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

We report on the high-pressure synthesis of ${\text{BiMn}}_7{\text{O}}_{12}$, 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 $6{\text{s}}^2$ lone pair, resulting in noncentrosymmetric 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, and the dielectric constant shows anomalies matching the magnetic transition temperatures. ${\text{BiMn}}_7{\text{O}}_{12}$ is therefore a promising multiferroic material, with evidence of coupling between magnetic and dielectric properties.

Figure 1

(Color online) (a) ${\text{BiMn}}_7{\text{O}}_{12}$ cell viewed along the [001] direction. The site definition corresponds to the general formula $A{A}_3^{\prime }{B}_4{\text{O}}_{12}$. (b) ORTEP perspective view of the ${\text{BiMn}}_7{\text{O}}_{12}$ structure refined in $Im$ (left) and $I2/m$ (right) space groups. Displacement ellipsoids are drawn in both cases at the 50% probability level. The arrows in the left panel point out the Bi displacement direction. (c) Distorted dodecahedral coordination around the Bi atom in ${\text{BiMn}}_7{\text{O}}_{12}$; the weakest interactions are indicated by dashed bonds.

Figure 2

Bright field (a) and high-resolution (b) TEM images taken in the [11$\overline{1}$] zone axis showing the presence of twinning domains in a ${\text{BiMn}}_7{\text{O}}_{12}$ sample. Fast Fourier transform of the HREM image is shown in (c).

Figure 3

ZFC (filled symbols) and FC (open symbols) dc magnetization curves (upper panel) and inverse susceptibility (lower panel) in applied magnetic field $H=10\text{Oe}$: (○) and (●) polycrystal; (△) single crystal. Upper inset: secondary transition in the magnetization curve at $T\approx 100\text{K}$. Lower inset: dielectric constant measured at selected frequencies: (◇) 3, (◻) 125, and (▽) 500 kHz.