Unusual Thermodynamic Properties and Nonergodicity in Ferroelectric Superlattices

The properties of $[Pb({Zr}_{1{-}{x}₁}{Ti}_{{x}₁}){O}₃{]}ₙ/[Pb({Zr}_{1{-}{x}₂}{Ti}_{{x}₂}){O}₃{]}ₙ$ superlattices, with a $2n$ period, are simulated using an ab initio based approach. The ${x}₁$ and ${x}₂$ compositions are chosen to be located across the morphotropic phase boundary of the corresponding disordered alloys, while the $({x}₁+{x}₂)/2$ average composition lies inside this boundary. These superlattices exhibit an unusual thermodynamic phase transition sequence, including a triclinic ground state. They also have the kind of peculiar free-energy landscape yielding nonergodicity. The effects responsible for these anomalies are discussed.