Non-d$^0$ Electric Dipole in FeO$_5$ bipyramid: a New Resource for   Quantum Paraelectrics, Ferroelectrics and Multiferroics
  Electric polarization in conventional ferroelectric oxides usually involves nonmagnetic transition-metal ions with an empty d shell (the d$^0$ rule). Here we unravel a new mechanism for local electric dipoles based on magnetic Fe$^{3+}$ (3d$^5$) ion violating the d$^0$ rule. The competition between the long-range Coulomb interaction and short-range Pauli repulsion in a FeO$_5$ bipyramid with proper lattice parameters would favor an off-center displacement of Fe$^{3+}$ that induces a local electric dipole. The manipulation of this kind of non-d$^0$ electric dipoles opens up a new route for generating unconventional dielectrics, ferroelectrics, and multiferroics. As a prototype example, we show that the non-d$^0$ electric dipoles in ferrimagnetic hexaferrites (Ba,Sr)Fe$_{12}$O$_{19}$ lead to a new family of magnetic quantum paraelectrics. 
Shi-Peng Shen, Yi-Sheng Chai, Jun-Zhuang Cong, Pei-Jie Sun, Jun Lu,   Li-Qin Yan, Shou-Guo Wang, and Young Sun
cond-mat.str-el