Thermal expansion behaviour of a versatile monazite phase with simulated HLW: A high temperature x-ray diffraction study

• The XPS characterization of the versatile monazite phase, Ce0.8Ca0.2PO4 (Ce0.63+Ce0.24+Ca0.22+PO4) confirms the mixed valence state of Ce.
• The monoclinic lattice parameters all increase to nearly the same extent with temperature displaying the isotropic behaviour for the monazite phase.
• The thermal expansion along the three crystallographic axes vary in the order b < a < c.

Synthetic analogues of monazite are considered as an alternate ceramic host for the immobilization of high level waste (HLW) generated in the nuclear fuel cycle. The Ca doped CePO4 (Ce0.8Ca0.2PO4) is expected to be a versatile monazite phase for this purpose due to the mixed valence state of cerium in this compound which assists in accommodating cations of any valence by initiating the internal redox reaction, viz., Ce3+ ⇌ Ce4+ + e−, as required by the incoming cation for charge neutrality of the lattice. The thermal expansion of this versatile monazite phase with 20 wt.% simulated HLW was measured using high temperature x-ray diffractometer (HTXRD) in the temperature range of 298–973 K and are reported here. The thermal expansion behaviour is found to be similar for CePO4, Ce0.8Ca0.2PO4 and Ce0.8Ca0.2PO4 with 20 wt.% simulated HLW with the average volume expansion coefficient of 27.4, 26.5 and 24.1 × 10−6 K−1 respectively in the temperature range of 298–973 K.