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 (Ln(3+): Ce, Nd) previously characterized by X-ray Diffraction (XRD), Environmental Scanning Electronic Microscopy (ESEM) and Energy Dispersive Spectroscopy. The thermal treatment at 700 degrees 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 D-5(3/2) -> F-2(5/2) and D-5(3/2) -> F-2(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 (2)G(9/2) -> I-4(9/2), P-2(1/2) -> I-4(9/2), (4)G(9/2) -> I-4(9/2) and (4)G(7/2) -> I-4(9/2) Nd3+ transitions and (iii) 400-490 nm is due to non-bridging oxygen hole centers related to the tetrahedral PO43- 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.