APb2(C7H3NO4)2I (A = K, Rb, Cs): rare stable nonlinear optical crystals with second-harmonic generation response and highly distorted lead core coordination polyhedra

APb 2 (C 7 H 3 NO 4 ) 2 I (A = K, Rb, Cs) features a 3D NCS cubic framework consisting of highly distorted [PbNO 5 ] and [PbNO 4 I] coordination polyhedra, a moderate SHG response, a wide transparent window and a high thermal stability above 300 °C.