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

Abstract

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 ferroelectrics, like KTaO3 (KTO) and SrTiO3 (STO), especially in the case of SHG induced by an external electric field. Another experimental manifestation of CTVE is related to a drastic red shift with temperature of the fundamental absorption (FA) edge in ferroelectric oxides SBN and KTO observed at high temperatures. This effect could be explained by thermal population of a low-lying bottom part of an additional valence band induced by the CTVE phase.Introduction. -A study of polaronic states in oxygen-octahedrical perovskites is one of the important topics of modern solid-state physics and chemistry [1][2][3][4][5]. The CTVE in ABO 3 ferroelectric oxides with mixed ionic-covalent type of chemical bonding (including KTO, KNbO 3 (KNO), STO) are new topics in this field. They consist of the correlated pairs or/and triads of electronic and hole polarons [6][7][8][9][10][11][12][13][14][15]. CTVE formation is possibile due to the electron and hole polaron vibronic interaction enhanced by the charge transfer. Such an effect is partly connected to the appearance of degenerate or pseudo-degenerate states of active ions in the final, CTVE state. The above-mentioned increase of the vibronic interaction induced by charge transfer leads to the formation of a new potential well of the potential for the CTVEactive vibration. The system's stability with such a new vibronic minimum is controlled by the potential barrier which is mainly of a vibronic origin, because electron-hole Coulomb attraction is strongly screened by the polarization effect in the ferroelectric soft matrices discussed. That is why this exciton state is called CTVE [6,7]. The CTVE state can be considered as a bipolaron comprising a pair of spatially correlated polaronic electron and polaronic hole. Their correlation is controlled by the "negative-U " effect. Moreover, this is

Intense blue and green photoluminescence emissions at room temperature in barium zirconate powders

“…This characteristic property is practically observed for all self-activated ABO 3 perovskites titanates [4]. The origin of green luminescence has been explained and discussed in many papers by different mechanisms, including self-trapped excitons [5], recombination of electron and hole polarons, charge transfer vibronic exciton [6], donor-acceptor recombination [7], transitions in MeO 6 complexes [8] and structurally disordered titanates [9][10][11]. However, few investigations on this phenomenon in zirconates have been reported [12][13][14][15][16].…”

“…The representation of an ordered BaZrO 3 -o model was built through 1 × 1 × 2 supercell (Fig. 5(a) 6 ], as shown in Fig. 5(b).…”

Modelling of defects and surfaces in perovskite ferroelectrics

“…This increases the vibronic interaction and induces a local lattice instability. This results in a formation of the excited state with a new equilibrium charge transfer and lattice relaxation [31,32]. As a result of modelling, we found that the CTVE consists of a pair of spatially well correlated electron and hole polarons localised mainly on the nearest Nb 1 and O 1 atoms.…”

“…The origin of this luminescence has been discussed more than once. Suggested mechanisms include donoracceptor recombination [29], recombination of electron and hole polarons, charge-transfer vibronic excitons (CTVE) [30][31][32] and transitions in MeO 6 complexes [33]. In this paper, we perform a modelling of electron polarons and triplet excitons in ABO 3 perovskites and compare calculated optical energies with relevant experiments.…”

Ab initio Calculations for SrTiO3 (100) Surface Structure

“…Such flat bands could make possible hole and exciton self-trapping. An exciton self-trapping in SrTiOa bulk was indeed discussed quite recently [27].…”