The thermal stability of calcium uranates in a hydrogen atmosphere

The stability and decomposition of CaUO ₄, Ca ₂UO ₅, and Ca ₃UO ₆ on heating in hydrogen were investigated by X-ray powder diffraction and thermogravimetry. Ca ₂UO ₅ decomposes at 450°C into Ca ₂UO _4.5 with a triclinic unit cell. At 850°C, it changes to monoclinic Ca ₂(Ca _0.67U _0.33)UO ₆ which loses some oxygen up to the composition Ca ₂(Ca _0.67U _0.33)UO _5.83. At 1100°C, it decomposs to UO ₂ solid solution and CaO. CaUO ₄ decomposes at 900°C to Ca ₂(Ca _0.67U _0.33)UO _5.83 and CaU ₂O ₆. The decomposition products of Ca ₃UO ₆ at 850°C are Ca ₂(Ca _0.67U _0.33)UO _5.83 and CaO.