Compressibility studies of RE₆UO₁₂ at extreme conditions of pressure

Rare-earth uranates-RE₆UO₁₂ are synthesized by heating mixture of uranium oxide and rare-earth oxides in 1:6 ratio above 1273 K. These compounds stabilize in rhombohedral structure at ambient. High-pressure (HP) X-ray diffraction studies reveal that the compounds are stable at lower pressures, beyond which disorder is seen to originate and compound has a tendency to amporphize at very HPs. The a-axis of the lattice is found to be more rigid as compared to c-axis because of corner sharing polyhedra along a-axis. Anomalous compressibility behaviour is seen in Gd₆UO₁₂, where sharp decrease in the bulk modulus is observed. The behaviour is against the normal trend of compressibility in RE₆UO₁₂ compounds along rare-earth cation series. (author)