INFORMATION OBSERVED IN Ti-Lα,β AND Ti-Ll,η EMISSION LINES OF Ti AND ITS OXIDES

摘要

X-rays originate from electronic transitions from valence bands (VB, bonding electron states) to inner-shell electron levels inform us of the energy states of the bonding electrons [1]. Thus, L-emissions of 3d transition metal elements, which range from 400 to 1,000 eV, are very important to assign the chemical states of those elements in compounds. La- and Lβ-emissions due to 3d_(5/2,3/2) →► 2p_(3/2) and 3d_(3/2) → 2p_(1/2) transitions, respectively, are suitable to probe valence states (bonding states). On the other hand, Ll- and Lη-emissions correspond to transitions from a shallow inner-shell level of 3_(S1/2) to 2p_(3/2) and 2p_(1/2) levels, respectively. Thus, Ll,η-lines can give us different information from that of La,β. Furthermore, charge state is also important to discuss the physical property because of the amount of 3d electrons closely related to magnetism and conductivity of 3d transition metal compounds. Figure 1 shows La,P- and Ll,η-emissions of metallic Ti, ε-TiO [2], and TiO_2 (rutile). These spectra were obtained from those bulk materials by using a soft-X-ray emission spectrometer attached to a scanning electron microscope [3]. The accelerating voltage was 5 kV. Spectral intensities are normalised at the peak height of the Ll-emission. In a simple ionic model, TiO_2 is considered as Ti~(4+)O_2~(2-) and cannot emit La,β-intensity because a Ti~(4+) ion does not have any 3d electrons. However, La,β-emissions are clearly observed. Thus, Ti atoms in TiO_2 are not pure Ti~(4+) ions via covalent bonding between Ti and O atoms. Intensity profiles of TiO_2 (rutile) and e-TiO are apparently different, reflecting different bonding states due to different crystal structures. The Ti atom is surrounded by six O atoms forming an octahedron in TiO_2, and surrounded by trigonal prismatic (six-fold coordination) and trigonal planar (three-fold coordination) arrangements of O atoms in ε-TiO [2]. Those intensity profiles should be compared with those of calculated Ti-3d component of density of state of valence bands. When discussing the relative position of those spectra, chemical shifts of Ti-2p level and band gap energies of those materials should be taken into account. Although information of chemical shift is important, it is difficult to deduce from different intensity profiles of La,β as seen in Fig. 1. The LI intensity profile is simply due to transitions between inner-shell levels. As seen in Fig. 1, the LI energy position of ε-TiO is almost the same as for metallic Ti. On the other hand, the LI of TiO_2 shows a shift to the higher energy side by about 0.7 eV compared to metallic Ti. As chemical shifts of core levels due to valence charge exhibit the same amount of shift [4], energy differences between core-levels do not show a chemical shift. Thus, the shift of Ll observed in Fig. 1 is interesting. By consulting a discussion on chemical shift in X-ray photoelectron spectroscopy, a shift of the Ll-line can be attributed to the relaxation energy of surroundings of the excited atom, which is due to a presence of a 3s hole in the final state. Thus, theoretical calculation on the binding energy of Ti- L3(2p_(3/2))- and Ti-Ml(3_(S1/2))-levels with a 3s hole were conducted for metallic Ti and TiO_2 by using the Wien2k code with a 3x3x3 supercell. The energy difference between Ti-L3 and Ti-Ml in TiO_3 is larger than that in metallic Ti by 0.64 eV. This value explains well the observed shift value of 0.7 eV. This core-hole effect in the final state is also important in inner-shell excitation electron energy-loss spectroscopy. As the effect originates from the screening of core-hole, the shift reflects dielectricity around the excited atom. Since ε-TiO should be a metal, it is reasonable that the LI energy positions of metallic Ti and s-TiO are almost the same. Shifts of other 3d transition metal elements in oxides will be also discussed. It should be noted that La,(3-intensities of metallic Ti and TiO_2 in Fig. 1, in which the spectral intensities are normalised to the peak height of the Ll-emission, are not so different. The number of 3 s electrons that can contribute to Ll-emission is the same for all Ti atoms, but the number of 3d electrons of a Ti atom can be different depending on the chemical bonding state. As La,β-emission originates from transitions of 3d electrons into the 2p core-hole states, those similar La,β-intensities of metallic Ti and TiO_2 can be attributed to similar d-electron numbers at the excited Ti atom with 2p hole. Thus, it is worth examining the distribution of 3d electrons around Ti atoms with and without 2p holes in metallic Ti and TiO_2 by theoretical calculation.