CaFeFeNbO 6 - an iron-based double double perovskite.

Ordering of cations is important for controlling properties of ABO 3 perovskites, and CaFeFeNbO 6 is the first example of an Fe-based AA'BB'O 6 double double perovskite, with Ca 2+ /Fe 2+ ordered on A-site columns, and Fe 3+ /Nb 5+ at the octahedral B-sites. Substantial (37%) antisite disorder of the latter cations leads to spin glassy magnetism below a freezing transition at 12 K. The CaMnFeNbO 6 analogue also shows substantial cation disorder and spin glassy behaviour. Comparison of synthesis pressures for ordered materials based on different A-site transition metals, suggests that pressures of at least 14-18 GPa will be required to discover the expected plethora of double double perovskites based on A' cations smaller than Mn 2+ .