O− bound small polarons in oxide materials

Abstract

Holes bound to acceptor defects in oxide crystals are often localized by lattice distortion atjust one of the equivalent oxygen ligands of the defect. Such holes thus form small polaronsin symmetric clusters of a few oxygen ions. An overview on mainly the opticalmanifestations of those clusters is given. The article is essentially divided into two parts:the first one covers the basic features of the phenomena and their explanations,exemplified by several paradigmatic defects; in the second part numerous oxidematerials are presented which exhibit bound small polaron optical properties. Thefirst part starts with summaries on the production of bound hole polarons andthe identification of their structure. It is demonstrated why they show strong,wide absorption bands, usually visible, based on polaron stabilization energies oftypically 1 eV. The basic absorption process is detailed with a fictitious two-wellsystem. Clusters with four, six and twelve equivalent ions are realized in variousoxide compounds. In these cases several degenerate optically excited polaronstates occur, leading to characteristic final state resonance splittings. The peakenergies of the absorption bands as well as the sign of the transfer energy dependon the topology of the clusters. A special section is devoted to the distinctionbetween interpolaron and intrapolaron optical transitions. The latter are usuallycomparatively weak. The oxide compounds exhibiting bound hole small polaronabsorptions include the alkaline earth oxides (e.g. MgO), BeO and ZnO, the perovskitesBaTiO3 andKTaO3, quartz, thesillenites (e.g. Bi12TiO20), Al2O3, LiNbO3,topaz and various other materials. There are indications that the magnetic crystals NiO, doped with Li,and LaMnO3, doped with Sr, also show optical features caused by bound hole polarons. Beyond beingelementary paradigms for the properties of small polarons in general, the defect speciestreated can be used to explain radiation and light induced absorption especially in laser andnon-linear oxide materials, the role of some defects in photorefractive compounds, thecoloration of various gemstones, the structure of certain catalytic surface centres,etc. The relation to further phenomena is discussed: free small polarons, similardistorted centres in the sulfides and selenides, acceptor defects trapping two holes.