • 70 V2O5 glass (left) from 0.70 to 0.55 mm s -1 after the annealing at 450 °C for 300 min.Figure 3 indicates that both CuO-and Cu2O-containing vanadate glasses showed a decrease of Δ by 0.12 -0.15 mm s -1 after annealing at 450 °C for only 30 min.Decrease of Δ was not so large in the case of 20 BaO • 5 CuO • 5 Fe2O3 • 70 V2O5 (middle) and 20 BaO • 5 Cu2O • 5 Fe2O3 • 70 V2O5 glasses (right) after the annealing for 300 min, probably because these glasses included Cu atoms in addition to Fe and V atoms.In this case, their complicated networks might be less effectively "relaxed" by the heat treatment.
All the Mössbauer spectra show a noticeable decrease in Δ values after the heat treatment, suggesting a decreased eqlat at the nuclear sites of 57 Fe, and hence a decreased distortion of FeO4 tetrahedra.This is also the case for distorted VO4 tetrahedra, since they are directly bonded to FeO4 tetrahedra through corner oxygen atoms.These Mössbauer results are in good agreement with the marked increase of σ, observed after 30 min-annealing (see Figure 1).
Figure 4 shows RT-Mössbauer spectra of 20 BaO•5 ZnO • 5 Fe2O3 • 70 V2O5 glass in which a decrease of Δ was observed from 0.68 mm s -1 to 0.61 and to 0.62 mm s -1 after isothermal annealing at 450 °C for 30 and 300 min, respectively.The experimental error of Δ is estimated to be ± 0.01 -0.02 mm s -1 .The decrease of Δ is ascribed to a decreased distortion of FeO4 and VO4 tetrahedra which are connected to each other through corner oxygen atoms to form distorted network structure.
Decrease of Δ value was observed when iron-containing vanadate glass was annealed for several ten minutes or more at temperatures higher than Tg or Tc. [4,5,10-13,15- 17,20,22,23] his behavior accompanied, without exception, significant increase or jump of the conductivity by several orders of magnitude, which primarily depended on the annealing temperature.It is interesting to correlate this behavior with the probability of the small polaron hopping, [3] which has successfully been utilized to understand the conduction mechanism of semiconducting vanadate glass with σ of ca. 10 -5 to 10 -7 S cm -1 .In the early study of 25 K2O • 10 Fe2O3 • 65 V2O5 glass annealed at 380 o C, as described above, enhancement of σ was observed from 6.3 × 10 -8 S cm -1 (ρ = 1.6 × 10 7 Ω cm) to 4.3 × 10 -4 S cm -1 (ρ = 2.3 kΩ cm), which was ascribed to the increased probability of the small polaron hopping in the less-distorted FeO4 and VO4 tetrahedra. [4,5]In such a case, 3d-orbitals of Fe III , V IV and V V atoms would be effectively overlapped with the 2p-orbitals of oxygen atoms.It was concluded that less-distorted FeO4 and VO4 tetrahedra were favorable for the small polaron hopping from V IV to V V and Fe III via oxygen atoms sharing VO4 and FeO4 tetrahedra.It was considered that the presence of Fe III atoms, which have symmetric electron configuration of 3d 5 , was favorable for the small polaron hopping from V IV to V V and Fe III via oxygen atoms. [4,5]All the Mössbauer results indicate that the structural relaxation of the glass network was well reflected as a decrease of Δ. [4-5,10-13,15- 17,20,22,23] The small polaron hopping will be promoted as the structural relaxation of the network goes on.
If the electrical conduction of highly conductive vanadate glass with σ higher than ca. 10 -4 S cm -1 were due to the structural relaxation of the network, increase in the σ value would be directly proportional to the decrease of Δ value.Mössbauer spectra revealed that the decrease of Δ was the most remarkable in the case of 20 BaO • 10 Fe2O3 • 70 V2O5 glass (Figure 3, left), while CuO-, Cu2O-(Figure 3, middle and right) and ZnO-containing vanadate glasses (Figure 4) showed smaller decreases.Nevertheless, σ of the latter group was one order of magnitude larger than that of the former (see Figures 1 and 2).6][17]20,22,23] This authors' group introduced n-type semiconductor model to the conduction mechanism in conjunction with the small polaron hopping theory, because the carrier of vanadate glass was the valence electrons originating from V IV atoms with 3d 1 -electron configuration. [12]In the case of 20 BaO • 10 Fe2O3 • 70 V2O5 glass, activation energy for the conduction (Ea) was calculated from the σ values obtained at different temperatures (T) between RT and 100 o C. [12] For the calculation of Ea, the small polaron hopping equation [3] was used instead of the Arrhenius equation;

exp( )
where T and k are the measuring temperature (K) and the Boltzmann constant, respectively.Conductivity (σ) of 20 BaO • 10 Fe2O3 • 70 V2O5 glass increased with an increasing temperature. [12]Natural logarithms of σT plotted against the reciprocal of the measuring temperature (T) showed a linear relationship with different slopes, which depended on the temperature and the time period of the isothermal annealing. [12]Activation energies (Ea) calculated from the slope of the ln σT vs. T -1 plot are shown in Figure 5, which indicates that Ea decreases when the annealing temperature is higher than Tc and when the annealing time is long.Figure 5 evidences that σ was linearly increased with decreasing value of Ea.The plot is composed of two components: one relationship obtained after isothermal annealing at 350 °C and the other obtained after the isothermal annealing conducted at temperatures of 350 -500 °C.The increase of σ was promoted by the annealing at higher temperatures probably because the glass network was significantly cleaved to induce recombination of the fragments for the "partial" crystallization or precipitation of crystalline particles.In the case of as-quenched 20 BaO • 10 Fe2O3 • 70 V2O5 glass with σ of 2.8 × 10 -6 S cm -1 , Ea was calculated as 0.38 eV, whereas it decreased to 0.13 eV (σ = 4.3 × 10 -2 S cm -1 ) after the annealing at 500 °C for 60 min. [12]s shown in Figure 1, 20 BaO • 5 CuO • 5 Fe2O3 • 70 V2O5 glass showed σ of 3.2 × 10 -1 S cm -1 after isothermal annealing at 450 °C for 30 min, while 20 BaO • 5 Cu2O • 5 Fe2O3 • 70 V2O5 glass had σ of 2.0 × 10 -1 S cm -1 .Both glasses showed identical σ value of 2.8 × 10 -1 S cm -1 after annealing at 450 °C for 300 min.It was noted that Ea's of copper-containing vanadate glasses annealed at 450 °C for 60 min were 0.09 -0.10 eV, [22] smaller than that of 20 BaO • 10 Fe2O3 • 70 V2O5 glass (0.13 eV) in which σ was reported to be 3.4 × 10 -2 S cm -1 after the same annealing.Conductivities of Li +and Na + -containing iron vanadate glasses, measured after the isothermal annealing at 400 -450 °C for 100 min, were respectively estimated to be 1.0 × 10 -1 and 2.6 × 10 -2 S cm -1 , together with the Ea's of 0.10 and 0.13 eV. [15]Alkaline iron tungsten vanadate glasses showed σ values of 6.2 × 10 -4 -4.5 × 10 -3 S cm -1 and Ea's of 0.15 -0.21 eV. [20]Barium iron manganese vanadate glasses exhibited σ of 1.4 × 10 -2 S cm -1 after annealing at 500 °C for 1000 min, and the Ea was calculated as 0.11 eV. [23]These results suggest that the conductivity of highly conductive vanadate glasses, prepared by the annealing at temperatures higher than Tg, favorably at temperatures higher than Tc, is closely related to the magnitude of Ea.
These Ea values are significantly smaller than the band gap (ΔE) between the valence band (VB) and the conduction band (CB), reported for typical semiconductors like  [12] GaSb (0.23 eV), Ge (0.68 eV) and Si (1.16 eV). [12]A photoluminescence study of 20 BaO • 10 Fe2O3 • 70 V2O5 glass annealed at 500 °C for 60 min showed the ΔE of 2.25 eV.ΔE of as-quenched vanadate glass was too large to be estimated.Figure 6 is a schematic view of 20 BaO • 10 Fe2O3 • 70 V2O5 glass in which Ea was correlated to the energy difference between the donor level and the CB, since it was much smaller than the band gap (ΔE).As described above, a decrease of Ea was recorded from 0.38 to 0.13 eV in 20 BaO • 10 Fe2O3 • 70 V2O5 glass after the isothermal annealing at 500 °C for 60 min. [12]Carrier (electron) density in the CB would increase along with a decrease of the energy gap between the donor level and the CB, hence giving high possibility of producing highly conductive vanadate glass.
Electron configurations of 3d 10 and 3d 9 in Zn II , Cu I and Cu II cause the valence electrons to occupy the CB, which is primarily composed of anti-bonding 4s-orbitals.This will result in an increase in the carrier density of the CB and hence an increase in the electrical conductivity.In such a case, shielding of nuclear charge by nine or ten 3d-electrons will be effective for the "smooth" carrier (electron) flow in the CB.Figures 1 and 2 indicate that the number of 3d-electrons was closely related to the highly conductive vanadate glass.If the 3d-orbitals were almost or fully occupied, valence electrons originating from V IV (3d 1 ) would occupy the CB primarily composed of anti-bonding 4s-orbitals.Figures 1  and 2 indicate that the transition metal oxides having 3d 10and 3d 9 -configurations are favorable for the fabrication of highly conductive vanadate glass.Substitution of "3d-block elements" having the electron configurations of 3d 10 (Cu I , Zn II ) and 3d 9 (Cu II ) for 3d 5 (Fe III ) is very effective, as 20 BaO • 5 ZnO • 5 Fe2O3 • 70 V2O5 glass annealed at 450 °C for 30 min proved a marked increase of σ from 2.5 × 10 -6 S cm -1 to 2.1 × 10 -1 S cm -1 , one order of magnitude larger than that of 20 BaO • 10 Fe2O3 • 70 V2O5 glass (3.4 × 10 -2 S cm -1 ).This was also the case for 20 BaO • 5 CuO • 5 Fe2O3 • 70 V2O5 and 20 BaO • 5 Cu2O • 5 Fe2O3 • 70 V2O5 glasses of which σ values increased from 3.9 × 10 -6 S cm -1 to 3.2 × 10 -1 S cm -1 and from 5.1 × 10 -6 S cm -1 to 2.0 × 10 -1 S cm -1 , respectively, by the isothermal annealing at 450 °C for 30 min. [22]All the experimental results for several vanadate glasses [4-5,10-13,15- 17,20,22,23] demonstrated that decrease of Ea accompanied an increase of carrier (electron) density in the CB, and the formation of highly conductive vanadate glasses.In other words, n-type semiconductor model is predominant over the small polaron hopping, which has been generally utilized to carry out the conduction mechanism of "semiconducting" vanadate glass with resistivity of "mega" Ω cm.
Silver-or lithium-ion containing iron vanadate glass exhibited ionic conduction due to mobile Ag + and Li + ions [15,[24][25][26][27][28]  glasses revealed that the structural change caused by Ag2O content was essentially the same as that by alkali ions in alkali vanadate glasses.Structural change was confirmed from 2Dto pseudo-1D with an increasing Ag2O content when the Ag2O/V2O5 ratio was less than unity, while the change occurred from pseudo-1D-to 3D when the ratio exceeded unity. [24]Electrical conductivity of "superionic conducting" AgI-containing vanadate glass showed a concordant change with the change of Δ values for Fe III in the Mössbauer spectra and with the Tg values, determined by DTA.][20][21] The electrical conduction of superionic conducting AgI-containing vanadate glass originated from the migration of mobile Ag + ions surrounded by I -ions, whereas Ag + ions originating from Ag2O played a role NWM, similar to several alkali and alkaline-earth ions.Low-temperature Mössbauer study of 5 Ag2O • 10 Fe2O3 • 85 V2O5 and 45 Ag2O • 10 Fe2O3 • 45 V2O5 glasses [25] was consistent with the structural change of the glass matrix described above.
In studying x AgI • (75 -x) Ag2O • Fe2O3 • 24 V2O5 and x LiI • (38 -x) Li2O • 6 Fe2O3 • 56 V2O5 glasses, Ikeda et al. calculated the fractions of ionic conduction and electronic conduction by combining the results of AC and DC measurements. [26]In the Mössbauer study of alkali iron vanadate glasses, x R2O • 10 Fe2O3 • (90 -x) V2O5 glasses (R = Li, Na, K; x = 20, 40), cationic conduction was effective in increasing the conductivity. [15]Li + -containing conductive vanadate glass, LiFeVPOx, was developed as a cathode active material for lithium ion battery (LIB). [12,27,28]LiFeVPOx glass annealed at 450 °C for 60 min showed an increase of σ from 1.3 × 10 -6 to 1.0 × 10 -3 S cm -1 , together with a marked increase of the specific discharge-and charge-capacities from 50 mAhg -1 to 150 mAhg -1 . [27]Mössbauer study of LiFeVPOx glass showed a decrease of Δ for Fe III from 0.99 mm s -1 to 0.50 mm s -1 , reflecting the structural relaxation of the network composed of distorted FeO4 and VO4 tetrahedra.These experimental results suggested that the structural relaxation, caused by the heat treatment, was effective even when the ionic conduction coexisted with the electronic conduction.
Highly conductive vanadate glasses have a lot of industrial applications, such as cathode active material for LIB, solid state electrolyte, sensor, electron-emitting needle, static electricity protecting material, conductive glass paste and the hyperfine processing material combined with FIB, electrons and laser.For these applications, vanadate glass is a very suitable material because of its flexibility in "tuning" the conductivity by the heat treatment.The authors wish this functional material could contribute to the development of science and technology all over the world.

SUMMARY
1. Electrical conductivity of conductive vanadate glass is promoted by the structural relaxation of the network composed of the structural units like VO4, FeO4 and VO5 units, causing an increased probability of the small polaron hopping.
2. In the case of highly conductive vanadate glasses with σ higher than 10 -4 S cm -1 , n-type semiconductor model becomes predominant over the small polaron hopping.
3. Heat treatment of vanadate glasses at temperatures higher than Tg or Tc resulted in a systematic decrease in the Ea for the conduction, which could be correlated to the energy gap between the donor level and the CB, mainly composed of anti-bonding 4s-orbitals.
4. Electron configurations of 3d 10 and 3d 9 are favorable for the highly conductive vanadate glass because the carriers (electrons) necessarily have to occupy the 4sorbitals which form isotropic chemical bonds.

5.
Valence electrons in the fully or almost fully occupied 3d-orbitals could effectively shield the nuclear charge.This might be effective for the smooth carrier flow in the CB.

Figure 3 illustrates
Figure 3 illustrates RT-Mössbauer spectra of 20 BaO • 10 Fe2O3 • 70 V2O5 (left), 20 BaO • 5 CuO • 5 Fe2O3 • 70 V2O5 (middle) and 20 BaO • 5 Cu2O • 5 Fe2O3 • 70 V2O5 glasses (right), measured before and after isothermal annealing at 450 o C.[22] A marked decrease of Δ was observed in 20 BaO • 10 Fe2O3 • 70 V2O5 glass (left) from 0.70 to 0.55 mm s -1 after the annealing at 450 °C for 300 min.Figure3indicates that both CuO-and Cu2O-containing vanadate glasses showed a decrease of Δ by 0.12 -0.15 mm s -1 after annealing at 450 °C for only 30 min.Decrease of Δ was not so large in the case of 20 BaO • 5 CuO • 5 Fe2O3 • 70 V2O5 (middle) and 20 BaO • 5 Cu2O • 5 Fe2O3 • 70 V2O5 glasses (right) after the annealing for 300 min, probably because these glasses included Cu atoms in addition to Fe and V atoms.In this case, their complicated networks might be less effectively "relaxed" by the heat treatment.All the Mössbauer spectra show a noticeable decrease in Δ values after the heat treatment, suggesting a decreased eqlat at the nuclear sites of 57 Fe, and hence a decreased distortion of FeO4 tetrahedra.This is also the case for distorted VO4 tetrahedra, since they are directly bonded to FeO4 tetrahedra through corner oxygen atoms.These Mössbauer results are in good agreement with the marked increase of σ, observed after 30 min-annealing (see Figure1).Figure4shows RT-Mössbauer spectra of 20 BaO•5 ZnO • 5 Fe2O3 • 70 V2O5 glass in which a decrease of Δ was observed from 0.68 mm s -1 to 0.61 and to 0.62 mm s -1 after isothermal annealing at 450 °C for 30 and 300 min, respectively.The experimental error of Δ is estimated to be ± 0.01 -0.02 mm s -1 .The decrease of Δ is ascribed to a decreased distortion of FeO4 and VO4 tetrahedra which are connected to each other through corner oxygen atoms to form distorted network structure.Decrease of Δ value was observed when iron-containing vanadate glass was annealed for several ten minutes or more at temperatures higher than Tg or Tc.[4,5,10-13,15-

Figure 6 .
Figure 6.Schematic image of the energy level for 20 BaO • 10 Fe2O3 • 70 V2O5 glass before and after the isothermal annealing conducted at 500 °C for 60 min.