CRITICAL-ASSESSMENT OF REPORTED PROCEDURES FOR THE SYNTHESIS OF THERMALLY STABLE VPI-5

Three procedures from the literature for the synthesis of a VPI-5 phas e that is stable at high temperatures in air have been tried. However, reproducible results were not obtained. Extensive washing or boiling of the products in water overnight did not improve the thermal stabili ty of the products. The thermal stabilities of the samples were assess ed in unsealed sample holders at 120 degrees C in static air and heate d in quartz trays to 400 degrees C at a heating rate of 5 degrees C/mi n in a flow of dry air. Additionally, the samples were heated under th ermal conditions specified in the relevant literature (in unsealed ves sels), but all samples transformed to AlPO4-8. Details of the XRD patt erns, earlier reported to be significant for the stable and unstable V PI-5 phases, are believed to be ambiguous.