V. S. Vikhnin

Semi-empirical calculations of the electronic and atomic structure of polarons and excitons in ABO3 perovskite crystals

Abstract Our quantum chemical calculations performed by means of the intermediate neglect of differential overlap confirm the existence of the self-trapped electrons in KNbO3, KTaO3 and BaTiO3 crystals. The relevant lattice relaxation energies are 0.21, 0.27 and 0.24 eV, whereas the optical absorption energies are 0.78, 0.75 and 0.69 eV, respectively. We suggest theoretical interpretation of the so-called green luminescence (2.2–2.3 eV) in ABO3 perovskite crystals as a result of the recombination of electrons and holes forming the charge-transfer-vibronic-excitons. The calculated luminescence energies for SrTiO3, BaTiO3, KNbO3 and KTaO3 perovskite crystals are in a good agreement with the experimentally observed energies.

Charge transfer vibronic excitons in ferroelectric oxides : experiment and theory

Abstract Review of the charge transfer vibronic excitons (CTVE) study on the basis of optical response investigations in ferroelectric oxides is presented. CTVE is the correlated pair of electron and hole small polarons whose stability is controlled by a pair pseudo-Jahn–Teller effect. The study is focused on the CTVE-induced critical dependence near the ferroelectric phase transition. The critical effect in absolute diffraction efficiency (ADE) formation of the laser-induced transient gratings near ferroelectric phase transition in KTa 1− x Nb x O 3 (KTN) is considered as an example. Critical concentration dependence for slow as well as for instantaneous (fast) optical response was detected. The peak positions for these x – dependences were different. The CTVE-model can explain these experiments taking into account not only the soft TO phonon mode contribution but also that of soft tunnel quasilocal modes of clusters of the second component.

A Model of Polar Clusters in Ferroelectric Relaxors of PMN-Type: Polaronic and Charge Transfer Effects

A model of polar clusters (PC) and of chemical fluctuation clusters (CFC) in PMN-like relaxors is proposed. The model is mainly based on three phenomena: on a charge transfer state formation corresponding to charge transfer reorienting dipole states induced by Mg-Nb disorder, on an appearance of electronic and hole polarons in CFC due to charge compensation, and on in-cluster Local Configuration Instability effects. The origin of co-operative phenomena, of the giant piezoelectric effect and characteristic dielectric losses, including Vogel-Fulcher law as well as peculiarities of EPR and NMR do find an explanation within this model. It is shown that mobile polaron clusters play an important role in the formation of PC reorienting polarization. The presence of small polarons in PMN was confirmed in this work by detection of the characteristic IR-absorption accompanied by light-induced polaron transport.

Ordering mechanism and dynamics of relaxor ferroelectrics

Abstract It is shown that the validity conditions of symmetry adapted random-bond-random-field model with rotation symmetry of local order parameter in the system of nanoscale polar domains in relaxors are fulfilled due to increasing of dispersion of soft TO-phonon. The latter is due to interaction of quasilocal modes of nanoscale domains with soft TO-phonons. As a result, long-wave contribution to domain-domain indirect interaction (DDII) and mean field effect are strongly increased. The quasi-rotation symmetry can be fulfilled due to intradomain disorder effects with high degree of quasi-degeneracy of orientation states of domain local order parameter. Local configuration instabilities of the first and of the second order for nanoscale domains are appeared due to softening of domain quasilocal modes essentially renormalized by dipole-dipole DDII. Compensation of extra-charge of nanoscale domains in relaxors due to ferroelectrically ordered charge transfer vibronic excitons in each cell with simultaneous domain polarization appearance is substantiated.

Critical effects in optical response due to charge transfer vibronic excitons and their structure in perovskite-like systems

Abstract A mechanism for bilinear interaction between high-frequency light-induced electronic polarization and low-frequency soft lattice polarization is proposed. It is based on the fluctuations of the charge transfer connected with charge transfer vibronic excitons (CTVE). This bilinear mechanism leads to the appearance of the critical peculiarities of the absolute diffraction efficiency of transient gratings near the ferroelectric phase transition. A semi-empirical Hartree–Fock INDO method was used for the evaluation of the energy parameters and the equilibrium displacements for the CTVE in KTaO 3 . This numerical study did confirm the proposed CTVE-model. It was shown that the CTVE-phase (with correlated CTVE in each cell) is characterized by new phonon spectra with a new set of soft lattice modes. It was also shown that metastable oxygen vacancy states are of dipole type in the CTVE-phase.

Congruent Sr 0.61 Ba 0.39 Nb 2 O 6 doubly doped with Ce and Cr: PHoto- and thermoluminescence investigations

Congruent Sr x Ba 1 m x Nb 2 O 6 (SBN, x=0.61) doped with Ce or Cr ions exhibits enhanced photorefractive properties and new spectral features like increased red sensitivity. Here special emphasis is placed on the luminescence features of doubly doped Ce+Cr SBN crystals. The luminescence excitation and emission spectra combined with the absorption of the impurities allow to draw conclusions about the origin of the charge carriers und their recombination. The well separated thermo-luminescence peaks detected and their spectral line shape in emission point to specific recombination processes following the thermal liberation of light-induced electron trapping centers: Nb 4+ polarons and VIS-centers created at low temperature under light irradiation. The thermal activation energy for the hopping motion of Nb 4+ polarons and of VIS-centers are estimated to be 0.18 - 0.02 v eV and 0.30 - 0.05 v eV respectively. Possible excitation and recombination mechanisms in SBN:Ce+Cr are discussed.

Polaron and charge transfer vibronic exciton phenomena in ferroelectrics

Abstract New phenomena induced by Jahn-Teller polarons (JTP) and charge transfer vibronic excitons (CTVE) in ferroelectric oxides are considered. The important role of CTVE was shown in the analysis of the drastic temperature shift of the fundamental absorption edge in ferroelectric oxides. This effect was explained on the basis of “band-intermediate radius CTVE” transitions. The critical increasing of the absolute diffraction efficiency of transient gratings near the ferroelectric phase transition (PT) was predicted and detected in KTN. It was shown that the mechanism of this phenomenon is connected with the excitation of CTVE. A new type of PT in solid solution with double substitution, like (SrTiO3)x(KTaO3)i-x is proposed.

Temperature induced red shift of the UV fundamental absorption edge in ferroelectric oxide crystals: effect of charge transfer vibronic excitons

Abstract A pronounced temperature induced red shift of the fundamental UV-absorption edge is observed in ferroelectric oxides at T > 300 K. The biggest shift value of—1.3 meV/K is found in tetragonal SBN (Sr x Ba1-x Nb2O6), where no peculiarities were detected in the region of the diffused phase transition (T c = 350 K). A model involving new in-gap states due to charge transfer vibronic excitons (CTVE) with a new correlated CTVE-phase is proposed to explain the experimental observations.

Photo- and thermoluminescence in congruent SBN crystals doped with Ce and Cr

Photo-refractive properties of Sr x Ba 1 m x Nb 2 O 6 (SBN, x = 0.61 congruent) are enhanced by doping with Cerium and Chromium ions. The luminescence excitation and emission spectra combined with the absorption of the impurities allow to draw conclusions about the origin of the charge carriers from these defect centers. The well separated thermo-luminescence peaks detected and their spectral line shape point to specific recombination processes following the thermal liberation of light-induced electron trapping centers: Nb 4+ polarons and VIS-centers created at low temperature under light irradiation. The thermal activation energy for the hopping motion of Nb 4+ polarons and of VIS-centers are estimated to be 0.18 - 0.02 eV and 0.30 - 0.05 eV respectively. Possible relaxation mechanisms of SBN: Ce, Cr are discussed.

Photo Dissociation of Blue-Light-Induced VIS-Centers in SBN:Ce, Cr and in BCT:Fe into Small Polarons at Low Temperatures

The promising photorefractive single crystal systems SrxBa1-xNb2O6 (SBN, congruent composition x = 0.61) [1] and Ba1-yCayTiO3 (BCT, congruent y = 0.23) [2], can be doped with polyvalent ions like Fe, Cr, Ce to enhance and optimize their photorefractive properties. Illumination with blue Ar+-laser light at low temperature creates light-induced NIR and VIS absorption bands [3, 4]. The NIR absorption has been identified as being due to electron polarons (Nb4+ in SBN, Ti3+ in BCT) and their properties have been investigated in some detail [3, 4]. The VIS-Centers (at about 2 eV) are less well understood and their nature is still under discussion [5]. We could show that illumination with red Kr+-laser light (647 nm) leads to a dissociation of the VIS-Centers and a simultaneous build-up of a transient NIR-polaron absorption [5]. These features give new insight into the dynamic nature of these VIS-Centers.