Weak Ferroelectricity in n = 2 Pseudo Ruddlesden–Popper-Type Niobate Li₂SrNb₂O₇

Li2SrNb2O7 (LSNO) crystallizes in a structure closely related to n = 2 Ruddlesden–Popper-type compounds, which are generally formed by intergrowth of two-dimensional perovskite-type blocks and rocksalt-type layers. The present study demonstrates the coexistence of spontaneous polarization and an anti-ferroelectric-like nonlinear response in LSNO at 80 K, suggesting weak ferroelectricity below the phase transition temperature of 217 K. A combination of first-principles calculations and single-crystal X-ray diffractions clarifies a polar P21cn structure for the ground state of LSNO, where an in-plane antiferroelectric displacement and an out-of-plane polar shift simultaneously take place. The present study offers a new perspective to design ferroelectric and antiferroelectric materials with Ruddlesden–Popper-type compounds.