Site selective spectroscopy of rare earth doped luminescent materials

In this disseration we focus on luminescent materials. They are able to emit light, which is not caused by heat and are used in light emitting diodes, displays, medical imaging... Most of these materials consist of a host lattice and an impurity ion. These impurities serve as luminescent centers in the material and are present in small fractions. The local structure and chemical neighborhood of the impurity ions, also called dopants, highly influence the luminescent properties. As such, the knowledge of these would enhance the development of more efficient luminescent materials. In this dissertation we use x-ray absorption spectroscopy (XAS) to probe the neighborhood of the dopant ions. The first part of this dissertation focuses on the application of x-ray absorption near edge spectroscopy (XANES) to luminescent materials. We will show that XANES is a powerful technique to probe the valence state of the impurity ions. Moreover we will demonstrate that XANES can not only be used as a probe to study the static valence state distribution of the dopant ions, but also to study the dynamics of luminescent processes. The aim of the second part of this dissertation is to study the incorporation of the dopant ions using extended x-ray absorption fine structure spectroscopy (EXAFS). We will show that EXAFS data analysis can be used to study the site occupancy by the dopant ions and the local distortion of the lattice