First-principles study of the structural, electronic, magnetic, and ferroelectric properties of a charge-ordered iron(ii)-iron(iii) formate framework

Abstract

Density functional theory calculations have been performed for the structural, electronic, magnetic and ferroelectric properties of a mixed-valence Fe(II)-Fe(III) formate framework [NH2(CH3)2][Fe III Fe II (HCOO)6] (DMAFeFe). Recent experiments report a spontaneous electric polarization and our calculations are in agreement with the reported experimental value. Furthermore, we shed light into the microscopic mechanism leading to the observed value and as well how to possibly enhanced the polarization. The interplay between charge ordering, dipolar ordering of DMA + cations and the induced structural distortions suggest new interesting directions to explore in these complex multifunctional hybrid perovskites.

Spin-density studies of the multiferroic metal-organic compound [NH2(CH3)2][FeIIIFeII(HCOO)6]

“…To shed light on this point an accurate structural model below 3.5 K will be needed. The local magnetic moments, considering collinear spins coupled antiferromagnetically, were also calculated by Tang et al (2019). The calculated values of the spin moments of Fe II and Fe III ions were |3.7| B and |4.3| B , respectively, which agree well with our results at 10 K [obtained considering the nuclear structure factor (F N ) derived from the structural model obtained in the space group R3c].…”

“…From the data refinement, the models discard the occurrence of any spin density on the counterion sites. Accordingly, the sign of the spin moments and the sum of the vertical contributions correspond well with those expected from the magnetometry measurements and the theoretical calculations (Tang et al, 2019).…”

“…Furthermore, a possible spin delocalization -that can only be detected by polarized neutron diffraction -could have implications for the magneto-electric behaviour. A possible delocalization towards the counterion may give an indication of the origin of the magneto-electric coupling, while a delocalization towards the oxygen atoms surrounding the magnetic ions could help to explain the observed behaviour at low temperature, in line with the mechanism proposed by Tang et al (2019), since it may be related to a distortion of the coordination octahedron around the iron atoms that could give rise to electric polarization.…”

“…The ferroelectric signal observed at low temperature in this compound was explained by Tang et al (2019), through density functional theory (DFT) calculations, by a symmetry breaking from R3c to R3c as a possible origin of the total polarization. Their study reveals that the small polarization originates mainly from the off-centred Fe III -O octahedron, and therefore has a structural origin.…”

“…Anomalies in the heat capacity and polarization measurements also suggest the onset of a ferroelectric phase below 3.5 K (Cañ adillas- Delgado et al, 2012;Guo et al, 2017). The electric behaviour of this compound has been the subject of alternative explanations, such as the presence of spinarrangement defects promoting a magnetoelectric coupling effect (Guo et al, 2017) or a symmetry breaking compatible with a small polarization mainly from the off-centred Fe III -O octahedron (Tang et al, 2019).…”