Huge shift of fundamental electronic absorption edge in Sr1−xBaxNb2O6 crystals at elevated temperatures

Luminescence in SrTiO3and LiNbO3Crystals Under High Density Pulsed Electron Excitation

The time-resolved luminescence as well as stationary excited luminescence in SrTiO3 (undoped, Nb and Pb doped) and LiNbO3 (congruent and stoichiometric) single crystals was studied. The charge-transfer vibronic exciton in the regular oxy-anion site is suggested to be responsible for luminescence bands at 2.8 eV and 2.75 eV for SrTiO3 and LiNbO3 crystals respectively. It is suggested that luminescence band observed in SrTiO3 at ∼2.2-2.3 eV above 80 K possibly is due to some defect common for all SrTiO3 crystals studied.

Dielectric Measurements on SBN:Ce

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-phas…

Theoretical Prediction and Experimental Confirmation of Charge Transfer Vibronic Excitons and Their Phase in ABO3 Perovskite Crystals

AbstractThe current theoretical and experimental knowledge of new polaronic-type excitons in ferroelectric oxides - charge transfer vibronic excitons (CTVE) is discussed. It is shown that quantum chemical Hartree-Fock-type calculations using a semiempirical Intermediate Neglect of Differential Overlap (INDO) method (modified for ionic/partly ionic solids) as well as photoluminescence studies in ferroelectric oxygen-octahedral perovskites confirm the CTVE existence. Our INDO calculations for KTaO3 and KNbO3 have demonstrated that the triplet exciton is a triad centre containing one active O atom and two Ta atoms sitting on the opposite sites from this O atom. The total energy of a system is …

OH/OD-IR absorption bands in SrxBa1−xNb2O6

Single crystals of Sr x Ba 1−x Nb 2 O 6 (SBN) with congruent composition (x = 0.61) and Sr 0.61 Ba 0.39 Nb 2 O 6 crystals doped with various concentrations of cerium are grown with the Czochralski method. A subsequent doping of the crystals with protons and deuterons is performed either by hydrothermal high temperature treatment or by field supported hydrogen diffusion at elevated temperatures in humid atmosphere. The presence of the protons is revealed by the infrared absorption of the OH stretching vibration. This broad OH absorption band is essentially the same for Sr 0.61 Ba 0.39 Nb 2 O 6 and Sr 0.61 Ba 0.39 Nb 2 O 6 :Ce (0.025 wt% CeO 2 ). It shows a polarization dependence with respec…

Pulsed electron beam excited transient absorption in SrTiO3

Abstract Transient absorption (TA) spectra and optical density relaxation kinetics in SrTiO3 (STO), STO–Nb and STO–V were studied. The electron polaron free or bound at intrinsic defect is suggested to be responsible for the TA band at ∼1.4 eV observed in STO. The band peaking below 0.75 eV in STO–Nb is proposed to be from Nb4+. The electron center V3+ or electron polaron bound in the vicinity of V4+ is responsible for the absorption band at 1.0 eV in STO–V. The absorption from hole polarons is suggested to be in the range 1.8–2.3 eV. It is shown that TA in pure STO at room temperature decays due to electron–hole polaron recombination.

Quantum chemical modelling of electron polarons and  green  luminescence in PbTiO3perovskite crystals

In an extension of our previous study on the electron polarons and excitons in KNbO3, KTaO3 and BaTiO3 (Kotomin E A, Eglitis R I and Borstel G 2000 J. Phys. Condens. Matter 12 L557; Eglitis R I, Kotomin E A and Borstel G 2002 J. Phys. Condens. Matter 14 3735) by the semiempirical Hartree–Fock method we present here results for free electron polarons in the PbTiO3 perovskite crystal. We discuss the origin of the intrinsic visible band emission of PbTiO3 perovskite oxides (so-called 'green luminescence') which has remained a topic of high interest during the last quarter of a century. We present a theoretical calculation modelling this emission in the framework of a concept of charge transfer…

Polaronic-type excitons in ferroelectric oxides: Microscopic calculations and experimental manifestation

We discuss the current experimental and theoretical understanding of new polaronic-type excitons in ferroelectric-oxides charge-transfer vibronic excitons ~CTVE’s!, which are pairs of strongly correlated electronic and hole polarons. It is shown that charge-transfer‐lattice distortion interactions are the driving forces for CTVE formation. Hartree-Fock-type calculations performed in the framework of the intermediate neglect of differential overlap ~INDO! method as well as photoluminescence, second-harmonic generation, and UVabsorption high-temperature studies performed for ABO3 ferroelectric oxides strongly support the CTVE existence. Both single CTVE and a phase of strongly correlated CTVE…

A new phase in ferroelectric oxides: The phase of charge transfer vibronic excitons

It is shown, by means of Hartree-Fock-type calculations using the intermediate neglect of the differential overlap (INDO) method, that polaronic-type charge transfer vibronic excitons (CTVE) in ferroelectric oxides could lead to the formation of a new phase. The ground-state energy of this phase of strongly correlated CTVE lies within an optical gap of pure crystal, and is characterized by a strong tetragonal lattice distortion, as well as ferroelectric and antiferromagnetic ordering. It is shown also that clusters of the CTVE phase being stabilized by oxygen vacancies could be responsible for the unusually strong optical Second Harmonic Generation (SHG) in nominally pure incipient ferroele…

X‐ray and pulsed electron beam excited luminescence and optical absorption in KTaO 3 crystals

Pulsed electron beam excited time-resolved transient absorption and luminescence have been studied in nominally pure and Li-doped KTaO3 single crystals. It is found that under electron beam excitation in nominally pure KTaO3 the absorption bands peaks at ∼0.8 eV, ∼1.1 eV and 1.7 eV whereas in KTaO3:Li at ∼1.1 eV and ∼1.7 eV. The transient absorption decay varies over the spectrum thus revealing several short-lived centers. The main luminescence peak is centred at ∼2.7 eV in all KTaO3 specimens. Luminescence decay kinetics observed for the two bands slightly differ indicating different kinds of excited states are involved. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Relaxation of electronic excitations in strontium titanate

The transient absorption spectra and kinetics were studied for undoped, lead doped and high purity SrTiO 3 single crystals. The pulsed electron beam induced transient absorption is studied in all crystals. The strong absorption at 0.8 v eV was observed only in high purity SrTiO 3 . This absorption is suggested to arise from intrinsic electron polaron. The bound electron polarons are likely responsible for absorption band at 1.4 v eV. The main luminescence band under excitation pulse is observed at 2.75 v eV. The luminescence decay is faster than that of transient absorption.