Ferroelectricity in the Dion-Jacobson CsBiNb$_2$O$_7$ from first   principles
  We have studied ferroelectricity in Dion-Jacobson CsBiNb$_2$O$_7$ from first principles. Using group-theoretical analysis and first-principles density functional calculations of the total energy and phonons, we perform a systematic study of the energy surface around a paraelectric prototypic phase. Our results suggest that CsBiNb$_2$O$_7$ is a ferroelectric with a polarization of P$_s$=40$\mu$C cm$^{-2}$. We propose further experiments to clarify this point. 
Craig J. Fennie and Karin M. Rabe