Ordered double perovskites (also known as elpasolites) have the general formula A2BB’O6 where A is generally a rare earth metal or alkaline earth metal and where B and B’ are typically transition metal or main group metal cations. There is a wide variety of structural and compositional diversity of such perovskites. As such this class of materials exhibits a wide range of physical properties which are controlled by doping, temperature and pressure. The aristotype double perovskite would be cubic, however the majority of double perovskites crystallise in a lower symmetry as a result of co-operative octahedral tilting. Changes in temperature and pressure are known to cause phase transitions in the ordered perovskites in that the thermodynamic variable is able to change the sample volume and the octahedral tilts are able to react to these volume changes.