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 (Al/Ga) with higher coordination numbers and lower valence states than Te into the reaction system decreases the connectivity among tellurite groups. By regulating the reaction conditions, CS Ga(TeO)Br and NCS M(OH)(TeO)(TeO)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 M(OH)(TeO)(TeO)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.