Giant spin-induced electric polarization in absence of orbital order in (Bi0.5Ag0.5)<…

Giant spin-induced ferroelectric polarization has been observed in the multiferroic CaMn₇O₁₂, 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 Bi³⁺/Ag⁺ ions through high-pressure and high-temperature methods, leading to the successful synthesis of single-phase isostructural (Bi0.5Ag0.5)Mn₇O₁₂. No orbital order is observed down to 30 K, while a giant electric polarization of ~19000.28em0exC0.16em0exm^-2 is realized below TN≈80 K when the system enters an antiferromagnetically ordered state. The low-temperature spin structure adopts the magnetic point group of 31^', the same as CaMn₇O₁₂. Our results strongly indicate that the giant ferroelectric polarization is primarily caused by exchange striction instead of orbital order.