Photo-stimulated luminescence and photo-induced infrared absorption in ZnWO4

Abstract

Reflection, luminescence-excitation, luminescence decay kinetics, photo-stimulated luminescence, and photo-induced infrared absorption have been measured for ZnWO 4 crystals in the temperature range of 6−300 K. The results are compared with the previous data. The optical properties of ZnWO 4 are discussed on the basis of the present results, as well as the result of the relativistic molecular orbital calculation.

Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radius

“…This dependence is typical of the low-energy tails of directabsorption edges with excitonic effects and has been attributed to the dissociation of excitons in the electric fields of polar phonons or impurities. The presence of the Urbach's tail is in agreement with the conclusion drawn from low-temperature measurements performed by Itoh et al [12]. These authors as well as others also concluded that the lowest band-gap of metal tungstates is direct [12,14,19].…”

“…Regarding, the steep absorption edge, as stated in the literature [12,14,21,28], we found that it exhibits an exponential dependence on the photon energy following the Urbach's law [29]. This dependence is typical of the low-energy tails of directabsorption edges with excitonic effects and has been attributed to the dissociation of excitons in the electric fields of polar phonons or impurities.…”

Static and dynamic structure of ZnWO4 nanoparticles

“…The red shift of the band allows to exclude the presence of the quantum size effect, which should cause, on the contrary, a blue shift. The luminescence decay kinetics in nanoparticles is faster than in microcrystalline ZnWO 4 [21] and cannot be described by three component exponential decay, associated with the three types of self-trapped excitons [34], opposite to the case of microcrystalline [21,26] or single-crystal [35] ZnWO 4 . The shift of the luminescence band position to lower energies and the acceleration of the luminescence kinetics can be explained by a significant deformation of WO 6 octahedra and by an influence of surface defects in nanoparticles.…”

Electronic excitations in ZnWO4 and ZnxNi1−x WO4 (x = 0.1 − 0.9) using VUV synchrotron radiation

“…In pure ZnWO 4 the origin of the main band at 2.5 eV has been previously assigned to radiative electron transitions within the [WO 6 ] 6− anions [11,12]. At the same time, the band at ∼ 2 3 eV has been attributed to recombination of e-h pairs localized at oxygen-atom-deficient tungstate ions [15,16] or distorted WO 6 octahedra [17]. The origin of the last band at 3.0 eV appearing in the solid solutions is attributed to the interference between the broad luminescence band of the WO 6 groups and the absorption band of the NiO 6 groups.…”

“…In particular, the intrinsic luminescence band, observed at room temperature at about 2.5 eV, has been attributed to a charge transfer between oxygen and tungsten ions in the [WO 6 ] 6− anions [11][12][13][14]. The contributions from defects and distorted tungsten-oxygen octahedra have been also observed [15][16][17]. Up to now, most fundamental studies on tungstates have been performed on single-crystal samples.…”