Hybrid Improper Ferroelectricity in Highly Cleavable Single Crystals of Dion-Jacobson-Compound CsBiNb2O7

Recently, “hybrid” improper ferroelectrics, a relatively new mechanism leading to ferroelectricity has drawn researchers' interests intensively. Among which, a subfamily of the layered perovskites, Dion-Jacobson (D-J) compounds with general chemical formula A[A'$_{\mathrm{n-1}}$B$_{\mathrm{n}}$O$_{\mathrm{3n+1}}$] is especially interesting because of its large polarization compared with all the other known hybrid improper ferroelectrics like the double-layered Ruddlesden-Popper perovskites with the chemical formula of A$_{\mathrm{3}}$B$_{\mathrm{2}}$O$_{\mathrm{7.\thinspace }}$We have grown high quality single crystals of Dion-Jacobson compound CsBiNb$_{\mathrm{2}}$O$_{\mathrm{7\thinspace }}$using self-flux method. Strong ferroelectric polarization in CsBiNb$_{\mathrm{2}}$O$_{\mathrm{7\thinspace }}$single crystal as well as its high coercivity has been confirmed and quantified during polarization-electric field measurement. Using Piezoelectric Force Microscopy, we perform a systematic study on the ferroelectric domain structure and dynamics of CsBiNb$_{\mathrm{2}}$O$_{\mathrm{7}}$. Further study on the high cleavability of Dion-Jacobson compounds indicates that they might be a good candidate to approach 2-D ferroelectric system.