Investigations on the charge compensation on Ca and U substitution in CePO4 by using XPS, XRD and Raman spectroscopy

This article discusses the charge compensation mechanism in Ca and U - substituted monazite (CePO4). The monazite compositions Ce0.6Ca0.2U0.2PO4, Ce0.8Ca0.2PO4 and CePO4 were prepared by solution chemistry route. XRD and μ-Raman studies confirm that the compounds are single phase materials. XPS studies on Ce0.8Ca0.2PO4 shows the presence of Ce4+ in addition to Ce3+ whereas in Ce0.6Ca0.2U0.2PO4 cerium is in +3 state with uranium in +4 valence state. Partial substitution of Ca in CePO4 (i.e. Ce0.8Ca0.2PO4), leads to the generation of Ce4+ which subsequently reverts to Ce3+ in Ce0.6Ca0.2U0.2PO4 upon U substitution. The Ce3+⇌Ce4+ intrinsic charge compensation existing in monazite, essential for the incorporation of higher valence cations, is demonstrated. This mechanism assists the incorporation of M2+/4+ in the monazite lattice without requiring coupled substitution.