EPR STUDY OF VANADIUM-CONTAINING AMORPHOUS SILICA FORMED BY SOL-GEL METHOD

Spectra of electron paramagnetic resonance (EPR) of sol-gel derived si lica glass doped with vanadium (1 wt%) were measured at similar to 9 G Hz and similar to 300 K after various stages of the process (sol --> f resh gel --> aged gel --> heat treated gel --> glass). We show that th e source of electron paramagnetic resonance spectra of the doped solut ion of aqua-complexes of vanadyl resides in the gel pores after heat t reatment at temperatures T-t less than or equal to 973 K. The number o f vanadyl ions in a sample decreases with increasing T-t and the remai nder is immobilized on the pore surface in local environments in which the spectra of vanadium species becomes undetectable. In the temperat ure range in which the pores collapse a symmetrically shaped single co mponent with observed g similar to 1.96 is attributed to V4+ clusters. After a treatment at T-t greater than or equal to 1023 K the width of this line increases at sample temperatures above 340 K. We assumed th at this line is due to the formation of crystalline inclusions of non- stoichiometric VO2 for which insulator-metal transition is commonly ob served at similar to 340 K. The volume of crystalline phase VO2 is les s than the detection limit of X-ray diffraction technique used in the present work. For the samples heat treated at T-t>1073 K the spectrum with hyperfine structure (HFS) attributed to isolated V4+ ions in tetr ahedral coordination appears at 77 K. The parameters of spin Hamiltoni an of orthorhombic symmetry indicate that the wave function of ground state of V4+ is the \3z(2) - r(2)] type. The absorption bands with max ima at similar to 770 and similar to 930 mm are observed in optical sp ectra of these samples. (C) 1998 Elsevier Science B.V. All rights rese rved.