Giant spin-induced ferroelectric polarization has been observed in the multiferroic $\mathrm{CaM}{\mathrm{n}}_7{\mathrm{O}}_{12}$, the origin of which remains controversial. Various theoretical models have been proposed in this regard, some of which put the preformed orbital order as a prerequisite factor, while others attribute it to the combination of Dzyaloshinskii-Moriya interactions and exchange striction. In this paper, we resolve this issue by replacing Ca with disordered $\mathrm{B}{\mathrm{i}}^{3+}/\mathrm{A}{\mathrm{g}}^{+}$ ions through high-pressure and high-temperature methods, leading to the successful synthesis of single-phase isostructural $\left(\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{A}{\mathrm{g}}_{0.5}\right)\mathrm{M}{\mathrm{n}}_7{\mathrm{O}}_{12}$. No orbital order is observed down to 30 K, while a giant electric polarization of $\sim 1900\mu \mathrm{C}{\mathrm{m}}^{-2}$ is realized below $T_{\mathrm{N}}\approx 80$ K when the system enters an antiferromagnetically ordered state. The low-temperature spin structure adopts the magnetic point group of ${31}^{\prime }$, the same as $\mathrm{CaM}{\mathrm{n}}_7{\mathrm{O}}_{12}$. Our results strongly indicate that the giant ferroelectric polarization is primarily caused by exchange striction instead of orbital order.

• Received 13 March 2024
• Revised 23 April 2024
• Accepted 24 April 2024

DOI:https://doi.org/10.1103/PhysRevB.109.174417