Evolution of different defect clusters in Eu3+ doped KMgF3 and Eu3+, Li+ co-doped KMgF3 compounds and the immediate consequences on the phosphor characteristics

Defects in lattice have both positive and negative implication on the optical properties of light emitting materials and a great effort is required to understand the defect structure surrounding the activator ion in a phosphor materials. Fluoride based perovskite class of compounds such as KMgF3 are always associated with cationic and anionic vacancies which may give rise to interesting optical properties. They may also play an important role in determining the phosphor characteristics, when activator such as Eu3+ ion is doped in KMgF3. This is because defects may alter the local structure of Eu3+ ions, whose characteristics transitions are very sensitive to local environment. In a fast attempt of its kind, we have tried to present a complete picture of the defect structure both in undoped and Li+, Eu3+ ions doped KMgF3 and their influence on phosphor characteristics. By analysing the changes in the defect structure due to changes in the dopant ions, using positron annihilation lifetime spectroscopy supported by EPR and theoretical calculation, it was observed that different defect cluster are being evolved upon changing the dopant ions from Eu3+ to Li+ as co-dopant ion. These changes have a direct impact on emission characteristics, especially the colour can be tuned to more deep red region. Using Time Resolved Spectroscopy two different Eu3+ components adjacent to different defect cluster are isolated. The cluster due to K-vacancies found to change their configuration upon co-doping Li+ ion; VC1 for Eu3+ doped and VC2 for Eu3+ and Li+ co-doped compounds. From the relative percentage of these cluster vacancies, it was possible to make a conclusion about the distribution of Eu3+ and Li+ ions among the different unit cells of KMgF3, which have different defect structure. The study will be helpful in tailoring the defect structure while designing an efficient phosphor material.