Lithium iron phosphate LiFePO4 triphylite is now one of the core positive electrode (cathode) materials enabling the Li-ion battery technology for stationary energy storage applications, which are important for broad implementation of the renewable energy sources. Despite the apparent simplicity of its crystal structure and chemical composition, LiFePO4 is prone to off-stoichiometry and demonstrates rich defect chemistry owing to variations in the cation content and iron oxidation state, and to the redistribution of the cations and vacancies over two crystallographically distinct octahedral sites. The importance of the defects stems from their impact on the electrochemical performance, particularly on limiting the capacity and rate capability through blocking the Li ion diffusion along the channels of the olivine-type LiFePO4 structure. Up to now the polyanionic (i.e. phosphate) sublattice has been considered idle on this playground. Here, we demonstrate that under hydrothermal conditions up to 16% of the phosphate groups can be replaced with hydroxyl groups yielding the Li1-xFe1+x(PO4)1-y(OH)4y solid solutions, which we term “hydrotriphylites”. This substitution has tremendous effect on the chemical composition and crystal structure of the lithium iron phosphate causing abundant population of the Li-ion diffusion channels with the iron cations and off-center Li displacements due to their tighter bonding to oxygens. These perturbations trigger the formation of an acentric structure and increase the activation barriers for the Li-ion diffusion. The “hydrotriphylite”-type substitution also affects the magnetic properties by progressively lowering the Néel temperature. The off-stoichiometry caused by this substitution critically depends on the overall concentration of the precursors and reducing agent in the hydrothermal solutions, placing it among the most important parameters to control the chemical composition and defect concentration of the LiFePO4-based cathodes.

Crystalline cerium(III) phosphate (CePO₄), cerium(IV) phosphates, and nanocrystalline ceria are considered to be promising components of sunscreen cosmetics. This paper reports on a study in which, for the first time, a quantitative comparative analysis was performed of the UV-shielding properties of CePO₄, Ce(PO₄)(HPO₄)_0.5(H₂O)_0.5, and CePO₄/CeO₂ composites. Both the sun protection factor and protection factor against UV-A radiation of the materials were determined. Ce(PO₄)(HPO₄)_0.5(H₂O)_0.5 was shown to have a sun protection factor of 2.9, which is comparable with that of nanocrystalline ceria and three times higher than the sun protection factor of CePO₄. Composites containing both cerium dioxide and CePO₄ demonstrated higher sun protection factors (up to 1.8) than individual CePO₄. When compared with the TiO₂ Aeroxide P25 reference sample, cerium(III) and cerium(IV) phosphates demonstrated negligible photocatalytic activity. A cytotoxicity analysis performed using two mammalian cell lines, hMSc and NCTC L929, showed that CePO₄, Ce(PO₄)(HPO₄)_0.5(H₂O)_0.5, and nanocrystalline ceria were all non-toxic. The results of this comparative study indicate that cerium(IV) phosphate Ce(PO₄)(HPO₄)_0.5(H₂O)_0.5 is more advantageous for use in sunscreens than either cerium(III) phosphate or CePO₄/CeO₂ composites, due to its improved UV-shielding properties and low photocatalytic activity.