Single- and multi-phase crystalline ceramics that are resistant to
radiation damage have been proposed as candidate materials for
encapsulating radioactive waste [1e4]. Monazite and xenotime are
naturally occurring rare-earth phosphate minerals that are being
considered as host-matrices for nuclearwaste because of the ability
of these structures to contain actinide elements [5e7]. These
mineral phases have been observed as solid solutions with one
another in igneous and metamorphic rocks [8]. Both the monaziteand
xenotime-type rare-earth phosphates possess the same general
formula REPO4, where RE represents the rare-earth element.
The rare-earth site in monazite and xenotime are occupied by
lighter (La to Gd) or heavier rare-earths (Lu to Yb and Y), respectively
[9]. Materials adopting the monazite-type structure crystallize
in a monoclinic (space group e P21/n) unit cell while materials
adopting the xenotime-type structure (YPO4) crystallize in a
tetragonal (space group e I41/amd) unit cell (Fig. 1) [9e13]. In
monazite (e.g., CePO4), the RE3þ ion is bonded to nine oxygen atoms
(REO9) in a non-symmetrical fashion (point group e Cs) whereas in
xenotime (e.g., YPO4), the RE3þ ion is coordinated to eight oxygen