Crystal Structure and Site Symmetry of Undoped and Eu Doped Ba LaSbO and BaLaMSbO Compounds (M=Mg,Ca): Tuning Europium Site Occupancy to Develop Orange and Red Phosphor.

Double perovskite antimonates of the type BaLaMSbO (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. BaLaMgSbO and BaLaCaSbO crystallized in monoclinic space groups (I2/m) and (P2 /n), respectively. In both compounds, La occupied the A-site of perovskite, which is 12-coordinated as compared to Ba LaSbO 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 Ba LaSbO was carried out upon doping with 2 atom% Eu ion, which confirmed that Eu occupies distorted 12-coordinated A-site in BaLaMSbO (M=Mg, Ca) and symmetrical octahedral B-site in Ba LaSbO . Furthermore, the emission spectrum corresponding to each Eu ion at different crystal site was successfully isolated through a TRES study. This site selective occupancy of Eu 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 Ba LaSbO  : Eu→BaLaCaSbO  : Eu→BaLaMgSbO  : Eu. Such an outcome will have high impact in designing new commercial Eu ion doped phosphor materials and tailoring of their optical properties.