O^– bound polarons in oxide materials

Abstract

The optical absorption of holes, self-trapped at one of the equivalent O 2-sites next to acceptor defects in crystalline oxide materials, is treated. The holes are localized by lattice distortion, forming small polarons with typical stabilization energies E P of 1 eV. The light-induced hole transfer from an initial O -site to an equivalent O 2-neighbour occurs under Franck-Condon conditions, leading to the widest possible homogeneous absorption bands, having high oscillator strength, typically 0.1, and peak energies between 8/3 E P and 1 E P , for 4 up to 12 equivalent neighbours. The basic absorption features, including characteristic excited state tunnelling splittings, are exemplified with representative defects of high point symmetry. Such polarons arise in numerous materials after excitation by light or ionizing radiation. They play important roles as sources of coloration in non-linear optic and photorefractive compounds, gemstones, as acceptor defects in oxide semiconductors and as catalytic agents, if at a surface. 1 Introduction Valence band holes created, e.g., by ionizing radiation or light-illumination, tend to be trapped at acceptor defects. If their ionization energy is high, as for cation vacancies or simple ions (Li + , Mg 2+ , Al 3+ etc.), the holes will localize at the O 2-ions neighbouring the cation impurity, rather than at the acceptor itself. EPR investigations have shown that lattice distortion leads to trapping at usually one of these oxygen sites, in spite of their equivalence in crystals. These holes thus form small polarons, characterized by localization at one site and a distortion field involving mainly the bonds of O -to its first neighbours. These objects are elementary paradigms for free small polarons, self-trapping at one of the translation-equivalent sites in a crystal. Mainly the optical absorption of bound polarons is treated as based on a configuration coordinate model. This simple approach allows to explain the typical features of those objects -wide absorption bands, high oscillator strengths, excited state tunnelling splittings -under a common guideline. This method was developed rather early [1]. New manifestations of O -bound small polarons have been found in many materials. Here some recent examples will be included.

Comparative Study of Photocarrier Dynamics in CVD-deposited CuWO4, CuO, and WO3 Thin Films for Photoelectrocatalysis

“…Our TRMC experiments performed in a resonant microwave cavity [26] allow for measurements at laser pump powers of at least a factor of ∼100 lower and possibly better reflect the impact of native defects in WO 3 with respect to trapping dynamics. The presence of traps has been reported by several groups [41,[46][47][48][49][50][51][52][53][54][55]. But also the CVD deposited WO 3 film studied here is stoichiometric [16] which possibly makes a big difference in carrier transport properties.…”

Infrared optical response of LuFe2O4under dc electric field

“…This photo-ionization process results in a broad and asymmetric absorption band in the optical conductivity as seen in, for example, La 2−x Sr x CuO 4+δ , La 2−x Sr x NiO 4+δ , MgO and BeO. 19,20 Considering the natural carrier doping due to non-stoichiometry in LFO, it is reasonable to expect that the polaron absorption will be present in ionic crystal LuFe 2 O 4 as well. D. Emin calculated the optical conductivity function σ 1 (ω) due to the small and large polaron absorption process theoretically.…”

Modeling of Intrinsic Electron and Hole Trapping in Crystalline and Amorphous ZnO

“…To test how the h‐DFT functional is reproducing the known polaronic states, we calculated the properties of ZnO:Li Zn center, which has been studied experimentally and theoretically . The thermodynamic charge transition level for the Li Zn defect, (Li–O) −/0 , was calculated using the usual expression where and are the total energies of the defective and non‐defective cells.…”