Synthesis, symmetry, and physical properties of cerium pyrophosphate

CeP₂O₇, a close structural relative of ZrP ₂O₇, and many other M^IVX₂O ₇ (X - P, V, As) phases, forms on heating Ce(HPO₄) ₂·xH₂O between ∼300 and 600°C and decomposes by oxygen loss at higher temperatures. In-situ X-ray diffraction measurements showed, for some precursor batches, the formation of CeP ₂O₇ in two distinct stages. At room temperature, CeP ₂O₇ is pseudocubic, but probably triclinic, displaying positive thermal expansion below ∼115°C. Above this temperature, there is a transition to cubic symmetry (Pa3̄) with the linear coefficient of thermal expansion going to zero at ∼450°C and becoming negative at higher temperatures. CeP₂O₇ probably undergoes two phase transitions on compression below ∼10.5 GPa. The phase existing between 0.65 and ∼5 GPa is soft with an average bulk modulus of ∼18 GPa.