Structural relaxation and crystallization of semiconducting vanadate glass accompanying a jump of the electrical conductivity

Abstract

The 57Fe Mossbauer spectrum of semiconducting xK,O (90-x)V205 * 10Fe203 glasses (20 < x d 30) consists of a quadrupole doublet peak. The linewidth and the quadrupole splitting decreased gradually after heat treatment at the crystallization temperature, reflecting an increased uniformity of Fe-0 bonds and a decreased distortion of FeO, tetrahedra, respectively. FTIR spectra of heat-treated vanadate glasses revealed an evident peak separation between VO, and FeO, tetrahedra. The FeO, units were not observed in non-treated glass because they occupy substitutional sites of VO,. An X-ray diffraction study of heat-treated glasses established the formation of a KV308 phase composed of VO, pyramids. Additional diffraction peaks due to a K3V5014 phase, composed of VO, tetrahedra and VO, pyramids, were observed in 30K20*60V205 lOFe,O, and 35K2O*65V2O5 glasses after the heat treatment. Weak peaks due to K4V10027 phase were also observed in 20K20.70V20, 10Fe,03 glass. A Kissinger plot of the differential thermal analysis (DTA) data yielded activation energies of 2.0-2.9 (d0.3) eV, indicating that cleavage of Fe-0 bonds with bond energies of ca. 2.6 eV triggers the crystallization. Magnetic susceptibility measurements of 25K20 * 65V,05 * 10Fe203 glass revealed an antiferromagnetic behaviour with Curie-Weiss constants of -13, -93 and -108 K after the heat treatment at 340 "C for 0,2100 and 5000 min, respectively. A jump of the electrical conductivity from 6.3 x lo-' S cm-' to 4.3 x lo-, S cm-' was observed when 25K20*65V20, 10Fe203 glass was heat treated at 380 "C for 10 min, and this can be ascribed to an increased probability of electron hopping from V4+ to V5+ which was brought about as a result of a decreased distortion of VO,, FeO, and V 0 5 units.Electrical conducting glasses have attracted much interest in the field of solid-state chemistry and materials science because they can be used as an electrode, solid-state electrolyte, sensor, etc. Several vanadate (V,O,-based) glasses show a semiconductivity with the electrical conductivity ( B ) of the order of about lo-' to Scrn-',' which is known to be due to the electron hopping from V4+ to V5+ in the 'skeleton' structure. The mechanism of semiconductivity was explained by Mott2 and by Austin and Mott3 with a small polaron hopping theory, in which the activation energy, W, was estimated to be equal to the summation of the electron-hopping energy, W', and half of the lattice disorder energy, W,. In the case of Sb2O3-Sr0-V2O5 glasses,' W was reported to be 0.41-0.61 eV and W, <0.1 eV.The electrical conductivity of insulating oxide glasses is generally less than lo-'' S cm-l and the alkali-metal ions are known to be representative 'carriers'. Crystallization of oxide glass generally results in a decrease in the electrical conductivity. By contrast, if a conducting crystalline phase like 'vanadium bronze', R,V205, precipitates in the matrix glass, the electrical conductivities of glass ceramics will become higher than those of non-treated glass. In the case of potassium vanada...

Characterization and Conduction Mechanism of Highly Conductive Vanadate Glass

“…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.…”

“…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.…”

“…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.…”

57Fe Moessbauer and DTA study of R2O 2FeO V2O5 P2O5 glasses (R = Li, Na)

“…5) Vanadate glasses are known to have semiconductivity caused by electron hopping from V IV to V V . 6) Vanadium pentoxide (V2O5) is known to have a layered structure composed of VO5 pyramids. 6 The different slope of the straight line suggests that the chemical bond of NWF-oxygen polyhedra is much more intense than that of NWM-oxygen polyhedra and also the chemical bond of NWF-oxygen tetrahedra is more intense than that of NWF-oxygen octahedra since the former is more covalent than the latter.…”

“…6) Vanadium pentoxide (V2O5) is known to have a layered structure composed of VO5 pyramids. 6 The different slope of the straight line suggests that the chemical bond of NWF-oxygen polyhedra is much more intense than that of NWM-oxygen polyhedra and also the chemical bond of NWF-oxygen tetrahedra is more intense than that of NWF-oxygen octahedra since the former is more covalent than the latter. In this paper, Mössbauer spectra of xLi2O·2FeO·V2O5·P2O5 and xNa2O·2FeO·V2O5·P2O5 glasses (x = 0, 0.5, 1.0 and 1.5), developed for a cathode material of Li-ion battery, were measured for determining the local structure Fe III .…”