Order Parameters and Phase Diagrams of Multiferroics

The symmetry properties, order parameters, and magnetoelectric phase diagrams of multiferroics are discussed. After brief reviews of Ni${}_3$V${}_2$O${}_8$, TbMnO${}_3$, and RbFe(MoO${}_4$)${}_2$, we present a detailed analysis of RMn${}_2$O${}_5$ (with R=Y, Ho, Dy, Er, Tb, Tm).