  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.
