Crystal Structure and Site Symmetry of Undoped and Eu3+ Doped Ba2 LaSbO6 and BaLaMSbO6 Compounds (M=Mg,Ca): Tuning Europium Site Occupancy to Develop Orange and Red Phosphor.

Double perovskite antimonates of the type BaLaMSbO6 (M=Mg, Ca) were synthesized by a standard solid-state route. The compounds were characterized by X-ray crystallography and the structures were refined using Rietveld method. BaLaMgSbO6 and BaLaCaSbO6 crystallized in monoclinic space groups (I2/m) and (P21 /n), respectively. In both compounds, La occupied the A-site of perovskite, which is 12-coordinated as compared to Ba2 LaSbO6 where La ion shifts to the B-site octahedral coordination due to the larger size of Ba as compared with Mg and Ca. The samples were further characterized using FTIR and the frequency of the octahedral vibration is correlated to the electronegativity of the B-site ions. Photoluminescence study of the title compounds and Ba2 LaSbO6 was carried out upon doping with 2 atom% Eu3+ ion, which confirmed that Eu3+ occupies distorted 12-coordinated A-site in BaLaMSbO6 (M=Mg, Ca) and symmetrical octahedral B-site in Ba2 LaSbO6 . Furthermore, the emission spectrum corresponding to each Eu3+ ion at different crystal site was successfully isolated through a TRES study. This site selective occupancy of Eu3+ ion also has a direct impact on the light emission, which was found to change from orange to red in a dark room in the order Ba2 LaSbO6  : Eu→BaLaCaSbO6  : Eu→BaLaMgSbO6  : Eu. Such an outcome will have high impact in designing new commercial Eu3+ ion doped phosphor materials and tailoring of their optical properties.