Highly conductive barium iron vanadate glass containing different metal oxides

Introduction

Highly conductive vanadate glasses have a lot of industrial applications such as cathode active material for LIB, solid state electrolyte, electric discharge needle, static electricity protecting material, conductive glass paste, hyperfine processing material with laser or FIB, etc. Some conductive vanadate glasses were reviewed in our previous paper [1], together with the results for semiconducting iron silicate glass prepared by recycling a mixture of fly ash (coal ash) and Fe₂O₃. Small polaron hopping theory has so far been applied to explain the conduction mechanism of semiconducting vanadate glass with a resistivity (ρ) of mega order (MΩcm) [2]. Highly conductive vanadate glass will be utilized as advanced materials because its conductivity (σ) is easily “tunable” in association with the heat treatment at a given temperature higher than glass transition temperature (T_g) or crystallization peak temperature (T_c) determined by differential thermal analysis (DTA) [1], [3], [4], [5], [6], [7], [8].

Isothermal annealing of potassium iron vanadate glass, 25K₂O·10Fe₂O₃·65V₂O₅ (composition in mol%), at 380°C for only 10 min caused a substantial decrease in ρ from 1.6×10⁷ Ωcm (σ=6.3×10⁻⁸Scm⁻¹) to 2.3 kΩcm (4.3×10⁻⁴Scm⁻¹) [3], [4]. The annealing was conducted at 340–380°C which was higher than T_g of 217°C, T_c(1) of 284°C and T_c(2) of 344°C. After the heat treatment at 340°C for 10 min, RT-Mössbauer spectra showed a marked decrease in quadrupole splitting (Δ) of Fe^III from 0.61 to 0.55 mms⁻¹, which reflected an increased symmetry or a decreased distortion of “distorted” FeO₄ and VO₄ tetrahedra that were involved with an increased structural relaxation of pseudo-1D network [3], [4]. Nishida et al. concluded that the lowering of ρ in potassium iron vanadate glass, amounting to several orders of magnitude, was ascribed to an increased probability of the small polaron hopping from V^IVto V^V. In case of potassium iron vanadate glass, 25K₂O·10Fe₂O₃·65V₂O₅, an isothermal annealing at 380°C for more than 150 min resulted in an increase in ρ due to the precipitation of “insulating” KV₃O₈ particles. Finally, the ρ became comparable to that of the as-cast potassium iron vanadate glass [3], [4], indicating that the structural relaxation of the network was involved with the high conductivity, as confirmed by a decrease in Δ of Fe^III in the Mössbauer spectra.

In case of 15BaO·15Fe₂O₃·70V₂O₅ glass (in mol%), further decrease of ρ was observed after heat treatment at temperatures close to T_c(1) of 372°C and T_c(2) of 468°C [5]. In this glass, ρ decreased from 1.0×10⁷ Ωcm (before annealing) to 250 Ωcm (4.0×10⁻³ Scm⁻¹) and 25 Ωcm (4.0×10⁻²Scm⁻¹) after the annealing at 370°C for 120 min and at 460°C for 30 min, respectively. It is noted that these ρ values are one- or two-orders of magnitude smaller than that of crystalline V₂O₅ [9]. It was reported that 15BaO·15Fe₂O₃·70V₂O₅ glass has a 3D-network structure in which several “pathways” or “route” were available for the small polaron hopping than in pseudo 1D-network of 25K₂O·10Fe₂O₃·65V₂O₅ glass [3], [4]. After the heat treatment of 15BaO·15Fe₂O₃·70V₂O₅ glass at 460°C for 60 min, Δ of Fe^III decreased from 0.67 to 0.60 mms⁻¹, reflecting decreased distortion of “distorted” FeO₄ and VO₄ tetrahedra or an increased structural relaxation of the 3D-network [5]. Representative Δ values for highly conductive BaO–Fe₂O₃–V₂O₅ glasses are summarized in Table 1.

Heat treatment of 20BaO·10Fe₂O₃·70V₂O₅ glass for 60 min at 500°C, which was higher than T_g of 312°C, T_c(1) of 376°C and T_c(2) of 468°C, resulted in a marked decrease in ρ from 3.6×10⁵ Ωcm (2.8×10⁻⁶Scm⁻¹) to 23 Ωcm (4.3×10⁻² Scm⁻¹) [6], [7]. Mössbauer spectra showed a remarkable decrease in Δ of Fe^III from 0.68 to 0.50 (±0.02) mms⁻¹ after the heat treatment. Heat treatment of this glass at 500°C for 1000 min caused a marked decrease in ρ from 6.3×10⁴ Ωcm (1.6×10⁻⁵Scm⁻¹) to 0.91 Ωcm (1.1 Scm⁻¹) [8]. X-ray diffraction (XRD) study of the glass ceramic prepared by annealing at 450°C for 2000 min showed the presence of small amounts of “insulating or semiconducting” BaFe₂O₄ and BaV₂O₆ particles. Crystal growth became maximal when annealed at temperatures around T_c(1) and T_c(2) [6], [7], at which the formation of BaFe₂O₄ and BaV₂O₆ particles were respectively detected, since the Fe^III–O bond energy was smaller than V^V–O bond energy of 3.9–4.9 eV [14], [15], and hence the precipitation of BaFe₂O₄ preceded that of BaV₂O₆. After the annealing at 500°C for 1000 min, precipitation of “semiconducting” FeVO₄ particles which intrinsically had ρ of 1.5×10⁶ Ωcm (σ=6.7×10⁻⁷ Scm⁻¹) was confirmed in “non-substituted” 20BaO·10Fe₂O₃·70V₂O₅ glass and in xR₂O·10Fe₂O₃·(90−x)V₂O₅ glasses (R=Li, Na, K; x=20, 40) [16]. These results proved that the remarkable decrease of ρ in several vanadate glasses caused by the annealing was not ascribed to the crystalline particles precipitated in the glass matrix, but to the structural relaxation of the 3D-network [1], [3], [4], [5], [6], [7], [8], [10], [11], [12], [13], [16], [17].

Change of the resistivity caused by a heat treatment of barium iron vanadate glasses, in which Fe₂O₃ was partially replaced by different metal oxides, are summarized in Table 1. Heat treatment of 20BaO·10Fe₂O₃·20WO₃·50V₂O₅ glass at 500°C for 240 min caused a decrease of ρ from 2.6×10⁵ Ωcm (3.9×10⁻⁶ Scm⁻¹) to 480 Ωcm (2.1×10⁻³ Scm⁻¹) in conjunction with a decrease of Δ from 0.82 to 0.76 mms⁻¹ [10]. In case of 20BaO·10Fe₂O₃·10WO₃·60V₂O₅ glass [11], an isothermal annealing at 500°C for 1000 min caused a decrease of ρ from 5.9×10⁴ Ωcm (1.7×10⁻⁵Scm⁻¹) to 10 Ωcm (1.0×10⁻¹ Scm⁻¹), simultaneously with a decrease in Δ of Fe^III from 0.73 to 0.59 mms⁻¹. XRD study of 20BaO·10Fe₂O₃·xWO₃·(70−x)V₂O₅ glasses revealed a precipitation of crystalline particles such as FeVO₄, BaFe₂O₄, BaFe₁₂O₁₉ and α-Fe₂O₃ [11]. It was concluded that these crystalline particles were not involved with the marked decrease of ρ, since the intrinsic resistivity of these compounds was much higher than that of WO₃-substituted vanadate glasses evaluated at RT after the annealing at 500°C. It proved that cleavage of Fe^III–O bonds triggered the crystallization of WO₃-substituted glasses [11], as observed in 25K₂O·10Fe₂O₃·65V₂O₅ glass [3]. Heat treatment of MnO₂-substituted vanadate glass, 20BaO·10Fe₂O₃·10MnO₂·60V₂O₅, at 500°C for 1000 min resulted in a decrease in ρ from 2.2×10⁶ Ωcm (4.5×10⁻⁷Scm⁻¹) to 71 Ωcm (1.4×10⁻² Scm⁻¹) in conjunction with a decrease in Δ of Fe^III from 0.76 to 0.49 mms⁻¹ which evidently showed the structural relaxation [12] (see Table 1).

20BaO·5CuO·5Fe₂O₃·70V₂O₅ and 20BaO·5Cu₂O·5Fe₂O₃·70V₂O₅ glasses showed a higher conductivity after the heat treatment at 450°C for 30 min [13]. Resistivity (ρ) of the former decreased from 2.6×10⁵ Ωcm (3.9×10⁻⁶ Scm⁻¹) to only 3.1 Ωcm (3.2×10⁻¹ Scm⁻¹) after the annealing, and the latter glass showed a comparable decrease of ρ from 2.0×10⁵ Ωcm (5.1×10⁻⁶ Scm⁻¹) to 5.0 Ωcm (2.0×10⁻¹Scm⁻¹) [13]. In 5 mol% CuO-substituted glass, Δ of Fe^III decreased from 0.66 to 0.54 mms⁻¹. In case of 5 mol% Cu₂O-substituted glass, Δ decreased from 0.69 to 0.54 mms⁻¹ (see Table 1). It is noted that Cu^II and Cu^I in CuO-and Cu₂O-substituted glasses respectively have the electron configuration of 3d⁹ and 3d¹⁰ in the outer-most orbital.

Heat treatment of 20BaO·5ZnO·5Fe₂O₃·70V₂O₅ glass at 450°C for 30 min, in which Zn^II has an electron configuration of 3d¹⁰ in the outer-most orbital, showed a marked decrease in ρ from 4.0×10⁵ Ωcm (2.5×10⁻⁶ Scm⁻¹) to 4.8 Ωcm (2.1×10⁻¹ Scm⁻¹), simultaneously with a decrease in Δ from 0.68 to 0.61 mms⁻¹ [1]. It is noted that elevation of the conductivity was most remarkable in case of Cu₂O- [13], CuO- [13] and ZnO-substituted glasses [1], and that an electron configuration of 3d¹⁰ or 3d⁹ was preferable to attain the high conductivity. The present study was carried out in order to verify this idea in Ga₂O₃- and GeO₂-substituted vanadate glasses, since Ga^III and Ge^IV of “p-block” elements also have the electron configuration of 3d¹⁰ in the outer-most orbital.

Mössbauer spectroscopy has successfully been utilized as a powerful tool for the local structural study of oxide glasses. Mössbauer nuclides like ⁵⁷Fe and ¹¹⁹Sn could play an effective role as a probe for the local structural study [1], [3], [4], [5], [6], [7], [8], [10], [11], [12], [13], [16], [17], [18], [19], [20], [21]. A “T_g-vs-Δ plot”, applied to several inorganic glasses like silicates, borate, borosilicate, aluminates, tellurite and gallate glasses, is very effective to determine the structural role of Fe^III in these glasses, i.e. “substitutional” sites as the network former (NWF) or “interstitial” sites as the network modifier (NWM) [4], [8], [11], [12], [17], [18], [21]. When T_g’s of several oxide glasses, ranging from 180 to 770°C, were plotted against the Δ of Fe^III (0.4–1.3 mms⁻¹), one “common” straight line with an identical slope of 680 K(mms⁻¹)⁻¹ was drawn when Fe^III atoms occupied substitutional “tetrahedral” NWF sites [4], [8], [11], [12], [17], [18]. For example, “T_g-vs-Δ plot” applied to xBaO·10Fe₂O₃·(90−x)V₂O₅ glasses (x=20–40) and 20BaO·10Fe₂O₃·xWO₃·(70-x)V₂O₅ glasses (x=10–50) yielded comparable slopes of 650 [8] and 680 K(mms⁻¹)⁻¹ [11], respectively. These results proved that Fe^IIIatoms occupied substitutional “tetrahedral” sites of V^IV or V^V as NWF. Essentially the same results were obtained in 20BaO·10Fe₂O₃·xMnO₂·(70−x)V₂O₅ glasses (x=0–30) which yielded a slope of 707 K(mms⁻¹)⁻¹ [12], and in 20R₂O·10Fe₂O₃·xWO₃·(70−x)V₂O₅ glasses (R=Na, K; x=0–50) with a slope of 670 or 680 K(mms⁻¹)⁻¹[17]. In contrast, Fe^III atoms in xNa₂O·Fe₂O₃·(99−x)WO₃ glasses (30≤x≤42) occupied substitutional “octahedral” sites of W^VI as NWF, in which a small slope of 260 K(mms⁻¹)⁻¹ was obtained because of its longer bond length with a weaker bond energy [21].

Distortion or local symmetry at the Mössbauer nuclear sites could be deduced from the Δ of Fe^III. Mössbauer spectra of heat-treated vanadate glasses containing Fe^III showed a marked decrease in Δ, reflecting a decreased distortion or an increased symmetry of “distorted” Fe^IIIO₄ tetrahedra [1], [3], [4], [5], [6], [7], [8], [10], [11], [12], [13], [16], [17], [18], [19], [20]. This is also the case for “distorted” VO₄ tetrahedra in the 3D-network, since one FeO₄ unit is randomly surrounded by three or four VO₄ units by sharing corner oxygen atoms. It is noted that the electric field gradient (EFG) caused by the valence electrons (eq_val) of high-spin Fe^III is “zero” since it has an isotropic electron configuration of 3d⁵ in the outer-most orbital. In this case, Δ of Fe^III reflects the EFG brought about by the lattice (eq_lat), i.e. by the steric configuration of oxygen atoms constituting the distorted FeO₄ and VO₄ tetrahedra.

Mössbauer study of heat-treated vanadate glasses indicated that a remarkable decrease of ρ was observed in conjunction with a decrease of Δ, which reflected a decreased distortion of Fe^IIIO₄ tetrahedra and VO₄ tetrahedra [1], [3], [4], [5], [6], [7], [8], [12], [13], [14], [15], [16], [17]. It was also considered that Fe^III atoms having an isotropic symmetric electron configuration of 3d⁵ in the outermost orbital was preferable for the remarkable decrease of ρ since half-occupied 3d-orbitals could effectively accept the carriers (polaron) in the remaining unoccupied 3d-orbitals [5], [6], [7], [8].

Experimental

Homogeneous glass samples with compositions of 20BaO·xGa₂O₃·(10-x)Fe₂O₃·70V₂O₅ and 20BaO·xGeO₂·(10-x)Fe₂O₃·70V₂O₅ (in mol%) were prepared by a melt-quenching method with weighed amounts of BaCO₃, Ga₂O₃, GeO₂, Fe₂O₃ and V₂O₅ of guaranteed reagent grade. Each reagent mixture placed in an alumina crucible was melted in an electric muffle furnace at 1300°C for 2.5 h in Ga₂O₃-substituted glass, and at 1100°C for 2 h in GeO₂-substituted glass. Glass samples of almost black color were prepared by quenching the melt in the air. Annealing of as-quenched glass sample was carried out at a given temperature using another electric furnace. For determining T_g and T_c values, DTA was conducted at a heating rate of 10 Kmin⁻¹ ranging from RT to 600°C in an N₂ atmosphere. A fixed amount of α-Al₂O₃ powder was used as a reference of the temperature. Electrical resistivity (ρ) of a rectangular glass sample was determined by a conventional dc-four probe method, in which a linear relationship was obtained by plotting the voltage against the electric current applied by the electrometer. Mössbauer measurement was conducted at RT by a constant acceleration method with a source of ⁵⁷Co(Rh). Small amount of enriched isotope, ⁵⁷Fe₂O₃, was used for the sample preparation of Mössbauer measurement. A foil of α-Fe was used as a reference of δ and for calibrating the velocity scale of the spectrometer. A software of Mösswinn 3.0i XP was used for the peak analysis of the Mössbauer spectrum.

Results

20BaO·xGa₂O₃·(10−x)Fe₂O₃·70V₂O₅ glass

DTA of Ga₂O₃-substituted vanadate glass showed a gradual lowering of T_g from 351°C (x=0) to 328 (x=5) and 324°C (x=10), reflecting lowered heat resistivity. Nishida et al. reported that gallate (Ga₂O₃-based) glass had higher heat resistivity [4], [18], [19], [20]. When the coordination number (CN) of Ga^III was four, Ga^III–O bond was reported to have a single bond energy of 2.9 eV [14], [15], which was larger than the Fe^III–O bond energy of 2.6 eV [21], [22]. A gradual lowering of T_g observed in the present study will be ascribed to the structural role of Ga^III that plays a role of NWM like Ba²⁺ at the “interstitial” sites of 3D-network of vanadate glass. In such case, six-fold coordinated Ga^III atoms have smaller single bond energy of 2.0 eV [14], [15]. Substitution of Ga₂O₃ for Fe₂O₃ will cause a lowering of T_g, since the increased fraction of NWM from 20 mol% BaO to 25 mol% (BaO+Ga₂O₃) is equivalent to a decreased degree of polymerization.

DTA study of Ga₂O₃-substituted vanadate glass showed T_c values of 410, 422 and 405°C when the Ga₂O₃ contents were 0, 5 and 10 mol%, respectively. Crystallization of oxide glass is closely related to the bond energy of NWF-oxygen polyhedra [3], [21], [22], [23], [24]. For example, crystallization of Fe^III-containing aluminate glass [22] was triggered by a cleavage of Fe^III–O bonds, since the Al–O bond energy (3.4–4.4 eV [14], [15]) was much larger than the Fe–O bond energy of 2.6 eV [21], [22]. The same conclusion was drawn in vanadate [3] and tungstate glasses [21]. In the crystallization study of IR-transmitting gallate glass in combination with the heat treatment, Ar⁺-laser and ⁶⁰Co-γ ray irradiations, FT-IR spectroscopy was effectively used to explore the kinetics and mechanism of crystallization [19], [23], [24]. Irradiation of Ga₂O₃-substituted vanadate glass with Ar⁺-laser and ⁶⁰Co-γ rays will also show the “memory effect” as observed in the IR-transmitting gallate glass described above.

DTA study of 20BaO·5CuO·5Fe₂O₃·70V₂O₅ and 20BaO·5Cu₂O·5Fe₂O₃·70V₂O₅ glasses showed a small decrease in T_g along with an increasing CuO or Cu₂O content, together with a lowering of T_c amounting to ca. 30 K [13]. Similar lowering of T_g and T_c amounting to ca. 30 K was observed in 20BaO·5ZnO·5Fe₂O₃·70V₂O₅ glass [1]. In case of 20BaO·10ZnO·70V₂O₅ glass, a decrease in T_g amounted to ca. 50 K and a decrease in T_c to ca. 30 K [1]. These DTA results proved that substitution of CuO, Cu₂O and ZnO for Fe₂O₃ was effective for the preparation of less heat-resistant vanadate glass with higher conductivity. Such conductive glass might be preferably utilized as sensors, electric discharge needle, conductive glass paste, static electricity-protecting material and hyperfine processing materials combined with FIB, electrons, laser, etc. This will be also the case for Ga₂O₃- and GeO₂-substituted glasses, if the conductivity were highly enough for each purpose.

Figure 1a depicts σ values of Ga₂O₃-substituted glasses measured at RT after the isothermal annealing at 450°C, which was higher than the T_c’s of 405–422°C. RT-conductivity (σ) of 5 mol% Ga₂O₃-substituted vanadate glass measured after the heat treatment for 120 min (solid square) increased from 2.2×10⁻⁶Scm⁻¹ (ρ=4.5×10⁵ Ωcm) to 9.6×10⁻³ Scm⁻¹ (104 Ωcm), which was one-order of magnitude lower than that of “non-substituted” 20BaO·10Fe₂O₃·70V₂O₅ glass measured in this study (3.5×10⁻² Scm⁻¹ or 28 Ωcm). If the heat treatment were made at temperatures higher than 450°C, the conductivity will be enhanced. After the heat treatment at 450°C, σ of 10 mol% Ga₂O₃-substituted vanadate glass (red solid circle) increased from 6.7× 10⁻⁷ Scm⁻¹(1.5×10⁶ Ωcm) to 6.2×10⁻² Scm⁻¹(16 Ωcm), which was comparable to that of non-substituted vanadate glass, i.e. 3.5×10⁻² Scm⁻¹(28 Ωcm).

Fig. 1:

(a) Electrical conductivity (σ) of 20BaO·10Fe₂O₃·70V₂O₅ (open circle), 20BaO·5Ga₂O₃·5Fe₂O₃·70V₂O₅ (solid square) and 20BaO·10Ga₂O₃·70V₂O₅ glasses (red solid circle) measured at RT after isothermal annealing at 450°C. (b) Electrical conductivity (σ) of 20BaO·10Fe₂O₃·70V₂O₅ (open circle), 20BaO·5GeO₂·5Fe₂O₃·70V₂O₅ (solid square) and 20BaO·10GeO₂·70V₂O₅ glasses (red solid circle) measured at RT after isothermal annealing at 450°C.

All the σ values plotted in Fig. 1a are one- or two-orders of magnitude lower than those of 5 mol% CuO-substituted vanadate glass [13], which showed a marked increase of σ from 3.9×10⁻⁶ Scm⁻¹ (2.6×10⁵ Ωcm) to 3.2×10⁻¹ Scm⁻¹(3.1 Ωcm) after the annealing at 450°C for 30 min (see Table 1). After the same annealing, 5 mol% ZnO-substituted vanadate glass also showed a remarkable increase in σ from 2.5×10⁻⁶ Scm⁻¹ (4.0×10⁵ Ωcm) to 2.1×10⁻¹ Scm⁻¹(4.8 Ωcm) [1]. Conductivities of related barium iron vanadate glasses are summarized in Table 1, together with the activation energy for the electrical conduction (E_a) and Δ values of Fe^III obtained from the Mössbauer measurement. As seen from Table 1, CuO-, Cu₂O- and ZnO-substituted vanadate glasses, in which Cu^II atom had an electron configuration of 3d⁹, and Cu^I and Zn^II atoms 3d¹⁰ configuration, were preferable for the preparation of highly conductive vanadate glass.

RT-Mössbauer spectra of 5 mol% Ga₂O₃-substituted vanadate glass, measured after the isothermal annealing at 450°C, are illustrated in Fig. 2a. A marked decrease in Δ from 0.69 to 0.55 (±0.01) mms⁻¹ was observed after the annealing at 450°C for only 30 min, and Δ showed a slight decrease from 0.55 to 0.52 (±0.01) mms⁻¹after the additional heat treatment up to 300 min. A larger decrease in Δ of 0.12–0.15 (±0.02) mms⁻¹was observed in CuO- and Cu₂O-substituted vanadate glasses after the annealing at 450°C for 30 min [13]. In case of ZnO-substituted glass, the decrease in Δ was 0.06–0.07 (±0.01) mms⁻¹ [1]. All the Mössbauer results are summarized in Table 1.

Fig. 2:

(a) RT-Mössbauer spectra of 20BaO·5Ga₂O₃·5Fe₂O₃·70V₂O₅ glass measured after isothermal annealing at 450°C. (b) RT-Mössbauer spectra of 20BaO·5GeO₂·5Fe₂O₃·70V₂O₅ glass measured after isothermal annealing at 450°C.

Figure 2a proved that the eq_lat at the nuclear sites of ⁵⁷Fe decreased as a result of heat treatment at 450°C, reflecting a decreased distortion of FeO₄ tetrahedra. This is also the case for VO₄ tetrahedra since they are directly bonded to FeO₄ tetrahedra. In several vanadate glasses, decrease in the resistivity or an increase in the conductivity has been observed in conjunction with a decrease in Δ of Fe^III [1], [3], [4], [5], [6], [7], [8], [10], [11], [12], [13], [16], [17]. As shown in Fig. 1a with solid squares, σ values of 5 mol% Ga₂O₃-substituted glass were smaller than those of non-substituted vanadate glass (open circle) despite that the Mössbauer spectra showed a large decrease from 0.69 to 0.55–0.56 mms⁻¹ after 30–60 min annealing (Fig. 2a). In case of CuO- and Cu₂O-substituted vanadate glasses, comparable decrease in Δ [0.12–0.15 (±0.02) mms⁻¹] was observed after the annealing at 450°C for 30 min, and σ showed much more remarkable increase from 3.9×10⁻⁶ to 3.2×10⁻¹Scm⁻¹(3.1 Ωcm) [13]. In case of ZnO-substituted glass, a remarkable increase of σ was observed from 2.5×10⁻⁶ to 2.1×10⁻¹Scm⁻¹(4.8 Ωcm) in spite that the decrease in Δ was not so large [0.06–0.07 (±0.01) mms⁻¹] [1] (see Table 1). These results prove that elevation of the conductivity is partially related to the decrease in the distortion of FeO₄ and VO₄ units. The conduction behavior of highly conductive barium iron vanadate glass should be discussed by combining the small polaron hopping theory and the n-type semiconductor model, as discussed below.

Discussion

Activation energy for the conduction (E_a) in the small polaron hopping theory [2] could be calculated with the following equation:

in which σ, T and k are the conductivity, measuring temperature (K) and the Boltzmann constant, respectively. Values of E_a could be calculated from the slope of the straight line obtained by plotting the natural logarithm of σT against the reciprocal of T, as illustrated in Fig. 3.

Fig. 3:

(a) Natural logarithm of σT for 20BaO·10GeO₂·70V₂O₅ glass plotted against the reciprocal of the measuring temperature (T) before and after the annealing at 450°C for 120 min. (b) Natural logarithm of σT for 20BaO·5ZnO₂·5Fe₂O₃·70V₂O₅ glass plotted against the reciprocal of the measuring temperature (T) before and after the annealing at 450°C for 30 min.

Non-substituted barium iron vanadate glass, 20BaO·10Fe₂O₃·70V₂O₅, showed a marked increase of σ from 2.8×10⁻⁶to 4.3×10⁻² Scm⁻¹ in conjunction with a decrease of E_a from 0.38 to 0.13 eV when annealed at 500°C for 60 min [7], [8]. Conductivities of this glass showed a linear increase along with a decrease of E_a [8], [11]. The σ values increased significantly when annealed at higher temperatures, since the glass network was partially cleaved to cause the structural relaxation after prolonged annealing. Several E_a’s obtained for a series of barium iron vanadate glasses are summarized in Table 1, in which latest results of Ga₂O₃-, GeO₂- and ZnO-substituted vanadate glasses are summarized together with those for non-substituted [7], [8], WO₃- [10], MnO₂- [12], CuO- [13] and Cu₂O- substituted glasses [13].

Figure 3a depicts the ln σT-vs.-T⁻¹ plot for 10 mol% GeO₂-substituted vanadate glass, and Fig. 3b for 5 mol% ZnO₂-substituted glass. In these plots all the σT values were normalized with the σT obtained at RT. Ten mol% GeO₂-substituted vanadate glass had E_a’s of 0.41 and 0.21 eV before and after the annealing at 450°C for 120 min, respectively (Fig. 3a). Ten mol% Ga₂O₃-substituted vanadate glass also had comparable E_a’s of 0.42 and 0.18 eV before and after the annealing, respectively (Table 1). In contrast, 5 mol% ZnO₂-substituted vanadate glass investigated for comparison (Fig. 3b) had much smaller E_a’s of 0.23 and 0.14 eV before and after the annealing at 450°C for 30 min, respectively. This glass had RT-conductivity of 2.1×10⁻¹ Scm⁻¹ (4.8 Ωcm) which was one-order of magnitude higher than that of non-substituted glass [1]. Smaller E_a’s were also obtained in CuO- (0.16 and 0.10 eV) [13] and Cu₂O-substituted glasses (0.22 and 0.09 eV) [13], of which σ’s after the heat treatment were 3.2×10⁻¹ Scm⁻¹ (3.1 Ωcm) and 2.0×10⁻¹ Scm⁻¹ (5.0 Ωcm), respectively.

These E_a values are significantly smaller than the band gap energy (E_g) between the valence band (VB) and the conduction band (CB), reported for typical semiconductors like GaSb (0.23 eV), Ge (0.68 eV) and Si (1.16 eV) [7]. A photoluminescence study of 20BaO·10Fe₂O₃·70V₂O₅ glass annealed at 500°C for 60 min yielded an E_g of 2.25 eV [1]. Carrier (electron) density in the CB would increase along with a decrease of E_g and also with the energy gap between the donor level and the CB of n-type semiconductor. It is considered that E_a is equivalent to the energy gap between the donor level and CB. All the experimental results obtained in different vanadate glasses [1], [6], [7], [8], [10], [11], [12], [13], [17] proved that a decrease in E_a caused an increase in σ due to an increased carrier (electron) density in the CB. It is noted that n-type semiconductor model becomes predominant over the small polaron hopping model that has been utilized for the conduction mechanism of “semiconducting” vanadate glass of which σ is of the order of “mega” Ωcm [1], [13].

In the present study, as-quenched Ga₂O₃- and GeO₂-substituted vanadate glasses respectively had E_a’s of 0.42 and 0.41 eV. After the annealing at 450°C for 120 min, E_a’s of Ga₂O₃- and GeO₂-substituted vanadate glasses were 0.18 and 0.21 eV, respectively. Conductivities of these glasses were 9.6×10⁻³Scm⁻¹ (104 Ωcm) and 2.5× 10⁻³ Scm⁻¹ (400 Ωcm), as shown in Fig. 1a and b. They were two-orders of magnitude lower than those of ZnO- [1], CuO- [13] and Cu₂O-substituted glasses [13]. All the E_a and σ values obtained for non-substituted, ZnO-, CuO, Cu₂O-, Ga₂O₃- and GeO₂-substituted vanadate glasses prove that smaller E_a’s obtained before and after the annealing could attain the higher conductivity that was comparable to that of “metal heater” like Ni–Cr alloy.

As described above, electron configuration of 3d¹⁰ proved to be preferable for the preparation of highly conductive vanadate glass like ZnO-, CuO- and Cu₂O-substituted glasses [1], [13]. It is interesting to discuss the σ value in association with the band gap energy (E_g) of the substituents. The E_g of α-Fe₂O₃ is reported to be 2.2 eV [25], whereas CuO and Cu₂O, respectively have smaller E_g’s of 1.2–1.5 [26], [27] and 2.1 eV [28]. It was reported that ZnO had comparable E_g of 3.3 [29] or 3.4 eV [30]. In contrast, much larger E_g of 5.3 eV was reported for α-Ga₂O₃ [31], and 4.7–4.9 eV for β-Ga₂O₃ [31], [32]. The former is known to be stable at relatively lower temperature like 470°C, whereas the latter is stable at relatively higher temperatures of 550–630°C [31]. In case of α-GeO₂, a large E_g of 5.95 eV was reported [33]. Larger E_a values obtained for Ga₂O₃- and GeO₂-substituted vanadate glasses before and after the heat treatment will be ascribed to the large E_g of Ga₂O₃ and GeO₂. These results prove that a partial replacement of Fe₂O₃ by metal oxide with small E_g is effective to attain the high conductivity of vanadate glass.

It was suggested that Fe^III atoms with a symmetric electron configuration of 3d⁵ were preferable for the small polaron hopping from V^IV to V^V (and Fe^III) via oxygen atoms. For example, 25K₂O·10Fe₂O₃·65V₂O₅ glass annealed at 380°C showed an increase of σ from 6.3×10⁻⁸Scm⁻¹(ρ=1.6×10⁷ Ωcm) to 4.3×10⁻⁴Scm⁻¹(ρ=2.3 kΩcm) as a result of an increased probability of the small polaron hopping among less-distorted FeO₄ and VO₄ tetrahedra [3], [4]. Structural relaxation of FeO₄ and VO₄ tetrahedra was confirmed as a marked decrease of Δ in the Mössbauer spectra [1], [3], [4], [5], [6], [7], [8], [10], [11], [12], [13], [16], [17], and the conductivity was enhanced by the structural relaxation. This is also the case for the vanadate glasses in which Fe₂O₃ was replaced by metal oxides in which each metal had 3d¹⁰ configuration [1], [13].

Small polaron hopping theory [2] has generally been utilized to explore the conduction mechanism of semiconducting vanadate glass of which σ is <10⁻⁴Scm⁻¹. This authors’ group suggested that n-type semiconductor model in conjunction with the small polaron hopping theory was most probable to explain the conduction mechanism of highly conductive vanadate glasses with σ higher than ca. 10⁻⁴ Scm⁻¹ [1], [13]. In conductive barium iron vanadate glasses, increase in σ was directly proportional to the decrease in Δ of Fe^III in the Mössbauer spectra. A marked decrease of Δ was observed in non-substituted [5], [6], [7], [8], WO₃- [10], [11], MnO₂- [12], CuO- [13], Cu₂O- [13] and Ga₂O₃- or GeO₂-substituted barium iron vanadate glasses (this study). In case of ZnO-substituted glass [1], a decrease in Δ was not remarkable (Table 1), but σ of this glass was one order of magnitude larger than that of non-substituted glass. These results evidently prove that n-type semiconductor model in conjunction with the small polaron hopping theory is most probable for the conduction mechanism of highly conductive vanadate glass of which σ value is comparable to that of metal heater like Ni–Cr alloy.

Summary

1. As for the conduction mechanism of highly conductive vanadate glasses with σ value larger than 10⁻⁴ Scm⁻¹, n-type semiconductor model combined with the small polaron hopping theory is most probable.

2. Heat treatment of barium iron vanadate glass at a given temperature higher than T_c causes a marked increase in the conductivity in conjunction with a decrease in E_a, which could be correlated to the energy gap between the donor level and the CB.

3. Substitution of Ga₂O₃ or GeO₂ for Fe₂O₃ was less effective to increase the conductivity, despite that Ga^III and Ge^IV atoms had 3d¹⁰ configuration that was preferable to attain the higher conductivity.

4. This will be ascribed to the intrinsic large E_g’s of Ga₂O₃ (5.3 eV) and GeO₂ (5.95 eV) which are much larger than that of Fe₂O₃ (2.2 eV).

5. All the experimental results suggest that replacement of Fe₂O₃ by the metal oxide with a small E_g is preferable to attain the higher conductivity, as recently discovered in CuO-, Cu₂O- and ZnO-substituted vanadate glasses.