Electron diffraction of U₄O₉

Thin foils of U₄O₉ prepared from bulk single crystals of UO₂ were investigated at 20 °C and between 300 °C and 1000 °C in the electron microscope. At 20 °C the complete reciprocal lattice of U₄O₉ has been established. Between about 550° and 700°C an order-partial disorder transformation has been found in most of the specimens investigated. This leads to a modified U-O phase diagram but at present it is not known if this new diagram or the old one represents metastable phase relations. Crystallographic relations between the low temperature phase and the partly disordered high-temperature phase of U₄O_{9 - y} have been deduced.