Ab initio investigation of the magnetic and ferroelectric properties of double perovskites LaPbMeSbO6 (Me = Mn, Fe, Co, Ni)

The results of ab initio calculations of the ferroelectric and magnetic properties of the recently synthesized double perovskites LaPbMeSbO$_6$ (Me$^{2+}$ = Mn$^{2+}$, Fe$^{2+}$, Co$^{2+}$, Ni$^{2+}$) have been presented. The ordered double perovskites with a layered ordering of cations $A$ and $A'$ and a checkerboard ordering of cations $B$ and $B'$ have been considered. The calculation of the lattice dynamics has revealed instabilities in the phonon spectra of the high-symmetry phase of these compounds. The condensation of unstable modes leads to a stable polar phase $P2_1$. The spontaneous polarization in this phase is found to be $\sim$ 30 $\mu$C/cm$^2$. The magnetic ground state is antiferromagnetic with an antiparallel direction of the magnetic moments in adjacent planes along the [001] direction. The Néel temperatures are estimated in the mean field approximation. The presence of the ferroelectric and magnetic order parameters in the structure suggests that the studied double perovskites with the simultaneous ordering of the cations are potential multiferroics.