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.