Polar vs chiral magneto-structural coupling in cubic quadruple perovskites

We report the high-pressure, high-temperature synthesis and characterisation of the quadruple perovskite CaMn₃(Cr₃Mn)O₁₂. Neutron powder diffraction experiments showed that below T_N = 155 K the B transition metal sublattice adopts long range G-type antiferromagnetic order while the A' transition metal sublattice remains paramagnetic until T₂ = 55 K, below which it also develops G-type order. Unlike the related multiferroic compound LaMn₃Cr₄O₁₂, which displays magnetically-induced ferroelectricity below T₂, we found that CaMn₃(Cr₃Mn)O₁₂ remained non-polar down to the lowest measured temperatures. We propose a phenomenological model of magneto-structural coupling based on competing free-energy terms that couple polar and chiral distortions, respectively, depending on the global direction of magnetic moments. The model naturally explains why some compounds in this family become ferroelectric below T₂, while others do not. The phenomenology is applicable to all cubic Im-3 quadruple perovskites that support antiferromagnetic G-type order on both A’ and B sites, implies a novel approach towards multiferroic functionality, and can be generalised to other multi-sublattice systems where the magnetic interaction between sublattices is prohibited by spatial inversion.