UV light‐induced absorption in pure KTaO ₃ at low temperature

Abstract

Pure KTaO 3 with cubic symmetry down to lowest temperatures is a model incipient ferroelectric exhibiting the existence of intrinsic inversion symmetry breaking defects in a wide range of experiments. At low temperature a strong second harmonic generation (SHG) of Nd:YAG laser light is observed, despite the overall cubic symmetry of the crystal. Under excitation with UV-light in the band gap region (about 330 nm (3.7 eV)) a strong green luminescence (GL, at 510 nm (2.43 eV)) appears with excitation maximum near 330 nm (3.7 eV). Both, SHG and GL depend strongly on previous oxidation/reduction treatments, increasing with the reduction. In absorption at low temperature broad bands peaking at 1.2 eV and at 1.5 eV are observed in reduced KTaO 3 samples. Additional UV-illumination in the band gap region at low temperature gives rise to broad light-induced absorption bands in the same 1-2 eV region. These observations are being discussed on the basis of polaronic holes, of charge transfer vibronic exitons (CTVEs) and of CTVE clusters interacting with oxygen vacancies.

Efficient water splitting over Na1−xKxTaO3 photocatalysts with cubic perovskite structure

“…Among the several mechanisms for discussing the origins of visible luminescence in perovskite-type tantalates (ATaO 3 ) [42][43][44][45] or titanates (ATiO 3 ), [49][50][51][52][53] researchers have recognized the recombination of trapped electrons and holes (excitons) as a characteristic feature for PL. [54][55][56][57][58][59][60] The luminescence emission at 440 nm (so-called ''blue luminescence,'' BL) can therefore be attributed to the intrinsic radiative emission 43 related to the electron-hole pair recombination of a localized exciton Ta 4+ -O À in a TaO 6 octahedron. The large Stokes shifts can be ascribed to a strong electron-lattice interaction and exciton relaxation.…”

An optical and dielectric spectroscopy study of Er3+‐doped KTaO3

“…In making the choice among possible candidates for the minor Er 3+ centres, we take into account the fact that the absorption band in Fig. 6 resembles, remarkably, a UV‐light‐induced wide absorption band with a maximum at 1.16 eV ( λ = 1068 nm, T = 52 K), which has been reported for nominally pure KTO and attributed to O − hole polarons 25. In our case, this indicates polaronic complexes as the most probable candidates for Er 3+ minor centres.…”

Photochromism and polaronic photocharge localization in diluted KTa1−xNbxO3

“…12͒ displaying absorption bands at ϳ9200 cm −1 ͑1.14 eV͒ attributed to O − hole polarons 12,29 and a very faint band in the 5000-6000 cm −1 region. 12͒ displaying absorption bands at ϳ9200 cm −1 ͑1.14 eV͒ attributed to O − hole polarons 12,29 and a very faint band in the 5000-6000 cm −1 region.…”

“…12 Additional UV illumination in the band-gap region gave rise to broad light-induced absorption bands in the same 1 -2 eV region, more pronounced for as grown and oxidized specimens. However, there are only a few of such optical experiments containing reliable data concerning incipient ferroelectrics at low temperatures.…”