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Thermochemical Study of the Relations of the Monazite and Zircon Structures in the Rare-earth Orthophosphates

• RE orthophosphates are of importance as actinide hosts in glass and ceramic radioactive waste forms and their alteration products.

• REE orthophosphates are under development as inert matrices for Pu fuel and as multilayered (with Al2O3 and ZrO2) weak-bonded ceramic composites.

• Pure anhydrous (RE)PO4 compounds change structure with increasing REradius: i.e., orthophosphates from Lu to Ho as well as Sc and Y exist only in the tetragonal zircon (xenotime) structure, while the orthophosphates from Eu to La exist only in the monoclinic monazite structure.

• The enthalpies of formations from oxides for zircon-type (Lu-Gd) andmonazite-type (Gd-La) orthophosphates show an almost linear dependence onthe RE radius. There was no significant discontinuity observed upon changing the structure type from xenotime to monazite.

• Existence of both zircon- and monazite-type polymorphs for Dy, Tb and Gd orhtophosphates has been previously reported, however, available structural and thermodynamic data are scarce. The goal of this work was to evaluate theenergetics of structural changes in these compounds.