Ferroelectric phase transition and spontaneous electric polarization in CaMn7O12 from first principles

Abstract

The phase transition mechanism and ferroelectric polarization of CaMn _7 O _12 are investigated by using the density functional theory. Our results show that the P 3 space group should be the ground-state structure with R 3 as the intermediate phase. It is the helicoidal magnetic order that induces the first transition to the R 3 phase while the second transition to P 3 structure results from soft modes under the constraint of the magnetic structure. The two phase transitions from $R\bar{3}$ through R 3 to P 3 are second order and first order, respectively, which is consistent with experimental observations. Group theoretical analysis shows that the particular domain states and multi-domain structure of the P 3 phase match with the observed coexistence of two magnetic modulations below T _N2 = 48 K. Our calculated electric polarization is also in good agreement with experiment. By analyzing the polarization contributions from both mode and atomic-decomposition viewpoints, we find that the Raman-type distortions give rise to a significant contribution to the total polarization. This unexpected result can be understood through the asymmetric change of the Born effective charges caused by the particular helicoidal magnetic order, which leads to an abnormal infrared character of the purely Raman-active modes.

Keywords

• magnetic multiferroics
• CaMn7O12
• phase transition
• electrical polarization
• first principles