Atomically sharp domain walls of Rb2Cd2(SO4)3 langbeinites: 4d electron effects

First-principle calculations are performed for the study of Rb2Cd2(SO4)3 langbeinites. Electronic bands, joint and partial densities of states as well as charge density distribution are analyzed systematically. We find that because oxygen atomic 2p electrons strongly hybridize with Rb or Cd 4d and S 2p states, there exist two-type ionic groups in the langbeinite structures. The couple strength between the cation bonds and the SO4 tetrahedra in the monoclinic phase is stronger than that in the orthorhombic. Hybridization effects of 4d electrons are the origin of monoclinic paraelectricity and orthorhombic ferroelectricity, relating to atomically shape domain wall rotations. As a result, the asymmetric cation bonds (RbO and CdO) and the subsequent rotations of the SO4 tetrahedra can lead to the driving force of the ferroelectric behaviors. It is shown that atomically shape domain walls originate from such coupling ionic groups, arising at the cell boundaries. The predicted pyroelectric current effects were observed experimentally in the ferroelectric phase.