[C(NH2)3]Pb(C2O4)Cl·H2O: A halogen- and lone-pair-tuned UV crystal with large birefringence originated from cooperative π-conjugation

Development of new birefringent crystals with enhanced properties remains an ongoing research priority. Through the exploration of guanidinium-containing oxalates with the structural modulation of Pb 2+ and Cl - , a new birefringent crystal, [C(NH 2 ) 3 ]Pb(C 2 O 4 )Cl·H 2 O (GPCCH), was successfully obtained. GPCCH was characterized by two types of π-conjugated building units, which contributes to the coexistence of a wide bandgap of 3.98 eV and a large birefringence of 0.24@546 nm. These findings highlight the synergistic role of π-conjugated building units and partially-chlorinated lead-containing units played in generating fascinating UV birefringent crystals.