Monoclinic LaPO4 (monazite) is a chemically stable compound with a high melting temperature, high radiation damage resistivity, and very low solubility in water; it has various applications arising from its unique structure. Low temperature solution synthesis usually results in formation of a hexagonal hydrate, LaPO4·½H2O (rhabdophane). The presence of rhabdophane in the compound results in formation of a liquid phase at high temperatures due to availability of excess phosphorus, thus making monazite less applicable. In this study, different batches of LaPO4 were synthesized at different temperatures using the direct precipitation synthesis method. The synthesis technique in this study coupled with washing and ball-milling the samples yields monoclinic monazite without the formation of rhabdophane, as confirmed via XRD.

Monoclinic LaPO4 doped with divalent elements such as strontium (Sr2+) is a material candidate for electrolytes in intermediate temperature proton conducting solid oxide fuel cells. Synthesis of monoclinic monazite usually results in the production of rhabdophane. Availability of liquid phase at high temperatures detrimentally scavenges Sr from the doped monazite. Here, monoclinic LaPO4, with up to 30% Sr doping, has been synthesized using a

direct precipitation synthesis, without any formation of rhabdophane, as investigated by XRD. The solubility limit of Sr in LaPO4 structure was determined to be around 30% via measuring Sr concentration through EDS. The XRD results showed that Sr(PO3)2 was formed when Sr doped LaPO4 powders were annealed in air, and Sr2P2O7 phase was formed when powders were sintered in water.