Different microscopic mechanisms found in type-I multiferroics. (a) In “mixed” perovskites with ferroelectrically active ions (green circles) and magnetic ions (red), shifts of ions from the centers of octahedra (yellow plaquettes) lead to polarization (green arrows), coexisting with magnetic order (red arrows). (b) In materials like and , the ordering of lone pairs (yellow ”lobes”) of and ions (orange), contributes to the polarization (green arrow). (c) In charge ordered systems, the coexistence of inequivalent sites with different charges, and inequivalent (long and short) bonds, leads to ferroelectricity. (d) The “geometric” mechanism of generation of polarization in  describes the tilting of a rigid block with a magnetic remaining at the center. Because of the tilting, the bonds form dipoles (green arrows), and there appears two “down” dipoles per one “up” dipole so that the system becomes ferroelectric (and multiferroic when spins order at lower temperatures).