SOL-GEL SYNTHESIS OF MICROCRYSTALLINE RARE-EARTH ORTHOPHOSPHATES

Studies of the reactions between rare earth salts and phosphoric acid in aqueous or ethanolic media have shown that in both cases stable gel s can be formed. Upon drying, gels prepared in aqueous environments yi eld macrocrystalline REPO(4) products similar to those produced by con ventional precipitation and drying. Gels prepared in ethanol, on the o ther hand, undergo dehydration to form dense microcrystalline products . This observation is based on optical and scanning electron microscop y, as well as on x-ray diffraction studies and infrared spectroscopy. These techniques, as well as differential thermal analysis, indicate t hat crystal growth of these products takes place around 600-700 degree s C. The composition of the dehydrated gels produced in both the aqueo us and ethanolic systems corresponds to an orthophosphate structure. O ther characteristics of the microcrystalline REPO(4) products include high resistance to attack by water, absence of coloration upon exposur e to gamma rays, and a high index of refraction.