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The exploration and improvement of excellent mid-infrared nonlinear optical (NLO) materials is still in great demand. The perovskite-type Sr2Nb2O7 is found to be a good predictor of NLO properties due to the similar band gap and NLO coefficients to KTiOPO4 (KTP). In this paper, we systematically investigated the electronic structures, linear and nonlinear properties of pure and V-doped Sr2Nb2O7 based on DFT. It is found that the V-doped Sr2Nb2O7 doubles the NLO coefficient and exhibits higher birefringence, which is moderate and sufficient for phase-matching. The results are helpful to the further development of the mid-infrared NLO crystals with perovskite structure.

In order to effectively utilize the visible solar light to catalyze and decompose water into hydrogen and oxygen, the ultraviolet light responsive photocatalyst Sr2Ta2O7 is engineered via co-doping of the anionic (S) and cationic (V/Nb) to shift the VBM (valence band maximum) upward and CBM (conduction band minimum) downward by approximately 1 eV, respectively. By first principles study (GGA-PBE), it is discovered that the VBM of (S, V) co-doped Sr2Ta2O7 is shifted upward by 1.14 eV while the CBM of this system is shifted downward by 0.68 eV, which is due to the appearance of impurity states near the VBM and CBM, respectively. For (S, Nb) co-doped Sr2Ta2O7, the VBM is shifted upward by 0.74 eV and the CBM is shifted downward by 0.63 eV. No impurity states of Nd are created, neither near the CBM nor in the band gap for (S, Nb) co-doped case, probably because the Sr2Ta2O7 is essentially isostructural with Sr2Nb2O7. For these two systems mentioned above, it could be a feasible way for Sr2Ta2O7 photocatalyst on water splitting under UV irradiation to convert into the visible light-utilizing material without introducing extra electrons or holes.