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The crystal structure, electronic structure and optical properties of Ba2RETaO6 (RE = Y, La, Pr, Sm, Gd) have been studied by first-principles calculation. The calculated lattice parameters are in good agreement with the previously reported values. With increasing the atomic number of RE (i.e., the number of 4f electrons), the energy level of RE 4f bands becomes lower continuously. The relationship between the electronic structure and optical properties is explored based on first-principles calculation. The electron transitions between O 2p states, RE 4f states and Ta 5d states have a key effect on optical properties such as dielectric function, refractive index, absorption coefficient and reflectivity. The phase structures have great influence on the optical properties of Ba2SmTaO6 and Ba2GdTaO6, and the big variation of reflectivity induced from phase transition makes them have potential applications in the infrared radiation protection area. [ABSTRACT FROM AUTHOR]