8,535 research outputs found
Metastability effects in strained and stressed SrTiO3 films
The sequence of ground states for SrTiO3 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 (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 SrTiO3 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.Comment: 14 pages, 5 figures, 1 table plus 12 tables of supplemental materia
On the nature of change in Ni oxidation state in BaTiO3-SrTiO3 system
XAFS studies of Ni-doped BaSrTiO solid solution reveal that
the Ni oxidation state changes from 4 in SrTiO to 2.5 in BaTiO when
varying . This change is accompanied by a noticeable change in the
interatomic Ni-O distances in the first shell. The first-principles
calculations show that nickel creates an impurity band in the forbidden band
gap of BaTiO and SrTiO, which explains the appearance of intense
absorption of Ni-doped samples in the visible region. The analysis of the
electronic structure of doped crystals and calculations of the oxygen vacancy
formation energy in them show that different oxidation states of Ni in
SrTiO and BaTiO can be explained by different formation energies of the
oxygen vacancies in these compounds.Comment: 5 pages, 2 figures, 1 tabl
An experimental and theoretical study of Ni impurity centers in BaSrTiO
The local environment and the charge state of a nickel impurity in cubic
BaSrTiO are studied by XAFS spectroscopy. According to the
XANES data, the mean Ni charge state is 2.5+. An analysis of the EXAFS
spectra and their comparison with the results of first-principle calculations
of the defect geometry suggest that Ni ions are in a high-spin state at
the sites of the perovskite structure and the difference of the Ni
and Ti charges is mainly compensated by distant oxygen vacancies. In
addition, a considerable amount of nickel in the sample is in a second phase
BaNiO. The measurements of the lattice parameter show a decrease
in the unit cell volume upon doping, which can indicate the existence of a
small amount of Ni ions at the sites.Comment: 8 pages, 3 figures, 3 table
First-principles study of ferroelectricity and pressure-induced phase transitions in HgTiO
Ground-state structure is found and pressure-induced phase transitions up to
210 kbar are studied in mercury titanate from first principles within the
density functional theory. It is established that the structure
experimentally observed in HgTiO is metastable at ambient pressure. With
increasing the hydrostatic pressure, the ground-state structure changes
following the sequence. It is shown that the
appearance of ferroelectricity in HgTiO at is associated with an
unstable phonon mode. Optical and elastic properties of different phases of
mercury titanate are calculated. The quasiparticle band gap calculated in the
\emph{GW} approximation ( eV) agrees with experimental data better
than the value obtained in the LDA approximation (1.49 eV). Analysis of the
thermodynamic stability explains why the synthesis of mercury titanate is
possible only at high pressures.Comment: 8 pages, 4 figures, 5 tables; to appear in Phase Transition
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