| Summary: | In recent experiments, a significant band gap widening was observed when Sb was substituted for Bi in the double-perovskite Ba _2 PrBiO _6 . In this work, we study a series of double-perovskites, Ba _2 Bi ^III Bi ^V O _6 , Ba _2 PrBiO _6 , and Ba _2 PrSbO _6 using the first-principles density functional theory with the Heyd-Scuseria-Ernzerhof hybrid functional to investigate the substitution effect on the structural and electronic properties. We find that the two topmost valence bands are disappeared on the substitution of Pr ^III for Bi ^III , and the two bottommost conduction bands are disappeared on the substitution of Sb ^V for Bi ^V , causing the significant band gap widening. Further, our calculation suggests that the Ba _2 PrPr _Bi O _6 is a possible candidate as a source of the Pr ^IV signal observed in the experiment. We find that the B -site disordering atomic configuration, Ba _2 B ″ ^V B ′ ^III O _6 , are restored to those of the original structures. On the other hand, our results also suggest the importance of the partial B -site disorder to explain the experimentally observed band gaps.
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