Electron paramagnetic resonance and other allied studies of sol–gel derived nanocrystallinechromium oxides in a silica glass matrix

Abstract

Monolithic silica gels doped with various concentrations (1, 5, 10 mol%) of chromium oxide areprepared by a sol–gel process and are subjected to calcination at different temperatures up to1200 °C. This has principally led to the formation ofCr2O3 nanocrystalsand also of CrO3 nanocrystals at the higher calcination temperatures. The sizes of theCr2O3 nanocrystals have been determined by x-ray diffraction (XRD) and transmission electronmicroscopy (TEM) studies and have been found to lie in the range 5–30 nm depending upon thedopant concentration as well as the calcination temperature. The formation of nonmagneticCrO3 crystals in 10 mol% chromium oxide doped silica glass calcined at1200 °C as a result ofoxidation of Cr5+ ions to Cr6+ ions has been confirmed from XRD studies. Optical, infrared and X-band electron paramagneticresonance (EPR) spectra of these nanocrystals have also been studied. EPR behaviours such as theg-factor andlinewidth of Cr2O3 nanocrystals investigated at room and liquid nitrogen temperatures havebeen found to be quite distinct from those of antiferromagnetic (AF) bulkCr2O3 crystals and may be manifestations of superparamagnetism/ferromagnetism in thesenanocrystals. A narrow anisotropic EPR line () has been detected for chromium oxide doped silica gel calcined at800 °C and has beenassigned to isolated Cr5+ ions in a tetragonally distorted tetrahedral environment. Studies of the thermaldependence of zero-field-cooled (ZFC) and field-cooled (FC) magnetization(σ) 10 mol% chromium oxide doped silica glasses calcined at800 °C by aSQUID magnetometer in the 5–300 K temperature range have revealed the occurrence of the blockingtemperature TB at about 250 K, which is a characteristic feature of superparamagnetism inCr2O3 nanocrystals. The superparamagnetic effects are almost quenched in the samples calcined at 1000and 1200 °C because of the presence of larger nanocrystals in these samples. Magnetichysteresis effects in these calcined samples at 300 K as well as at 5 K in the ± 6 T magnetic field range have also been investigated. The existence of ferromagnetic order atlow temperature (5 K) as well as at room temperature (300 K) (for samples calcined athigher temperatures) has been confirmed.