Formation of localized hole states in complex oxides: I. Hole states in

Abstract

Defect electrons (holes) play an important role in most technologically important complex oxides. In this contribution we present the first detailed characterization of localized hole states in such materials. Our investigations employ advanced embedded-cluster calculations which consistently include electron correlations and defect-induced lattice relaxations. This is necessary in order to account for the variety of possible hole-state manifestations.PACS numbers: 71.55Ht, 71.50.+t I. INTRODUCTIONThe basic structural building units of solid oxides are MO 6 metal-oxygen octahedra and, possibly, additional MO 4 tetrahedra. The complete crystal structure is built up of corner-, face-and/or edge-sharing connections of these structural elements. Additional cations (A) can be incorporated at interstitial lattice sites. These are increasingly formed, the more open-structured the whole network of MO n units appears to be. Generally, open crystal structures imply high formal M-cation charge states (referring to formal O 2− anions) leading to mixed ionic-covalent (or semiionic) material properties. The M-cations are in most cases transition-metal (TM) ions. Examples of such complex materials are given by AMO 3 perovskite-structured oxides such as barium titanate (BaTiO 3 ). Perovskite oxides are frequently ferroelectric and possess important electrooptic applications based on the photorefractive effect. Also high-T C oxides resemble the perovskite structure.Charge carriers, either valence-band (VB) holes or conduction-band (CB) electrons, are created by doping with impurities, annealing treatments or by light-induced charge-transfer excitations which, for example, take place during photorefractive processes in the appropriate oxides. Combined optical-absorption-and electron-spin-resonance measurements (Photo-ESR) [1] proved that the created and afterwards trapped holes influence the light-induced charge-transfer reactions in photorefractive BaTiO 3 . These holes, which are either para-or diamagnetic, probably induce the observed sublinear dependence of photoconductivity on the light intensity (e.g. [2]). A further example highlighting the relevance of holes refers to high-T C oxides: Pairs of doped holes give rise to superconductivity in these materials. The possible pairing scenarios are still a matter of active debate.The present work initiates a systematic and detailed characterization of localized hole states in complex oxides. Our investigations employ real-space embedded-cluster calculations, which consistently combine electron correlations and defect-induced lattice relaxations. This procedure is indispensable in order to predict the richness of possible hole states. Here, we demonstrate simulations for trapped holes in BaTiO 3 , but many results, ranging from stabilization of cationic charge states to the formation of bipolarons, can be extrapolated to other oxides.The trapping of holes at acceptor defects leads to a localization of hole states, which is further aided by defectinduced lattice distortions. In near-...

Surface Reconstruction with a Fractional Hole:(5×5)R26.6°LaAlO3(001)

“…3. This behavior is atypical for most bulk oxides since hole trapping occurs through acceptor defect sites in open lattice perovskite structures [45]. This effect leads to small hole mobilities in bulk materials, and is reminiscent of the Mott-Wannier exciton model in more ionic crystals (despite the increased covalency here).…”

O−bound small polarons in oxide materials

“…Among other models, such situations can point to the fact that an O − hole is delocalized over two neighbouring oxygen ions, as e.g. for a hole next to Al 3+ in BaTiO 3 [13,14]. Such cases, however, occur rather seldom for oxides.…”

The Energy Spectrum of Single-Hole States in Transition Metal Oxides

“…The system is then considered as a quasi-crystal constructed from metaloxygen clusters with the four p-d single-hole states of distinct energies, attached to each of them [2,3]. The clusters, labelled by their TM cations, are separated from one another, so the compensating holes get trapped and can only be released by the direct ρ-ρ hopping between oxygens belonging to different clusters.…”

“…The crystal is then considered as constructed of these simple structural elements which can be either mutually-separated, or form corner-, face-, and/or edge-sharing connections [2,3]. The Μ cations are in most cases transition-metal (TM) trivalent ions.…”