The incorporation of tetravalent cerium into the monazite structure via Ca(II)-coupled substitution was investigated using a solid state and a co-precipitation route for the synthesis of the La1-2xCaxCexPO4 solid solution. Based on powder XRD measurements and elemental mappings an optimised synthesis procedure was developed that averts the formation of secondary phases and allows the stabilisation of tetravalent cerium with a ratio of up to 0.21 Ce(IV)/Ce(III). In-situ HERFD-XANES measurements at the Ce L3 edge at up to 800 °C were performed to study the cerium oxidation state during the phase transformation from rhabdophane to monazite, revealing an unexpected non-linear behaviour as well as a charge-directing effect of the lanthanum cation.
