Synthesis, Symmetry, and Physical Properties of Cerium Pyrophosphate

Abstract

CeP 2 O 7, a close structural relative of ZrP 2 O 7, and many other M IV X 2 O 7 (X – P, V, As) phases, forms on heating Ce(HPO 4 ) 2 · x H 2 O 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 2 O 7 in two distinct stages. At room temperature, CeP 2 O 7 is pseudocubic, but probably triclinic, displaying positive thermal expansion below ∼115 °C. Above this temperature, there is a transition to cubic symmetry ( Pa 3̅) with the linear coefficient of thermal expansion going to zero at ∼450 °C and becoming negative at higher temperatures. CeP 2 O 7 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.