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

Abstract

The 57Fe Mossbauer 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.