Theoretical Prediction and Experimental Confirmation of Charge Transfer Vibronic Excitons and Their Phase in ABO 3 Perov
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Theoretical Prediction and Experimental Confirmation of Charge Transfer Vibronic Excitons and Their Phase in ABO3 Perovskite Crystals
R. I. Eglitisa, V. S. Vikhnina, E. A. Kotomina,b, S. E. Kapphana, and G. Borstela a Department of Physics, University of Osnabrueck, D-49069 Osnabrueck, Germany b Institute of Solid State Physics, University of Latvia, 8 Kengaraga str., Riga LV-1063, Latvia ABSTRACT The 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 lowered by the combination of Coulomb attraction between electron and hole and the vibronic effect in this charge transfer vibronic exciton. It is shown by means of our INDO calculations that polaronic-type CTVE in ferroelectric oxides could lead to the formation of a new crystalline phase. The ground state energy of this phase consisting of strongly correlated CTVEs lies within an optical gap of a pure crystal, and is characterized by a strong tetragonal lattice distortion, as well as by the ferroelectric ordering. INTRODUCTION Charge transfer excitons interacting with the crystalline latice are the subject of intensive theoretical studies performed during the last 25 years. The pioneering Agranovich-Reineker approach [1,2] was based on a linear exciton-phonon interaction which is induced by modulation of the Coulomb electron-hole attraction. Their basic assumptions are valid for many real systems but turn out to be inadequate for ABO3 ferroelectric oxides and other strongly polarisable, partly covalent matrices with linear and significant non-linear vibronic interaction and anharmonicity. The description of electronic excitations in these systems needs another approach. Development of such an approach based on polaronic-type models [3,4] is the main purpose of the present work. Our main idea is the Charge Transfer Vibronic Exciton (CTVE) [5,6]. The CTVE in ABO3 ferroelectric oxides with partly covalent chemical bonding consist of spatially correlated pairs of electronic and hole polarons [6-8]. Oxygenoctahedral ABO3 perovskite ferroelectrics and related oxides serve as good examples of advanced materials with partly covalent chemical bonding, where, on the one hand, charge transfer effects are rather strong, and, on the other hand, the vibronic interactions are also large. KTaO3 and KNbO3 crystals are considered below as illustrative examples of matrices revealing new type polaronic excitons (CTVE) in ferroelectric and other oxides. We presen
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