Appropriate value of permittivity for depolarization field and universal instability of insulating ferroelectric phase in single-domain state

We redefine the permittivity for the depolarization field and show that in most situations the permittivity for depolarization field should be unity, and show a proper expression for depolarization field. Based on this expression a generic consideration of ferroelectrics undergoing the first order transition shows that free standing two-dimensional ferroelectric phase cannot exist in a single domain state without a non-dielectric screening mechanism.