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

Abstract

The ⁵⁷Fe Mössbauer spectrum of semiconducting xK₂O·(90–x)V₂O₅· 10Fe₂O₃ glasses (20⩽x⩽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–O bonds and a decreased distortion of FeO₄ tetrahedra, respectively. FTIR spectra of heat-treated vanadate glasses revealed an evident peak separation between VO₄ and FeO₄ tetrahedra. The FeO₄ units were not observed in non-treated glass because they occupy substitutional sites of VO₄. An X-ray diffraction study of heat-treated glasses established the formation of a KV₃O₈ phase composed of VO₅ pyramids. Additional diffraction peaks due to a K₃V₅O₁₄ phase, composed of VO₄ tetrahedra and VO₅ pyramids, were observed in 30K₂O · 60V₂O₅· 10Fe₂O₃ and 35K₂O · 65V₂O₅ glasses after the heat treatment. Weak peaks due to K₄V₁₀O₂₇ phase were also observed in 20K₂O ·70V₂O₅· 10Fe₂O₃ glass. A Kissinger plot of the differential thermal analysis (DTA) data yielded activation energies of 2.0–2.9 (± 0.3) eV, indicating that cleavage of Fe–O bonds with bond energies of ca. 2.6 eV triggers the crystallization. Magnetic susceptibility measurements of 25K₂O · 65V₂O₅· 10Fe₂O₃ 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 × 10^–8S cm^–1 to 4.3 × 10^–4 S cm^–1 was observed when 25K₂O · 65V₂O₅· 10Fe₂O₃ 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 VO₄, FeO₄ and VO₅ units.