Ultra-high entropy rare earth phosphate against environmental corrosion.

Environmental particulate deposits are the most severe factor causing high-temperature corrosion failure in advanced structural materials. Yet, the primary mechanisms of molten material corrosion under extreme conditions remain unclear, hindering the development and improvement of new crucial corrosion-resistant materials. "Ultra-high entropy" rare earth orthophosphates (REPOs) are one such promising class of materials. Here, we investigate the high-temperature structural state and properties of a wide range of substitutions involving 15 (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) rare earth elements in phosphates. Using stepwise stoichiometric compositional design, we aim to systematize our understanding of the corrosion resistance behavior of these complexes RE phosphates. We observe that high compositional complexity slows the reaction kinetics. Increasing rare-earth ionic radius, correlates positively with the chemical inertness of the synthesized material. This is overlain by phase state considerations whereby single-phase compositions exhibit the highest corrosion resistance. In summary, we infer that 1) the grade of complexity of rare earth mixtures; 2) the proportions of the individual rare earths and 3) the phase state of solid solution stoichiometries are the key factors in determining the corrosion resistance of these complex phosphate materials.