Ferroelectric Response Induced in <i>cis</i>-Type Anion Ordered SrTaO₂N Oxynitride Perovskite

Oxynitride perovskite SrTaO₂N has been attracting attention as a possible new dielectric material owing to the <i>cis</i>-type anion ordering in its crystal structure. It is currently difficult to obtain it in bulk form because it easily loses a part of its nitrogen to become semiconducting during densification at high temperature. We found that its surface layer recovered its original orange color and dielectric properties after postannealing in ammonia. Piezoresponse force microscopy measurements clearly revealed a ferroelectric behavior on the entire orange surface layer peeled off from the black sintered body inside. The present sample was free-standing at about 8 μm thick and clearly distinct from the compressively strained SrTaO₂N thin films for which ferroelectricity was recently reported in small domains (10–10² nm) in the locally assumed <i>trans</i>-type anion ordering. The present results represent the first experimental observation of ferroelectric response in a bulk oxynitride perovskite with <i>cis</i>-type anion ordering