Effect of trivalent rare earth doping in cadmium silicates hosts: a theoretical study

Abstract

The objective of the present work is to investigate the possibilities for extrinsic defects in the three cadmium silicate matrices, CdSiO3, Cd2SiO4 and Cd3SiO5 and understand the final geometry of the most energetically favourable defects, which are generators of the luminescence centres in the material. This analysis will be performed out through classical and static computer modelling, using the code GULP. A number different mechanisms were modelled and their related solid state reactions were devised allowing the energetic calculations for the incorporation of trivalent rare Earth ions (R3+) in these matrixes. The results demonstrated that the trivalent rare Earth ions are preferentially incorporated substitutional at Cd sites with cadmium vacancy being created in order to compensate charge unbalance. Careful investigations of the different distances and angles between the ions involved in the defects formation for CdSiO3 found that all investigated R3+ ions have the same lower energetic defect configuration. The lowest energetic costs were found for a configuration where the rare Earth ions and the accompanying vacancy are almost aligned with a distance of around 7 Å and an angle closer to 180°. These results predict a possible set of two dipoles caused by the position of the cadmium vacancy in defect. Cadmium vacancies were expected to be present in the cadmium silicate due to the low vapor pressure of CdO; however, no previous work was able to pre-establish the final geometric structure of the cadmium silicate matrix in the defective region that can modify the luminescent centres in the material. The study of a wide range of possibilities for the insertion of defects and the proposal for the final configuration of these defects is presented, in order to assist in the identification of the sources and the specific chracteristics of the luminescent emissions presented in cadmium silicates doped with R3+.