Ab initio studies of dielectric,piezoelectric, and elastic properties of BaTiO 3 /SrTiO 3 ferroelectric superlattices

The phonon spectrum; crystal structure of the polar phase; spontaneous polarization; dielectric constant, piezoelectric, and elastic moduli tensors for free_standing and substrate_supported superlattices mBaTiO₃/nSrTiO₃ (with m = n = 1–4) were calculated within the density functional theory. The simulation of properties of the disordered Ba_0.5Sr_0.5TiO₃ solid solution using two special quasirandom SQS-4 structures and their comparison with the properties of the superlattices revealed a tendency of the BaTiO₃-SrTiO₃ system to superstructure ordering and showed that the superlattices are thermodynamically quite stable. The ground state of the free-standing superlattice corresponds to the monoclinic polar phase Cm, which transforms to the tetragonal polar phase P4mm under in-plane compressive strain of the superlattice and to the orthorhombic polar phase Amm2 under in-plane tensile strain. With a change in the in-plane lattice parameter, in the vicinity of boundaries between neighboring polar phases, some optical and acoustic modes soften and some components of the static dielectric constant, piezoelectric, and elastic moduli tensors diverge critically.