arXiv subjects

A. I. Lebedev

Publications and source records attributed to A. I. Lebedev.

At least 19 recordsLinked to original sources

arXiv2026

EXAFS studies of the local environment of lead and selenium atoms in PbTe$_{1-x}$Se$_x$ solid solution

EXAFS spectroscopy is used to study the local environment of lead and selenium atoms in PbTe$_{1-x}$Se$_x$ solid solution. In addition to a bimodal distribution of bond lengths in the first shell, unusually large Debye--Waller factors for the Pb--Pb interatomic distances in the second shell and a substantial deviation of these distances from Vegard's law are observed. Valence force field (VFF) calculations show that these observations are due to the complex structure of the distribution function for Pb--Pb distances. It is found that the number of Se--Se pairs in the second shell surpasses the statistical value, which indicates that chemical factors play an important role in the structure of the solid solution. The contribution of these chemical factors to the enthalpy of mixing of the solid is estimated to be approximately 0.5 kcal/mole, which is comparable to the strain contribution.

cond-mat.mtrl-sci↗

arXiv2024

A new approach to analyzing the spinor wave functions: Effect of strain on the electronic structure and optical transitions in bulk CdSe

An approach to analyzing the spinor wave functions that appear in the electronic structure calculations when taking the spin-orbit interaction into account is developed. It is based on the projection analysis of angular parts of wave functions onto irreducible representations of the point group and analysis of the evolution of the energy levels upon the adiabatic turning on the spin-orbit interaction. The technique is illustrated by an example of the changes in the valence band structure in strained bulk CdSe with zinc-blende structure. An analysis of the character of mixing of various branches of the valence band supports the Luttinger-Kohn model of the valence band. It is shown that the above calculations complemented by the Zeeman splitting in a magnetic field make it possible to unambiguously determine the polarization of all optical transitions. Using the spinor wave functions, matrix elements of optical transitions between the valence subbands and the conduction band are calculated.

cond-mat.mtrl-sci↗

arXiv2023

On the possibility of incorporation of the iron impurity into $A$ sites in SrTiO$_3$

The local environment and oxidation state of the Fe impurity in strontium titanate are studied using XAFS spectroscopy. The influence of annealing temperature and deviation from stoichiometry on the possibility of incorporation of the impurity into the $A$ and $B$ sites of the perovskite structure is studied. The results obtained from the X-ray diffraction, XANES spectra, and EXAFS spectra suggest that at high annealing temperature the iron atoms, at least partially (up to 30%), enter the $A$ sites in SrTiO$_3$. The obtained results agree with results of first-principles calculations, according to which the iron at the $A$ site exhibits strong off-centering (the displacement of $\sim$1 Å), similar to that previously established in SrTiO$_3$ samples doped with Mn and Co.

cond-mat.mtrl-sci↗

arXiv2023

First-principles calculations of vibrational spectra of CdSe/CdS superlattices

The vibrational spectra of CdSe/CdS superlattices (SLs) with different layer thicknesses are calculated from first principles within the density functional theory. It is shown that, along with folded acoustic and confined optical modes, a number of confined acoustic modes appear in SLs. In structures with a minimum thickness of one of the layers, microscopic interface modes similar to local and gap modes in crystals appear. An analysis of projections of the eigenvectors of vibrational modes in SLs onto the orthonormal basis of normal modes in binary compounds enables to establish the details of formation of these vibrational modes and, in particular, to determine the degree of intermixing of acoustic and optical modes. A comparison of the frequencies of vibrational modes in CdSe/CdS SLs and CdSe/CdS nanoplatelets enables to separate the influence of size quantization and surface relaxation on the vibrational frequencies in the nanoplatelets.

cond-mat.mtrl-sci↗

arXiv2021

Cobalt in strontium titanate as a new off-center magnetic impurity

The local structure and oxidation state of the cobalt impurity in SrTiO$_3$ is studied by X-ray absorption fine structure (XAFS) spectroscopy. The synthesis conditions, under which cobalt predominantly (up to 76%) substitutes the atoms at the $A$ site of the perovskite structure, is found for SrTiO$_3$(Co) samples. By varying the synthesis conditions, it is possible to appreciably change the ratio between the concentrations of cobalt atoms incorporated into the $A$ and $B$ sites. It is established that the oxidation state of cobalt is +2 at the $A$ site and +3 at the $B$ site. It is revealed that the Co impurity at the $A$ site is off-center, and its displacement from the lattice site is $\sim$1.0 Å. First-principles calculations show that an isolated Co$^{3+}$ ion at the $B$ site is diamagnetic, whereas the Co$^{2+}$ ion at the $A$ site is in a high-spin state ($S = 3/2$).

cond-mat.mtrl-sci↗

arXiv2020

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

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 occur were established. It was 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 the ordering and tunneling of off-center ions.

cond-mat.mtrl-sci↗

arXiv2019

Electronic and magnetic properties of structural defects in SrTiO$_3$(Co)

The synthesis conditions of SrTiO$_3$(Co) samples in which cobalt predominantly enters the $A$ or $B$ sites of the perovskite $AB$O$_3$ structure are found. EXAFS studies show that the Co impurity at the $A$ site is off-center and is displaced from the site by 1.0 Å. XANES studies reveal two predominant oxidation states of Co: Co$^{2+}$ at the $A$ site and Co$^{3+}$ at the $B$ site. First-principles calculations of a number of possible cobalt-containing structural defects reveal defects whose properties are compatible with the experimentally observed Co oxidation state, its local structure, magnetic, electrical, and optical properties.

cond-mat.mtrl-sci↗

arXiv2018

EXAFS and electrical studies of new narrow-gap semiconductors: InTe$_{1-x}$Se$_x$ and In$_{1-x}$Ga$_x$Te

The local environment of Ga, Se, and Tl atoms in InTe-based solid solutions was studied by EXAFS technique. It was shown that all investigated atoms are substitutional impurities, which enter the In(1), Te, and In(2) positions in the InTe structure, respectively. The electrical measurements revealed that In$_{1-x}$Ga$_x$Te and InTe$_{1-x}$Se$_x$ solid solutions become semiconductors at $x>0.24$ and $x>0.15$, respectively.

cond-mat.mtrl-sci↗

arXiv2017

An experimental and theoretical study of Ni impurity centers in Ba$_{0.8}$Sr$_{0.2}$TiO$_3$

The local environment and the charge state of a nickel impurity in cubic Ba$_{0.8}$Sr$_{0.2}$TiO$_3$ are studied by XAFS spectroscopy. According to the XANES data, the mean Ni charge state is $\sim$2.5+. An analysis of the EXAFS spectra and their comparison with the results of first-principle calculations of the defect geometry suggest that Ni$^{2+}$ ions are in a high-spin state at the $B$ sites of the perovskite structure and the difference of the Ni$^{2+}$ and Ti$^{4+}$ charges is mainly compensated by distant oxygen vacancies. In addition, a considerable amount of nickel in the sample is in a second phase BaNiO$_{3-δ}$. 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$^{4+}$ ions at the $B$ sites.

cond-mat.mtrl-sci↗

arXiv2016

On the nature of change in Ni oxidation state in BaTiO3-SrTiO3 system

XAFS studies of Ni-doped Ba$_{1-x}$Sr$_x$TiO$_3$ solid solution reveal that the Ni oxidation state changes from 4 in SrTiO$_3$ to 2.5 in BaTiO$_3$ when varying $x$. 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$_3$ and SrTiO$_3$, 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$_3$ and BaTiO$_3$ can be explained by different formation energies of the oxygen vacancies in these compounds.

cond-mat.mtrl-sci↗

arXiv2016

Nonlinear optical properties of undoped and doped with Zr and Nb KTiOPO4 crystals

The structure of the ferroelectric phase of undoped KTiOPO$_4$ and its solid solutions with zirconium and niobium is studied from first principles within the density functional theory. The second-order nonlinear susceptibility tensor and the spontaneous polarization of these materials are obtained. It is shown that an improvement in nonlinear optical properties of KTiOPO$_4$ upon its doping with Zr and Nb cannot be explained by a systematic change in the composition of crystals and is apparently associated with the occurrence of defects. Possible structures of such defects are discussed.

cond-mat.mtrl-sci↗

arXiv2016

Ferroelectric phenomena in CdSnO3: a first-principles study

The phonon spectrum of cubic cadmium metastannate and the crystal structures of its distorted phases were calculated from first principles within the density functional theory. It is shown that the phonon spectrum and the energy spectrum of distorted phases in $α$-CdSnO$_3$ are surprisingly similar to the corresponding spectra of CdTiO$_3$. The ground state of $α$-CdSnO$_3$ is the ferroelectric $Pbn2_1$ phase; the energy gain accompanying the phase transition from the nonpolar $Pbnm$ phase to this phase is $\sim$30 meV and the spontaneous polarization in it is 0.25 C/m$^2$. An analysis of the eigenvector of the ferroelectric mode in $α$-CdSnO$_3$ and calculations of the partial densities of states indicates that the ferroelectric instability in this crystal, which does not contain $d$ transition elements, is associated with the formation of a covalent bonding between Cd and O atoms.

cond-mat.mtrl-sci↗

arXiv2015

Combined first-principles and EXAFS study of structural instability in BaZrO3

Phonon spectrum of cubic barium zirconate is calculated from first principles using the density functional theory. Unstable phonon mode with the $R_{25}$ 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 $I4/mcm$. In order to find the manifestations of the predicted instability, EXAFS measurements at the Ba $L_{\rm III}$-edge are used to study the local structure of BaZrO$_3$ at 300 K. An enhanced value of the Debye-Waller factor for the Ba-O atomic pair ($σ^2_1 \sim 0.015$ Å$^2$) revealed in the experiment is associated with the predicted structural instability. The average amplitude of the thermal octahedra rotation estimated from the measured $σ^2_1$ value is $\sim$4 degrees at 300 K. The closeness of the calculated energies of different distorted phases resulting from the condensation of the $R_{25}$ mode suggests a possible structural glass formation in BaZrO$_3$ when lowering temperature, which explains the cause of the discrepancy between the calculations and experiment.

cond-mat.mtrl-sci↗

arXiv2015

Local environment and oxidation state of Mn impurity in SrTiO3 determined from XAFS data

The local environment and oxidation state of Mn impurity in strontium titanate doped with 3% Mn were studied by X-ray absorption fine structure spectroscopy. The influence of the preparation conditions on the incorporation of the impurity into the $A$ and $B$ sites was studied. It was established that Mn ions substituting for Ti are in the Mn$^{4+}$ oxidation state and on-center. Mn ions substituting for Sr are in the Mn$^{2+}$ oxidation state, are off-center and displaced from the lattice sites by $\sim$0.32 Å. It was demonstrated that studies of the X-ray absorption near-edge structure can be used to determine the concentration ratio of Mn atoms located at the $A$ and $B$ sites.

cond-mat.mtrl-sci↗

arXiv2015

Effect of local environment on crossluminescence kinetics in SrF2:Ba and CaF2:Ba solid solutions

Spectral and kinetic properties of extrinsic crossluminescence (CL) in SrF2:Ba(1%) and CaF2:Ba(1%) are compared with those of intrinsic CL in BaF2 and are analyzed taking into account EXAFS data obtained at the Ba LIII edge and results of first-principles calculations. The CL decay time was revealed to be longer in SrF2:Ba and CaF2:Ba compared to BaF2. This fact contradicts the expected acceleration of luminescence decay which could result from an increased overlap of wave functions in solid solutions due to shortening of the Ba-F distance obtained in both EXAFS measurements and first-principles calculations. This discrepancy is explained by the effect of migration and subsequent non-radiative decay of the Ba(5p) core holes in BaF2 and by decreasing of the probability of optical transitions between Ba(5p) states and the valence band in SrF2:Ba and CaF2:Ba predicted by first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2015

Ferroelectric properties of RbNbO3 and RbTaO3

Phonon spectra of cubic rubidium niobate and rubidium tantalate with the perovskite structure are calculated from first principles within the density functional theory. Based on the analysis of unstable modes in the phonon spectra, the structures of possible distorted phases are determined, their energies are calculated, and it is shown that $R3m$ is the ground-state structure of RbNbO3. In RbTaO3, the ferroelectric instability is suppressed by zero-point lattice vibrations. For ferroelectric phases of RbNbO3, spontaneous polarization, piezoelectric, nonlinear optical, electro-optical, and other properties as well as the energy band gap in the LDA and GW approximations are calculated. The properties of the rhombohedral RbNbO3 are compared with those of rhombohedral KNbO3, LiNbO3, and BaTiO3.

cond-mat.mtrl-sci↗

arXiv2012

Studies of eta-mesic nuclei at the LPI electron synchrotron

A brief review of searches for eta-mesic nuclei is presented with emphasis on photoreactions and results of a new experiment done at the LPI electron synchrotron are reported. Decay products of eta-mesic nuclei presumably formed by photons in a carbon target, namely correlated back-to-back (pi+ n) and (p n) pairs, have been studied with a two-arm time-of-flight setup. The obtained data show that apart from previously observed (pi+ n) pairs from one-nucleon annihilation of stopped etas (via eta N --> pi N) correlated (p n) pairs from two-nucleon absorption of eta in the nucleus (via eta NN --> NN) are also ejected with a comparable rate. The cross section of eta-mesic nuclei photoproduction off carbon was estimated from the obtained data to be less than 10 microbarn.

nucl-ex↗

arXiv2012

Crystal structure, local structure, and defect structure of Pr-doped SrTiO$_3$

X-ray diffraction studies showed that the structure of (Sr$_{1-x}$Pr$_x$)TiO$_3$ solid solutions at 300 K changes from the cubic $Pm3m$ to the tetragonal $I4/mcm$ with increasing $x$. The analysis of XANES and EXAFS spectra of the solid solutions revealed that Pr ions are predominantly in the 3+ oxidation state, they substitute for Sr atoms and are on-center regardless of the preparation conditions. The weak dependence of the lattice parameter in (Sr$_{1-x}$Pr$_x$)TiO$_3$ on the Pr concentration was explained by the competition between the relaxation of the Sr--O bond length, which results from the difference in ionic radii of Sr and Pr ions, and the repulsion of positively charged Pr$^{3+}$ and Ti$^{4+}$ ions. It was shown that the most important defects in the crystals are charged Sr vacancies and SrO planar faults; praseodymium does not enter the Sr sites in the planar faults.

cond-mat.mtrl-sci↗