Noncentrosymmetric tellurite halides created by a depolymerization strategy: toward strong SHG intensity and wide bandgap†

The design and synthesis of nonlinear optical (NLO) materials with a 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 (3.89–4.35 eV). And M₂(OH)(TeO₃)(Te₂O₅)X (M = Ga, Al; X = Br, Cl) are the only examples of metal tellurites with band gap exceeding 4.0 eV and SHG intensity exceeding 4 × KDP. This work provides an effective strategy for the design and synthesis of tellurite-based NLO materials with a strong SHG effect and wide bandgap.