Structural Phase Transitions in Layered Perovskite CsFeF4${\rm CsFeF}_{4}$ and Weak Polar Response in CsFeF4${\rm CsFeF}_{4}$/RbFeF4${\rm RbFeF}_{4}$ Superlattice

Abstract In the search for novel magnetically and ferroelectrically active compounds, layered materials have served as an ideal playground for understanding and engineering such properties. Within this type of compounds, the fluoride‐based Dion‐Jacobson (DJ) family ( = 1) are poorly explored and several issues related to the group‐to‐subgroup phase transitions remain unclear. Here, the symmetry‐allowed structural instabilities of , as a prototype among this magnetically active family of compounds, are examined and highlighted. Based on soft modes present at the high‐symmetry ‐points of the Brillouin zone, the sequence of allowed structural phase transition is re‐examine using group theory analysis and first‐principles calculations within the density‐functional theory framework. The physical reasons behind the appearance of in‐phase octahedral rotations over the out‐of‐phase ones are also described. Although this member of the DJ family has not shown any polar structure in its subsequent transitions, which are generally present when 1, the [001] (/( superlattice as a potential route is investigated to engineer polarization. The results show that the competing low‐energy phases may disrupt the polarization; nevertheless, it is showed that the appearance of weak polar displacements is symmetry‐allowed.