Ferroelectricity driven by soft phonon and spin order in multiferroic BiMn 3 Cr 4 O 12

BiMn 3 Cr 4 O 12 shows an unusual joint multiferroicity, which facilitates the coexistence of considerable ferroelectric polarization and remarkable magnetoelectric coupling in a single‐phase multiferroic material. Based on first‐principles calculations, we investigate the two different types of ferroelectric phase transitions in the BiMn 3 Cr 4 O 12 material. Our results show that the first ferroelectric phase transition is driven by soft mode and leads BiMn 3 Cr 4 O 12 into the Cm space group. The predicted ferroelectric polarization in single crystal is about ~9.8 μC/cm 2 . With the emergence of spin order on both Mn and Cr sublattices, it is the polar Cm structure that triggers the exchange striction mechanism and therefore results in a large type‐II multiferroicity (~1.1 μC/cm 2 ). In addition, the intrinsic direction of the spin‐driven ferroelectric polarization is always opposite to that of the existing Cm phase structure. Our results imply a feasible strategy in searching/designing novel type‐II multiferroics with large ferroelectric polarization.