ELECTRON-PARAMAGNETIC-RESONANCE AND DIFFERENTIAL SCANNING CALORIMETRYSTUDIES OF THORIUM AND TIN PHOSPHATE XEROGELS

Different transparent phosphate xerogels were synthesized using concen trated solutions of metal chlorides and phosphoric acid with a proper mole ratio of both components. By this method we prepared bulk samples of thorium and tin(IV) phosphate xerogels by drying at room temperatu re or at 350 K. Some properties of these amorphous materials were stud ied by means of differential scanning calorimetry (DSC) and electron p aramagnetic resonance (EPR) techniques. Depending on mole ratio metal/ phosphate, these xerogels show, near 180 K, inflection points which we interpret as T-g Samples dried at 425 K lose their transparency and h ave no T-g Thus, it seems that the ''glassy'' state is due to water mo lecules remaining in the material. The same properties were confirmed by EPR studies of the xerogels doped with Cr3+ and Fe3+ ions as probes . These results show the existence of two different phases in the xero gels: a liquid-like one, in the range from 190 K to 350 K and a solid- like one, in the range from 4 K to 190 K.