Tailoring defect structure and dopant composition and the generation of various color characteristics in Eu3+ and Tb3+ doped MgF2 phosphors

A novel approach to generate a wide range of color characteristics such as near white, yellow, orange and red in MgF2, by proper tailoring of the defect structure and varying the composition of Eu3+ and Tb3+ dopant ions have been presented here. It has been observed from positron annihilation lifetime spectroscopy (PALS) study that various defect centers such as mono vacancies and their cluster forms exist in the system, whose amount varies upon varying the dopant ion's composition. The experimentally observed positron lifetime values of the defect centers also matched well with the theoretically calculated lifetime values using the MIKA-DOPPLER package. It has been found that a few vacancies or defect centers act as color centers, while the cluster vacancies change the local symmetry of the rare earth ion by inducing more distortion surrounding them thereby resulting in different emission characteristics in the photoluminescence (PL) study. The defect-related host emission in combination with the green and red emission from Tb3+ and Eu3+ ions generated near-white-light in some of the compounds, while other compounds showed a variety of other color characteristics due to the Tb3+ → Eu3+energy transfer dynamics. The various defect-related emissions, the role of the defect-related trap state in the decay kinetics and the energy-transfer dynamics were also understood by analyzing the electronic structure using HSE06 hybrid functional calculation.