Structural and spectroscopic features of Ca9M(PO4)7 (M = Al3+, Lu3+) whitlockites doped with Pr3+ ions

In this work we present the structural and spectroscopic properties of Ca9M(PO4)7 (M = Al, Lu) whitlockite compounds doped with Pr3+ ions. The Al containing samples were prepared by the citrate route, while the Lu based samples were prepared by standard solid state reaction. The structural properties were investigated by XRD measurements and Rietveld analysis. Detailed spectroscopic properties like emission and excitation spectra, luminescence kinetics and luminescence temperature quenching were measured to determine the influence of different trivalent host metal on the Pr3+ ions. Pr3+ enter in the three Ca2+ sites in the Ca9Al(PO4)7 compounds, creating some defect to compensate the charge mismatch, whereas in the Ca9Lu(PO4)7 Pr3+ ions occupy four Ca2+/Lu3+ sites, where no charge compensation is needed. The emission spectra are similar for both materials. Efficient quenching of the 1D2 emission was observed, while the 3P0 emission remains stable for all dopant concentrations. Decay times were found to be non-single exponential due to the occupation of different sites by the Pr3+ ions. The luminescence temperature quenching measurements have revealed that two different mechanisms (multi-phonon relaxation and cross-relaxation processes) are responsible for the emission quenching