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 (4d) based chloride double perovskites: a 3-D rock-salt ordered CsNaMoCl, a 1-D chain (MA)AgMoCl and a Dion-Jacobson type 2-D layered (1,4-BDA)AgMoCl (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 ions in all these systems. CsNaMoCl shows stronger coupling with a frustration index of 5 which we attribute to the geometrically frustrating fcc lattice of Mo 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.