Hydrothermal treatment of nanoscale amorphous ceric phosphate gel in KOH aqueous solutions was found to result in a new KCe 2 (PO 4 ) 3 phase. The refinement of the KCe 2 (PO 4 ) 3 structure showed that it was isostructural to recently reported (NH 4 )Ce 2 (PO 4 ) 3 . For the KCe 2 (PO 4 ) 3 phase, the unit cell parameters (sp. gr. C2/c) were a = 17.3781(3) Å, b = 6.7287(1) Å, c = 7.9711(2) Å, β = 102.351(1) • , V = 910.53(4) Å3 , Z = 4. The thermal decomposition of KCe 2 (PO 4 ) 3 at 800 • C resulted in the mixture of crystalline CePO 4 and KPO 3 .

Nanocrystalline cerium dioxide is a promising inorganic UV filter for sunscreen applications due to its high UV absorbance and non-toxicity to normal cells. Nanoscale CeO 2 also showed selective cytotoxicity to cancer cells, thus ceria-containing materials are now regarded for the creation of both preventive and therapeutic compositions. At the same time, the interaction of ceria nanoparticles with cell membranes and phosphate-rich components of sunscreen compositions arise the interest to biocompatibility of ceric phosphates. Crystalline cerium(IV) phosphates can be a promising alternative for nanoscale CeO 2 due to their low solubility, high redox stability and UV protective property. However, to date, there is no information on their toxicity to cancer cells. In this work, using the MTT, Live/Dead and MMP assays, we demonstrated for the first time that the inhibitory impact of ceric phosphates Ce(PO 4 )(HPO 4 ) 0.5 (H 2 O) 0.5 and NH 4 Ce 2 (PO 4 ) 3 on murine melanoma B16/F10 cell line in vitro is comparable to that of nanoscale CeO 2 , at high (500-1000 µg/ml) concentrations. KEYWORDS ceric phosphates, ceria, metabolic activity, reactive oxygen species, UV protectors. ACKNOWLEDGEMENTS This work was supported by Russian Science Foundation (Grant no. 21-73-00294, https://rscf.ru/en/project/21-73-00294/) using the equipment of the JRC PMR IGIC RAS.