Holes bound to acceptor defects in oxide crystals are often localized by lattice distortion at
just one of the equivalent oxygen ligands of the defect. Such holes thus form small polarons
in symmetric clusters of a few oxygen ions. An overview on mainly the optical
manifestations 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 oxide
materials are presented which exhibit bound small polaron optical properties. The
first part starts with summaries on the production of bound hole polarons and
the identification of their structure. It is demonstrated why they show strong,
wide absorption bands, usually visible, based on polaron stabilization energies of
typically 1 eV. The basic absorption process is detailed with a fictitious two-well
system. Clusters with four, six and twelve equivalent ions are realized in various
oxide compounds. In these cases several degenerate optically excited polaron
states occur, leading to characteristic final state resonance splittings. The peak
energies of the absorption bands as well as the sign of the transfer energy depend
on the topology of the clusters. A special section is devoted to the distinction
between interpolaron and intrapolaron optical transitions. The latter are usually
comparatively weak. The oxide compounds exhibiting bound hole small polaron
absorptions include the alkaline earth oxides (e.g. MgO), BeO and ZnO, the perovskites
BaTiO3 and
KTaO3, quartz, the
sillenites (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 being
elementary paradigms for the properties of small polarons in general, the defect species
treated can be used to explain radiation and light induced absorption especially in laser and
non-linear oxide materials, the role of some defects in photorefractive compounds, the
coloration of various gemstones, the structure of certain catalytic surface centres,
etc. The relation to further phenomena is discussed: free small polarons, similar
distorted centres in the sulfides and selenides, acceptor defects trapping two holes.
An overview of the properties of electron small polarons and bipolarons is given, which can occur in the congruently melting composition of LiNbO(3) (LN). Such polarons influence the performance of this important optical material decisively. Since coupling to the lattice strongly quenches the tunnelling of free small polarons in general, they are easily localized at one site even by weak irregularities of a crystal. The mechanism of their optical absorptions is thus shared with those of small polarons localized by binding to selected defects. It is shown that the optical properties of free electrons in LN as well as those bound to Nb(Li) antisite defects can be attributed consistently to small polarons. This is extended to electron pairs forming bipolarons bound to Nb(Li)-Nb(Nb) nearest neighbours in the LN ground state. On the basis of an elementary phenomenological approach, relying on familiar concepts of defect physics, the peak energies, lineshapes, widths of the related optical absorption bands as well as the defect binding energies induced by lattice distortion are analysed. A criterion universally identifying small polaron absorption bands in oxide materials is pointed out. For the bipolarons, the dissociation energy, 0.27 eV, derived from a corresponding study of the mass action behaviour, is shown to be consistent with the data on isolated polarons. Based on experience with simple O(-) hole small polaron systems, a mechanism is proposed which explains why the observed small polaron optical absorptions are higher above the peak energies of the bands than those predicted by the conventional theory. The parameters characterizing the optical absorptions are seen to be fully consistent with those determining the electrical conductivity, i.e. the bipolaron dissociation energy and the positions of the defect levels as well as the activation energy of mobility. A reinterpretation of previous thermopower data of reduced LN on the basis of the bipolaron model confirms that the mobility of the free polarons is activated by 0.27 eV. On the basis of the level scheme of the bipolarons as well as the bound and free polarons the temperature dependence of the electronic conductivity is explained. The polaron/bipolaron concept also allows us to account for the concentrations of the various polaron species under the combined influence of illumination and heating. The decay of free and bound polarons dissociated from bipolarons by intense short laser pulses of 532 nm light is put in the present context. A critical review of alternative models, being proposed to explain the mentioned absorption features, is given. These proposals include: single free polarons in the (diamagnetic) LN ground state, oxygen vacancies in their various conceivable charge states, quadpolarons, etc. It is shown why these models cannot explain the experimental findings consistently.
The optical absorption of amorphous
WO3
electrochromic display layers is explained as small polaron absorption. The necessary W5+ 5d‐electron localization is favored by the lattice disorder. This is concluded from the change of the optical properties to free‐electron‐like behavior upon crystallization of the layers. The increased electrocoloration stability of amorphous layers and the blue shift of the absorption peak in Mo‐doped
WO3
films can also be explained within the model given.
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