Orbital Hybridization of the Ferroelectric Rb₂Cd₂ (SO₄)₃: Origin of Domain Walls

We perform first-principle calculations for the study of the orthorhombic Rb₂Cd₂ (SO₄)₃ structure. Electronic energy bands, total and partial densities of states are reported and analysed. It is found that oxygen atomic 2p electrons strongly hybridize with Rb/or Cd 4d and S 2p states, resulting in two-type ionic groups with weak couplings. It is shown that macroscopic domain walls originate from such weak-coupling ionic groups, arising at the cell boundaries. The asymmetric cation bonds (Rb--O and Cd--O) and the subsequent rotations of the SO₄ tetrahedra can lead to the driving force of the ferroelectric behaviour. The predicted pyroelectric current effects are observed experimentally in the ferroelectric phase.