Intrinsic Defects And Non-Stoichiometry In Undoped Cadmium Silicate Hosts

Cadmium silicates, mainly the CdSiO3 phase, are interesting materials due to their persistent and intrinsic luminescence, making them possible candidates for a number of applications. Although many of the luminescence properties of these materials are known from the experimental point of view, the current problem that still remains is to understand the origin of the intrinsic luminescence. Different types of point defects were considered in the literature and, most of them are contradictory and/or controversial and lack direct evidence for the assumptions. The aim of the present work is to investigate in depth the possible origin of the luminescence properties of cadmium silicates, focusing on the CdSiO3 phase, considering not only the possibilities reported in the literature but all other mechanisms involving intrinsic defects in these materials. Our results predicted that CdSiO3 tends to show stoichiometric deviation due to CdO pseudo-Schottky defects and the simulated structure for the deficient matrix is much closer to the experimental X-ray diffraction pattern than the stoichiometric one. This feature indicates that most of the available CdSiO3 phases are truly CdO deficient. Additionally, it was found that this defect induces the formation Si-Si bonds that are responsible for the intrinsic luminescence in this matrix. The CdO deficiency also accounts for the presence of additional cadmium silicate phases, like Cd2SiO4 or Cd3SiO5, during the synthesis of CdSiO3, a common feature found in many works in the literature. These results strongly indicate that CdO pseudo-Schottky defects, that trigger the CdO deficiency naturally present in cadmium silicate structures, should be considered not only in all luminescent models proposed, including intrinsic and extrinsic matrix, but also for the structural properties of these materials. (C) 2020 Elsevier B.V. All rights reserved.