Major improvement of ferroelectric and optical properties in Na-doped Ruddlesden-Popper layered hybrid improper ferroelectric compound, Ca^sub 3^Ti^sub 2^O^sub 7

Ruddlesden-Popper layered Ca3Ti2O7 (CTO) and Ca2.85Na0.15Ti2O7 (CNTO) ceramics were successfully prepared by a conventional solid-state reaction process. The ferroelectric coercive field greatly decreased via Na doping. Interestingly, a diode-like electric transport effect was observed in the CNTO sample, which is most likely due to a newly formed p-n junction caused by the applied electric field. Optical characterization by UV–visible absorption showed a decrease of band gap from 3.55 to 3.41eV via Na doping. First-principles calculation showed a similar decreasing trend which confirmed the experimental results.