Noncentrosymmetric Tellurite Halides Created by Depolymerization Strategy: Toward Strong SHG Intensity and Wide Bandgap

Design and synthesis of nonlinear optical (NLO) materials with strong second harmonic generation (SHG) effect and wide band gap is a long-standing challenge because of their inverse relationship, especially for metal tellurites. We found that a lower degree of polymerization of tellurite groups corresponds to a higher proportion of non-centrosymmetric (NCS) structures formed. Based on Pauling's rule #4, introduction of cations (Al3+/Ga3+) with higher coordination numbers and lower valence states than Te4+ into the reaction system decreases the connectivity among tellurite groups. By regulating the reaction conditions, CS Ga(Te3O7)Br and NCS M2(OH)(TeO3)(Te2O5)X (M = Al, Ga; X = Br, Cl) were successfully synthesized, which represent the first examples of Ga/Al-Tellurite-Cl/Br. The NCS compounds have presented strong SHG intensities (4.0-9.8 × KDP) and wide bandgaps (3.78-4.30 eV). And M2(OH)(TeO3)(Te2O5)Cl (M = Ga, Al) are the only examples of metal tellurites with band gap exceeding 4.0 eV and SHG intensity exceeding 4 × KDP. In addition, their ultraviolet (UV) cut-off edges are below 280 nm (271, 261 nm), even reaching the solar blind UV range (200-280 nm). This work provides an effective strategy for design and synthesis of tellurite-based NLO materials with strong SHG effect and wide bandgap.