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

Abstract

To cite this article: V. S. Vikhnin , S. Kapphan & J. Seglins (1999) Temperature induced red shift of the UV fundamental absorption edge in ferroelectric oxide crystals: effect of charge transfer vibronic excitons, Radiation Effects and Defects in 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,Ba,-rNb20& where no peculiarities were detected in the region of the diffused phase transition ( T , = 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.

Compositional dependence of optical and vibrational properties of strontium barium niobate (SrxBa1–xNb2O6)

“…Polarized absorption spectra in a wide temperature range have been reported [13][14][15][16] and the lattice coupling has been interpreted by charge transfer vibronic exciton-states [14] or a one-oscillator model [16]. Both gave good fits, but the first suffers from parameter sets, which are not unique, preventing a physical interpretation.…”

Charge transfer vibronic excitons

“…The CTVEs here could be considered as metastable intrinsic dipole defects. Above mentioned concept for such type excitons have been confirmed later by experimental results (photoluminescence -see, for instance, [13,15,19], X-ray induced luminescence [20], Second Harmonic Generation [14,15], significant red shift of the fundamental absorption edge at high temperatures [21] ) and theoretical (phenomenological theory -see, for instance, [16,22], and semi-empirical Hartree-Fock calculations in the INDO approach [11,12,14,15,17]) studies. Ferroelectric oxides KTaO 3 , KNbO 3 , SrTiO 3 , BaTiO 3 , SBN, PbTiO 3 , PbNb x O y , and PMN should be mentioned among topical systems for the CTVE manifestation.…”

“…Special interest could be related with a new, CTVE-phase, where CTVEs are in each cell and under the action of a strong correlation field from neighbouring CTVEs. Such a phase was proposed in [8] and some of its manifestations were shown experimentally [14,15,21] and by semi-empirical Hartree-Fock calculations in the INDO approach [14,15]. For instance, the CTVE-related polar clusters of this new phase could explain the nature of polar clusters [14,15] detected by Second Harmonic Generation in paraelectric states of ferroelectric oxides.…”

“…For instance, the CTVE-related polar clusters of this new phase could explain the nature of polar clusters [14,15] detected by Second Harmonic Generation in paraelectric states of ferroelectric oxides. Thermal occupation of CTVE-phase low lying states explains also the above mentioned strong shift at high temperature of the fundamental absorption edge in ferroelectric oxides [21]. It was also shown that the charge transfer mechanism [22] of giant piezoelectricity based on the CTVE-like states in ferroelectric relaxors can be effective for the explanation of some important features of this phenomenon.…”

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

“…Special attention is paid to the idea of the existence of a new CTVE phase of ABO 3 crystals. 4,19 The CTVE phase is expected to consist of the strongly correlated CTVE's occupying each unit cell of a crystal. That is, in fact the CTVE phase corresponds to a new state of the crystal which is characterized by a new equilibrium charge transfer as well as new lattice parameters.…”