Paramagnetic radiation-induced radicals in calcium pyrophosphate polymorphs

Calcium pyrophosphate (Ca2P2O7) is a promising material for biomedical and optical applications; however, relatively little is documented on the radiation-induced point defects of the material. This study reports on the formation, identity, and properties of X-ray-induced radicals in gamma-, beta-, and alpha-polymorphs of Ca2P2O7. Complementary thermally stimulated luminescence (TSL), electron paramagnetic resonance (EPR), and solid-state nuclear magnetic resonance (NMR) spectroscopy techniques were combined to reveal the effects of ionising radiation on each polymorph. The high temperature polymorphs beta-, and alpha-Ca2P2O7 exhibited diminishing variety and intensity of EPR signals, whereas the opposite trend was observed in the TSL response. TSL glow curves consisted of multiple peaks in the temperature range of 20-350 degrees C, with maximum emission intensity located at approximately 600 nm. Multiple electronic spin 1/2 paramagnetic centres were identified on the basis of spinHamiltonian parameters determined from EPR spectra simulations. Annealing kinetics of the individual paramagnetic centres were determined to provide an interpretation of the complex nature of TSL glow curves. A decrease of the 31P spin-lattice relaxation T1 times after irradiation was detected for all Ca2P2O7 polymorphs. The results highlight the importance of structure, morphology, and synthesis conditions on the formation of charge traps in Ca2P2O7 and expand the knowledge base on the variety of radiation-induced radicals in phosphate materials.