Ruddleson-Popper phase SnO(SnTiO₃)_n: Lead-free layered ferroelectric materials with large spontaneous polarization

Perovskite phase SnTiO₃ was predicted to have a large spontaneous polarization of 1.1 C/m², but its synthesis has been unsuccessful so far. Here, we calculated the ferroelectric properties of a series of Ruddleson-Popper (RP) phase SnO(SnTiO₃)_n (n = 1 ∼ 6), with perovskite SnTiO₃ as the ending structure (n =-∞), using the first principles calculations. An analysis of RP phase tolerance factor indicated that the fabrication of the RP phase SnO(SnTiO₃)_n may be easier than that for SnTiO₃. For bulk SnO(SnTiO₃)₁, the most stable phase is Aba2 with a spontaneous polarization of 0.53 C/m² along [110]. Aba2 is also the most stable phase for SnO(SnTiO₃)₁ under biaxial strain. For RP phase SnO(SnTiO₃)_n on a SrTiO₃ substrate, there is also a phase transition with the increasing layer thickness n. The polarization increases with strain or layer thickness. The ferroelectric properties and size effect in the free standing RP phase SnO(SnTiO₃)_n nanosheets were also investigated, and the in-plane polarization is 60% larger than that of the bulk.