Optimisation of synthesis procedures and structural investigation of monazite-cheralite solid solutions

The monazite-cheralite solid solutions Ln PO 4 -Ca 0.5 Th 0.5 PO 4 with Ln = La, Gd were prepared via a co-precipitation route, showcasing an optimised, scalable synthesis procedure for a possible waste form accommodating high level liquid waste streams. A distortion of the cheralite structure with respect to the monazite structure was observed throughout both solid solutions as evidenced by a deviation of the lattice parameters from the linear behaviour known from other monazite solid solutions. Using a high temperature flux method, cheralite single crystals were grown for the first time for in-depth structural investigations. Both thorium and calcium were found to deviate from the central position of the Ln O 9 polyhedron, supporting previous neutron diffraction investigations of identical cheralite samples. Graphical Abstract