Synthesis and characterization of multiferroic BiMn$_7$O$_{12}$

We report on the high pressure synthesis of BiMn$_7$O$_{12}$, a manganite displaying a quadruple perovskite structure. Structural characterization of single crystal samples shows a distorted and asymmetrical coordination around the Bi atom, due to presence of the $6s^{2}$ lone pair, resulting in non-centrosymmetric space group Im, leading to a permanent electrical dipole moment and ferroelectric properties. On the other hand, magnetic characterization reveals antiferromagnetic transitions, in agreement with the isostructural compounds, thus evidencing two intrinsic properties that make BiMn$_7$O$_{12}$ a promising multiferroic material.

Structural anomalies at the magnetic transition in centrosymmetric BiMnO$_3$

The structural properties of BiMn$O_3$ were determined from neutron powder diffraction data as a function of temperature and magnetic field. The structure at all temperatures was found to be centrosymmetric with space group $C2/c$, which is incompatible with ferroelectricity. At T$_Csimeq 100 K$, we observed the onset of a large magnetoelastic strain, proportional to the square of the magnetization. We interpret this structural rearrangement, together with the previously observed magnetodielectric anomalies, as due to the need to optimize the partially frustrated magnetic interactions.