Nature of WO4 tetrahedra in blue light emitting CaWO4 probed through the EXAFS technique

Abstract

Blue emission due to self trapped exciton recombination in CaWO₄ is believed to be very sensitive to the nature of the WO₄ structural units. The present manuscript deals with the probing of structural differences existing in WO₄ tetrahedra of CaWO₄ particles having varying average crystallite sizes and different blue light emission characteristics. Based on XRD, W L₁ edge XANES and W L₃ edge EXAFS studies, it is inferred that factors like average W–O and Ca–O bond lengths, average number of oxygen atoms around W⁶⁺ ions and disorder in WO₄ tetrahedra do not have any effect on the blue luminescence intensity from the samples. The lifetime value of excitons is lower for the nanocrystals/nanoparticles of CaWO₄ compared to the bulk sample. The lower lifetime of self trapped excitons and the associated decrease in blue luminescence intensity for nanoparticles/nanocrystals (compared to bulk) has been explained based on competing non-radiative processes involving the interaction of holes with surface hydroxyl groups and an associated decrease in the extent of radiative exciton recombination.