Structure and UV Emission Characteristics of Gd3+-activated X2P2O7 (X = Ca, Sr, Ba) Pyrophosphates

: Ultraviolet-emitting phosphors deliver higher-energy photons than visible or infrared sources, which is a prerequisite for efficient photoionisation and energy transfer processes. In this work, gadolinium-activated alkaline-earth pyrophosphates (X 2 P 2 O 7 , X = Ca, Sr, Ba) are comparatively investigated to determine how host composition and structure influence Gd 3+ incorporation, irradiation-induced defect formation, and UV-B persistent luminescence characteristics. Reflection electron energy-loss spectroscopy (REELS) indicates that all compounds possess suitable bandgap values for UV emission. X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) analyses confirm successful incorporation of Gd 3+ ions at low-to-moderate doping concentrations into the X 2 P 2 O 7 hosts. All materials exhibit the characteristic Gd 3+ 6 P 7/2 → 8 S 7/2 UV-B photoluminescence emission at 312 nm, which is also observed in thermally stimulated luminescence (TSL) measurements following X-ray excitation, with each host showing distinct glow peak temperatures. Spin-Hamiltonian parameters derived from EPR spectra simulations reveal that X-ray irradiation induces the formation of oxygen-related hole centres and phosphorus-related radicals. Among the studied compounds, Sr 2 P 2 O 7 : Gd 3+ exhibits optimal trap depths for intense UV-B persistent luminescence, maintaining radiance above 5 ⋅ 10 -4 mW m -2 sr -1 for nearly two hours after excitation. These results establish Gd 3+ -doped pyrophosphates as efficient UV-B phosphors with tuneable defect structures suitable for phototherapy, sterilisation, and photocatalytic applications.