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Ionic Crystal Structures

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Molten-Salt Synthesis of Complex Metal Oxide Nanoparticles

Molten-Salt Synthesis of Complex Metal Oxide Nanoparticles

Published on: October 27, 2018

Solid-state synthesis of monazite-type compounds containing tetravalent elements.

Damien Bregiroux¹, Olivier Terra, Fabienne Audubert

¹Commissariat à l'Energie Atomique, DEN/DEC/SPUA/LTEC, Cadarache, 13108 Saint Paul Lez Durance, France. damien.bregiroux@ccr.jussieu.fr

Inorganic Chemistry

|October 30, 2007

Summary

This study investigated the high-temperature synthesis of phosphate ceramics for immobilizing tetravalent actinides. Thorium and uranium successfully formed brabantite phases, while cerium did not stabilize in the tetravalent state.

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Area of Science:

Materials Science
Nuclear Chemistry
Ceramics Engineering

Background:

Phosphate-based ceramics are explored for nuclear waste immobilization.
Stabilizing tetravalent actinides (Th, U, Pu) in ceramic structures is crucial for safe disposal.
Brabantite (CaPO4-based) is a potential host phase for these actinides.

Purpose of the Study:

To determine the optimal heating conditions for preparing brabantite and monazite/brabantite solid solutions.
To investigate the incorporation of tetravalent thorium, uranium, and cerium into these phosphate ceramics.
To evaluate the stability of these actinide-doped ceramics at elevated temperatures.

Main Methods:

Optimized grinding and heating cycles were developed.
X-ray diffraction (XRD), thermogravimetric/differential thermal analysis (TG/DTA), electron probe microanalyses (EPMA), and micro-Raman spectroscopy were employed.
Initial mixtures included actinide dioxides (AnO2)/cerium dioxide (CeO2), calcium hydrogen phosphate (CaHPO4·2H2O) or calcium oxide (CaO), and ammonium dihydrogen phosphate (NH4H2PO4).

Main Results:

Thorium (Th) was successfully incorporated into brabantite at 1100°C, forming a pure, single-phase material (Ca0.5Th0.5PO4).
Uranium (U) was stabilized as tetravalent uranium in uranium-brabantite (Ca0.5U0.5PO4) after heating at 1200°C.
Cerium(IV) could not be stabilized in brabantite due to reduction to Ce(III) above 840°C, resulting in polyphase materials. This inability to stabilize Ce(IV) mirrors challenges with Pu(IV) incorporation.

Conclusions:

Brabantite is a viable host for tetravalent thorium and uranium under specific high-temperature synthesis conditions.
The stability of tetravalent cerium and plutonium in brabantite structures is limited due to redox instability.
Understanding these incorporation and stability limits is essential for selecting appropriate waste forms for actinide immobilization.