Disorder stabilisation of high pressure Mn2−xCoxScSbO6 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 Mn_2−xCo_xScSbO₆ (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 behaviour is consistent with disorder (configurational entropy) assisted stabilisation of the A₂BB′O₆ double perovskite phase under the reported high-pressure synthesis conditions, and neutron diffraction reveals substantial cation disorder with approximate distribution (Mn_2−xSc_0.5Co_x−0.5)(Sc_0.5Co_0.5)SbO₆ for 0.5 < x < 1.5. The long range antiferromagnetic order previously reported for x = 0 at T_N = 22 K is increasingly suppressed by Co-doping. The x = 0.5 sample also orders antiferromagnetically (T_N = 18 K) with ordered moments rotated and reduced in magnitude relative to x = 0, but x = 1 and 1.5 exhibit probable spin-freezing transitions (at T_g = 15 and 8 K respectively) without observed long-range magnetic order. This study demonstrates that interesting new magnetic materials may be accessed through a combination of high pressure synthesis and disorder-stabilisation of mixed-cation phases.