One of the most promising groups of molecular photomagnetic materials are Co-Fe Prussian blue analogues (i.e. compounds with the same structure and similar chemical make up to Prussian blue.) A Prussian blue analogue has a chemical formula M 1-2x Co 1+x [Fe(CN) 6 ]•zH 2 O where x and z are variables (z may be zero) and M is an alkali metal. Prussian blue analogues have a face centre cubic structure.