Phenomenological Landau analysis of predicted magnetoelectric fluorides: KMnFeF${}_6$ and Ba${}_2$Ni${}_7$F${}_{18}$

Recently, we predicted based on symmetry considerations that KMnFeF${}_6$ and Ba${}_2$Ni${}_7$F${}_{18}$ are likely magnetoelectric multiferroic materials. In this contribution, we investigate with Landau theory and crystal structure considerations the polarization and the linear magnetoelectric effect in these materials. Based on these two examples, we show that any magnetoferroelectric will display additional electrical polarization below its magnetic ordering temperature. This additional electrical polarization is not related to the linear magnetoelectric effect. Its magnitude depends on the dielectric susceptibility.