  Li2SrNb2O7 (LSNO) crystallizes in a structure closely related to n = 2
Ruddlesden-Popper-type compounds, which is gen-erally formed by intergrowth of
2-dimensional perovskite-type blocks and rocksalt-type layers. The present
study demonstrates a coexistence of spontaneous polarization and
anti-ferroelectric-like nonlinear response in LSNO at 80 K, suggesting a weak
ferroelectricity below the phase transition temperature of 217 K. A combination
of first-principles cal-culations and single crystal x-ray diffractions
clarifies a polar P21cn structure for the ground state of LSNO, where an
in-plane anti-ferroelectric 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.
