Ferroelectric control of altermagnetism in momentum space has been studied widely, while the control of magnetism in real space of altermagnets is still rare. We present a design rule to identify multiferroicity in $n=2$ Ruddlesden-Popper halides. Our results show that a Jahn-Teller distortion can cooperate with oxygen octahedral rotations to break inversion symmetry, which we demonstrate in ${\mathrm{K}}_3\mathrm{C}{\mathrm{r}}_2{\mathrm{F}}_7$ and cation-ordered ${\mathrm{KAg}}_2{\mathrm{Cu}}_2{\mathrm{Cl}}_7$, and leads to a ferrielectric-to-ferroelectric phase transition in ${\mathrm{K}}_3\mathrm{C}{\mathrm{r}}_2{\mathrm{F}}_7$. Altermagnetic spin order in the ferrielectric phase of ${\mathrm{K}}_3\mathrm{C}{\mathrm{r}}_2{\mathrm{F}}_7$ transforms into a conventional antiferromagnetic order in the ferroelectric phase, at which strain/pressure engineered sizable changes of weak ferromagnetism can occur. Our study is not only conducive to realize strong magnetoelectric coupling in multiferroics but also reveals more functionalities in altermagnetic materials.

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• Ferroelectricity
• Magnetism
• Multiferroics
• First-principles calculations