Paramagnetic defects and luminescence in praseodymium-activated alkaline earth pyrophosphates

Abstract

Establishing defect-property systematics in complex materials remains an elusive challenge, requiring complementary spectroscopic techniques and correlated investigations across chemically related host lattices. This study presents a systematic overview of defect-mediated charge trapping and recombination luminescence in Pr 3+ -activated alkaline earth pyrophosphates (X 2 P 2 O 7 ; X = Ca, Sr, Ba). Phase composition and dopant incorporation are analysed using powder X-ray diffraction (XRD), X-ray absorption (XANES), and photoluminescence spectroscopy techniques. All X 2 P 2 O 7 :Pr 3+ materials exhibit optical transitions in both the ultraviolet (UV) and visible (VIS) spectral regions, while Sr 2 P 2 O 7 and Ba 2 P 2 O 7 additionally show blue-to-UV-C upconversion potential. Charge trapping can be initiated by either X-ray or UV-C excitation, filling trapping states over a broad range of activation energies, with differences between the hosts reflected in the relative populations of shallow and deep traps. In Sr 2 P 2 O 7 :Pr 3+ , 444 nm excitation can additionally populate charge traps and generate UV–VIS persistent luminescence via an excited-state absorption (ESA) mechanism. Electron paramagnetic resonance (EPR) measurements reveal multiple phosphorus-related radicals, which, based on multifrequency spectra simulations, are assigned to PO 4 2− -type and PO 3 2− -type centres. The annealing kinetics of these radicals span over a 25–300 °C range and are partially correlated with maxima observed in TSL glow curves. These results establish a link between phosphorus-centred defects and recombination luminescence, providing a framework for defect-engineered phosphate materials.