Pre-edge Peak of Ti K XANES: Determined by Short- or Medium-Range Structure?
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1152-TT01-06
Pre-edge Peak of Ti K XANES: Determined by Short- or Medium-Range Structure? Nan Jiang Department of Physics, Arizona State University, Tempe AZ 85287-1504, U.S.A. ABSTRACT The calculations of Ti K-edge X-ray absorption near-edge structures (XANES) in fresnoite Sr2TiSi2O8 are carried out using multiple-scattering method. The origins of the pre-edge peak along with the fine structures in the main peak of Ti K-edge XANES are interpreted by comparing the calculations in various modified structure models. It concludes that the pre-edge peak is dominantly determined by the structure (including both coordination and bond lengths) within the nearest-neighbor distance, while the fine structures beyond the pre-edge peak region are sensitively determined by the structure extending to the medium-range distance. INTRODUCTION Ti K-edge XANES have been intensively studied both experimentally and theoretically for several decades. The attention has been particularly focused on its pre-edge features. A survey in a quite large group of Ti-containing silicates and oxides confirmed that the near-edge features are sensitive to the symmetry and coordination of Ti polyhedron [1]. The efforts have also been made to quantify the coordination of Ti in amorphous materials utilizing the height and energy of pre-edge peaks of Ti K-edge XANES [2]. However, our recent theoretical study shows that the coordination is not the only factor that determinates the appearance of the pre-edge features of Ti K XANES, while a reduction of Ti – O bond length has definitely a great impact as well [3]. In electron energy-loss spectroscopy (EELS), which is the electron version of X-ray absorption, it is found that the near-edge structure is also sensitive to the structure in the medium-range order [4]. This phenomenon can be easily understood within the real-space multiple scattering (MS) theory. In principle, an absorption spectrum is due to quantummechanical transitions that excite a particular inner-shell electron of a specific atom to the unoccupied states, i.e. to the states with an excited electron above the Fermi energy, leaving behind a core hole. According to MS theory, the outgoing excited electron can be considered as a quantum electron wave that spreads out over the material [5]. The surrounding atoms can act as scattering centers which scatter the outgoing electron wave back towards the original atom. The amplitudes of the outgoing electron wave and all the reflected waves add up at the absorbing atom either constructively or destructively, and hence modulate the density of states (DOS). Due to the limited lifetime of the excited electron, the excited electron state decays rapidly as a function of time and distance. Therefore, absorption fine structure is determined by the local atomic structure and chemistry (or composition) over a medium-range distance, typically on an order of several nanometers or less [6]. Although the pre-edge peak of Ti K XANES was frequently used in literature as an indicator of Ti coordination, there has bee
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