A. I. Lebedev

Ab initio Calculations of Phonon Spectra in A TiO 3 Perovskite Crystals ( A = Ca, Sr, Ba, Ra, Cd, Zn, Mg, Ge, Sn, Pb)

First-principles calculations of phonon spectra based on the density functional theory are carried out for calcium, strontium, barium, radium, cadmium, zinc, magnesium, germanium, tin, and lead titanates with a perovskite structure. By analyzing unstable modes in the phonon spectrum, the possible types of lattice distortion are determined and the energies of the corresponding phases are calculated. From analyzing the phonon spectra, force constants, and eigenvectors of TO phonons, a conclusion is drawn concerning the nature of ferroelectric phenomena in the crystals studied. It is shown that the main factors determining the possible appearance of off-center atoms in the A position are the geometric size and electronic configuration of these atoms.

Direct evidence for off-centering of Mn impurity in SrTiO3

Mn-doped SrTiO3 samples (3% Mn) with different deviations from stoichiometry have been investigated using XAFS spectroscopy. It was shown the under various preparation conditions manganese atoms can sub-stitute A and B sites of the perovskite structure and are in them in different charge states. Impurity Mn4+ ions that substitute Ti sites occupy on-center position in the lattice, whereas Mn2+ ions that substitute Sr sites are off-center and are displaced from the lattice sites by a distance of 0.32Å.

Ground state and properties of ferroelectric superlattices based on crystals of the perovskite family

The crystal structure of the ground state of ten free-standing ferroelectric superlattices based on crystals with the perovskite structure (BaTiO3/SrTiO3, PbTiO3/SrTiO3, PbTiO3/PbZrO3, SrZrO3/SrTiO3, PbZrO3/BaZrO3, BaTiO3/BaZrO3, PbTiO3/BaTiO3, BaTiO3/CaTiO3, KNbO3/KTaO3, and KNbO3/NaNbO3) was calculated from first principles within the density functional theory taking into account criteria for stability of the structures with respect to acoustic and optical distortions. It was shown that the ground state in all the considered superlattices corresponds to the ferroelectric phase. It was found that the polarization vector has a tendency toward a tilt to the plane of the superlattice layers, which makes it possible to decrease the electrostatic and elastic energy in the superlattices consisting of materials with different ferroelectric properties. The importance of the inclusion of structural distortions due to unstable phonons at the Brillouin zone boundary, which, in a number of cases, lead to significant changes in ferroelectric and dielectric properties of the superlattices, was demonstrated.

Ferroelectric phase transitions in IV-VI semiconductors associated with off-center ions

Abstract New ferroelectric phase transitions were found in ternary and quaternary solid solutions of IV-VI semiconductors. Their analysis shows that offcenter ions play important role in the appearance of ferroelectricity in these crystals.

Low-temperature phase transitions in some quaternary solid solutions of IV-VI semiconductors

Abstract Samples of PbS x Se y Te 1- x - y , Pb 1- x Sn x Te 1- y Se y and Pb 1- x Sn x Te 1- y S y quaternary solid solutions were investigated in the 4–200 K temperature range using electrical and X-ray methods. The regions where low-temperature phase transitions take place were established. It is shown that phase transitions in these solid solutions are associated with off-center S and Sn ions. The dependence of the phase transition temperature on the composition of solid solutions can be qualitatively described taking into account the influence of substitutional disorder on ordering and tunneling of off-center ions.

Structural instability in BaZrO3 crystals: Calculations and experiment

Phonon spectrum of cubic barium zirconate is calculated from first principles using the density functional theory. Unstable phonon mode with the

Crystal structure, local structure, and defect structure of Pr-doped SrTiO3

R_{25}

Structural position and charge state of nickel in SrTiO 3

symmetry in the phonon spectrum indicates an instability of the cubic structure with respect to rotations of the oxygen octahedra. It is shown that the ground-state structure of the crystal is

XAFS Studies of the Local Environment of Pb Impurity Atoms in Barium, Strontium, and Calcium Titanates

I4/mcm

Ferroelectric phase transition in orthorhombic CdTiO3: First-principles studies

. In order to find the manifestations of the predicted instability, EXAFS measurements at the Ba