DIELECTRIC,PIEZOELECTRIC,AND ELASTIC PROPERTIES OF BaTiO_(3)/SrTiO_(3)FERROELECTRIC SUPERLATTICES FROM FIRST PRINCIPLES

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 thefirst-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 shortrange ordering and shows that these superlattices are thermodynamically quite stable.

Metastability effects in strained and stressed SrTiO_(3) films

The sequence of ground states for SrTiO_(3) film subjected to epitaxial strain as well as to mechanical stress along the[001]and[110]axes is calculated from first principles within the density functional theory.Under the fixed-strain boundary conditions,an increase in the lattice parameter of a substrate results in the I4cm→I4/mcm→Ima2→Cm→Fmm2→Ima2(II)sequence of ground states.Under the fixed-stress boundary conditions,the phase sequence is different and depends on how the stress is applied.It is revealed that the simultaneous presence of competing ferroelectric and antiferrodistortive instabilities in SrTiO_(3) gives rise to the appearance of metastable phases,whose number increases dramatically under the fixed-stress conditions.In the metastable phases,the octahedral rotation patterns are shown to differ substantially from those in the ground state.It is suggested that in systems with competing instabilities,each polar phase has its optimal octahedral rotation pattern which stabilizes this phase and creates a potential barrier preventing this phase to be transformed into other structures.

Structural position and oxidation state of nickel in SrTiO_(3)

The properties of Ni-doped strontium titanate are studied using X-ray diffraction and XAFS spectroscopy.It is shown that regardless of the preparation conditions,the SrTi_(1-x)Ni_(x)O_(3) solid solution and the NiTiO_(3) phase are the most stable phases which can coexist.According to the EXAFS data,in the single-phase sample of SrTi_(0.97)Ni_(0.03)O_(3),the Ni atoms substitute for the Ti ones and are oncenter.The distortion of the oxygen octahedra is not observed.The XANES spectra analysis shows that the oxidation state of nickel in NiTiO_(3) is 2+,and in the SrTi_(1-x)Ni_(x)O_(3) solid solution it is close to 4+.It is shown that the strongest light absorption in doped samples is associated with the presence of tetravalent nickel in the SrTi_(1-x)Ni_(x)O_(3) solid solution.This doping seems the most promising one for solar energy converters that exploit the bulk photovoltaic effect.