DIELECTRIC, PIEZOELECTRIC, AND ELASTIC PROPERTIES OF BaTiO_3/SrTiO_3 FERROELECTRIC SUPERLATTICES FROM FIRST PRINCIPLES

摘要

The effect of epitaxial strain on the phonon spectra, crystal structure, spontaneous polarization, dielectric, piezoelectric, and elastic properties of (001)-oriented ferroelectric (BaTiO_3)_m/ (SrTiO_3)_n superlattices (m = n = 1-4) was studied using the first-principles density functional theory. The ground state of free-standing superlattices is the monoclinic Cm polar phase. Under the in-plane biaxial compressive strain, it transforms to tetragonal P4mm polar phase, and under the in-plane biaxial tensile strain, it transforms to orthorhombic Amm2 polar phase. When changing the in-plane lattice parameter, a softening of several optical and acoustic modes appears at the boundaries between the polar phases, and corresponding components of dielectric, piezoelectric, and elastic tensors diverge critically. The comparison of the mixing enthalpy of disordered Ba_(0.5)Sr_(0.5)TiO_3 solid solution modeled using two special quasirandom structures SQS-4 with the mixing enthalpy of the superlattices reveals a tendency of the BaTiO_3-SrTiO_3 system to short-range ordering and shows that these superlattices are thermodynamically quite stable.