Disorder stabilisation of high pressure Mn 2-x Co x ScSbO 6 double perovskites with complex magnetic properties

Abstract

High-pressure synthesis provides a powerful route to stabilise metastable oxides with unconventional structural and magnetic properties. Here we show that Mn2-xCoxScSbO6 (x = 0.5, 1 and 1.5) solid solutions synthesised at 6 GPa pressure form double perovskites (space group P21/n) whereas the previously-reported x = 0 and 2 end members adopt the corundum-derived Ni3TeO6 structure-type (R3) at comparable pressure. This is attributed to entropy-stabilisation of the A2BB'O6 double perovskite phase, and neutron diffraction reveals substantial cation disorder with approximate distribution (Mn2-xSc0.5Cox-0.5)(Sc0.5Co0.5)SbO6 for 0.5 < x < 1.5. The long range antiferromagnetic order previously reported for x = 0 at TN = 22 K is increasingly suppressed by Co-doping. The x = 0.5 sample also orders antiferromagnetically (TN = 18 K) with ordered moments rotated and reduced in magnitude relative to x = 0, but x = 1 and 1.5 have spin-freezing transitions at Tg = 15 and 8 K respectively. This study demonstrates that interesting new magnetic materials may be accessed through a combination of high pressure synthesis and entropy-stabilisation of cation-disordered phases.2