Hybrid improper ferroelectricity in B-site substituted Ca3Ti2O7: The role of tolerance factor

Abstract

Hybrid improper ferroelectricity is induced by the coupling of oxygen octahedral rotation and tilting, and a promising way to create multiferroic materials with a strong magnetoelectric effect can be proposed. Although some hybrid improper ferroelectric materials with Ruddlesden-Popper structures have been discovered experimentally, the small polarizations and high coercive fields will severely limit their practical applications. To deal with this issue, an alternative way is proposed by the first-principles calculations in the present work. By calculating the energies of phases with oxygen octahedral rotation and tilting in Ca3TiMO7 (M = Si, Ge, Mn, Sn, Hf, and Zr) compounds with five B-site ordering configurations, it is found that the tolerance factor has a considerable impact on their ferroelectric properties. The energy barriers and ferroelectric polarizations increase with decreasing tolerance factor. Moreover, the individual-layer contributions to the total polarizations depend on the space group, which is also decided by the tolerance factor of the compound. Therefore, to enhance the ferroelectric polarization in B-site substituted Ca3Ti2O7, the tolerance factor should be decreased according to the calculation results of this work.