Modulating the optical and electrical properties of oxygen vacancy-enriched La2Ce2O7:Sm3+ pyrochlore: role of dopant local structure and concentration

This work presents the important role of defects in achieving efficient luminescence and electrical conduction in pyrochlore materials. Undoped and Sm3+-doped La2Ce2O7 (LCO and LCOS) were stabilized in the defect fluorite structure. LCO depicted visible light emission under ultraviolet irradiation endowed by the defect states with a predominance of oxygen vacancies (V-O). LCOS depicted color-tunable emission from blue to orange on Sm3+ doping and to white on changing the excitation wavelength from 413 to 254 nm. Raman spectroscopy shows an enhancement of the oxygen vacancy density on Sm3+ doping, which is seen in the increased electrical conduction of LCOS compared to that of LCO. Further, based on the DFT-calculated formation energy and cell volume, it was postulated that Sm3+ prefers Ce4+ sites over La3+ sites with the formation of V-O as charge-compensating defects and the most favorable configurations are the Sm-Ce-V-O defect complexes. The higher luminescence lifetime of LCOS is because of the Sm-Ce far away from the oxygen vacancies, while the lower lifetime, on the other hand, is attributed to the same with nearby oxygen vacancies. Positron annihilation lifetime spectroscopy (PALS) suggested the highest defect density in the case of the 7.5% Sm3+-doped LCOS sample. Thermoluminescence measurements on the other hand also show the highest density of deep traps associated with V-O in the 7.5% Sm3+-doped LCOS sample. This was manifested in the enhanced conductance and lowest bulk resistance for this particular sample. Based on the results of this work, the high potential of LCOS for tunable light emission and as an afterglow phosphor and electrical conductor can be clearly seen and future work would further enrich knowledge on its multifunctionality.