(Invited) Local Environment Analysis of Dilute Rare-Earth Dopants in Phosphor Materials

Abstract

Doping technique, i.e., incorporation of dilute amount of additional element, has been widely applied to add new functions for various kinds of materials. Appropriate doping can change their specific properties significantly, e.g., electric, magnetic, optical, mechanical and chemical properties, etc. It is, of course, fundamental to know the crystal and electronic structures of such materials. In addition to such studies, it is mandatory to investigate the local environment of dopants on an atomic scale to understand the mechanism of appearance of new properties by doping and to design new materials with desired properties by such doping technique. Although crystal structure analysis has been widely carried out by the X-ray diffraction (XRD) technique, it is very difficult to determine the local environment of dilute dopants only by XRD. There are some experimental methods to investigate such local environment of dilute dopants. Among these methods, X-ray absorption near-edge structure (XANES) analysis by using the synchrotron radiations is one of the most powerful methods, which enables us to determine the local environment of dopant at an ultra-dilute concentration level, such as atomic ppm level of concentration [1]. We have reported quantitative analysis of XANES from many different kinds of functional materials [2, 3] with the aid of the first-principles calculations within the density functional theory (DFT). The fingerprint type analysis is widely accepted for various kinds of analysis, but such strategy requires reliable fingerprints to be compared with the newly observed result. However, in the case of local environment analysis by XANES, observed XANES spectrum from dopant often shows different profile from any of the standard ones, which yields that fingerprint type of analysis does not work for the local environment analysis. From such reason, theoretical fingerprint prepared by using first-principles calculation plays a key role for the local environment analysis of dopant by XANES. In the current study, we focus upon local environment analysis of dilute dopants in phosphor materials. After introduction of our analytical method of the local environment analysis with XANES and first-principles calculation, several results on such dopants, rare-earth ions and 3d-transition metal ions in perovskite structured oxides, will be shown. As an example, Ga XANES analysis of Ga doped SrTiO3:Pr, in which Ga codoping enhances red luminescence of Pr doped SrTiO3 [3], is shown in Fig. 1.