Paramagnetic Intrinsic Point Defects in Alkali Phosphate Glasses: Unraveling the P3 Center Origin and Local Environment Effects

In this work, we carry out a first-principles investigation of intrinsic paramagnetic point defects in P2O5 and in Na2O−P2O5 glasses as a representative of alkali phosphate glasses. Glass models are generated by combining classical molecular dynamics and Monte Carlo simulations and validated by comparing their corresponding structure factors with the available X-ray and neutron scattering experiments. We use density functional theory to calculate the electron paramagnetic resonance parameters for a large set of paramagnetic oxygen-vacancy configurations. Our investigation, also by unveiling the effect of the local environment and disorder on the hyperfine tensor, enables us to propose a new model for the much debated P3 center. In particular, we establish the occurrence of two variants, which we name Pa3 and Pb3 centers, that are instrumental to explaining the experimental shifts of the hyperfine splittings observed in alkali phosphate glasses as a function of the alkali content x in the phosphate glass. Our scenario predicts that for low to intermediate alkali contents (0 < x < 50%), a mixture of P1 and Pa3 centers should be generated under irradiation. For x > 50%, essentially only Pa3 and Pb3 centers would be generated, while P1 will be absent. Therefore, our findings, by providing an improved mapping of P centers in phosphate glasses, pave the way for fine-controlling/tuning the optical absorption in a wide range of technological applications.