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Ba(MoO2F)2(XO3)2 (X = Se and Te): First Cases of Noncentrosymmetric Fluorinated Molybdenum Oxide Selenite/Tellurite Through Unary Substitution for Enlarging Band Gaps and Second Harmonic Generation

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posted on 22.10.2020, 16:33 by Lin Lin, Xingxing Jiang, Chao Wu, Longhua Li, Zheshuai Lin, Zhipeng Huang, Mark G. Humphrey, Chi Zhang
Nonlinear optical (NLO) materials have critically important applications in advanced laser technologies. However, achieving a good balance between the mutual competing NLO properties and band gap within one molecular structure remains a great challenge. In this study, two alkaline earth metal fluorinated molybdenum oxide selenite/tellurite, Ba­(MoO2F)2(XO3)2 [X = Se (BMFS) and Te (BMFT)], were synthesized through a facile unary substitution: BMFS was obtained by partial substitution of oxygen atoms with highly electronegative fluorine in the parent compound BaMo2O5(SeO3)2 (BMS), while BMFT was achieved by further replacing lone-pair Se4+ cations in BMFS with heavier Te4+ cations in the same main group. By partially replacing oxygen with fluorine, BMFS shows a broadened band gap and enhanced second harmonic generation (SHG) response compared to BMS owing to the high electronegativity of fluorine anions and the favorable orientation and alignment of NLO-active [MoO5F]5– and [SeO3]2– groups, which is relatively rare for unary anion substitution. BMFS and BMFT are isostructural and both belong to the polar space group Aba2, featuring a three-dimensional (3D) double-layered framework composed of 2D [MoO4F­(XO3)] anionic layers interconnected by divalent barium cations. Both BMFS and BMFT exhibit good optical performance, including large SHG responses (3× and 4× KH2PO4), wide band gaps (3.30 and 3.27 eV) and optical transparency window, and high laser damage thresholds (60× and 53× AgGaS2), demonstrating their potential applications as promising second-order NLO crystals. DFT calculations have elucidated the crucial role of the [MoO5F]5– groups in the enlarged band gaps and enhanced SHG responses in BMFS and BMFT. This work proposes a feasible unary substitution strategy for synthesizing the first polar fluorinated molybdenum oxide selenite/tellurite with synchronously enlarged band gaps and SHG efficiency.