Molybdenum chloride double perovskites: dimensionality control of optical and magnetic properties.

Halide double perovskites are a promising class of semiconducting materials for applications in solar cells and other optoelectronic devices. Recently, there has been a surge of interest in these materials to study phenomena beyond optoelectronics, especially magnetism. Here, we report three new Mo 3+ (4d 3 ) based chloride double perovskites: a 3-D rock-salt ordered Cs 2 NaMoCl 6 , a 1-D chain (MA) 2 AgMoCl 6 and a Dion-Jacobson type 2-D layered (1,4-BDA) 2 AgMoCl 8 (MA = methylammonium; 1,4-BDA = 1,4-butanediammonium). Their structures and dimensionalities can be tuned by means of the A-cation. The measured bandgaps are relatively narrow (2.0-2.1 eV) which show a blueshift on reducing the dimensionality. At low temperatures, we observe antiferromagnetic coupling between the nearest-neighbour Mo 3+ ions in all these systems. Cs 2 NaMoCl 6 shows stronger coupling with a frustration index f of 5 which we attribute to the geometrically frustrating fcc lattice of Mo 3+ ions. This work expands the scope of halide double perovskites beyond main group metals and beyond optoelectronics, and we hope that it will lead to future developments in magnetic halide perovskites.