Stronger Polar Ferromagnetism and Narrower Optical Band Gaps in Two-Dimensional Hybrid Copper Perovskites Induced by Bromine-Substitution

A ‘dual-action’ halogen-substitution strategy, a heavier atom Br substituted in organic moieties and within inorganic octahedral moieties, results in structures (3/4-BrbaH) 2 CuBr 4 with the highest Curie temperatures and the narrowest optical band gaps.