Cathodoluminescence response of natural and synthetic lanthanide-rich phosphates (Ln³⁺: Ce, Nd)

This paper reports on the cathodoluminescence (CL) emission of both natural and synthetic lanthanide-rich phosphates (Ln³⁺: Ce, Nd) previously characterized by X-ray Diffraction (XRD), Environmental Scanning Electronic Microscopy (ESEM) and Energy Dispersive Spectroscopy. The thermal treatment at 700 °C performed on the synthetic sample obtained by chemical precipitation, promotes increasing of the crystallinity degree giving rise to a phase transition from the hexagonal (comprising monazite and rabdophane) into the monoclinic (cerianite and monazite) structures detected by XRD. Despite the size and the morphology of the grains are similar under ESEM, it could be appreciated significant differences among CL signals attending to the shape (with well-defined peaks for the annealed sample) and intensity (with lower emission for the non-thermally pretreated synthetic phosphate). The main wavebands centered at (i) 360, 380 and 490 nm are associated respectively with ⁵D_3/2 → ²F_5/2 and ⁵D_3/2 → ²F_7/2 transitions as well as a redox reaction assigned to the presence of Ce³⁺, (ii) 276, 424, 516 and 531 nm are linked respectively to ²G_9/2→⁴I_9/2, ²P_1/2→⁴I_9/2, ⁴G_9/2→⁴I_9/2 and ⁴G_7/2→⁴I_9/2 Nd³⁺ transitions and (iii) 400–490 nm is due to non-bridging oxygen hole centers related to the tetrahedral PO₄^3- groups or structural defects for the heated synthetic samples. The natural sample from Madagascar, with a very complex CL spectrum, displays a characteristic band emission in the green-yellow and red regions corresponding to [UO₂]²⁺ groups and Sm³⁺ respectively. - Highlights: • Synthetic Monazite were synthesized by Chemical precipitation method. • Cathodoluminescence emission displays spectra with well-defined maxima. • Thermal treatment (700 °C) induces an increase of the crystallinity degree.