Coordination structures of implanted Fe, Co, and Ni ions in silica glass by x-ray absorption fine structure spectroscopy
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Coordination structures of implanted Fe, Co, and Ni ions were studied in 1.78–2.00-MeV 5 × 1016 ions/cm2-implanted silica glasses by x-ray absorption and optical absorption spectroscopies. It was found from x-ray absorption spectra that the implanted Fe, Co, and Ni ions are coordinated by ca.3 oxygen atoms. The implanted ions dispersed in glass matrix and did not form crystals. The valence of the implanted ions was about 1.5. The Fe–O, Co–O, and Ni–O interatomic distances were 190, 191, and 192 pm, respectively. In addition, it was found from optical absorption spectra that one-fifth of implanted Co ions were present as Co2+ ions in tetrahedral symmetry.
I. INTRODUCTION
Ion implantation is a useful method for the doping of ions in materials because of its control of ion beam position and of the ion concentration. Various kinds of ions have been implanted in silica glass to modify surface properties of glasses,1,2 and it has been proposed that implanted ions form elemental colloid particles in silica glass when the free energy of formation of oxides of implants is greater than that of silica at 3000 K.3 On the other hand, studies also show that although implanted ions are in a metallic state at high local concentration, they are in an ionic state at low local concentration.4 – 6 These findings indicate that the chemical state of implanted ions depends on their transport process in glass and on the thermodynamic stability of the system of matrix and implanted ions.6 Thus far the chemical state of a number of implanted ions has been studied by Mo¨ssbauer,4 electron spin resonance,8 and x-ray photoelectron spectroscopies;5,6,9 however, the coordination state, such as coordination number and bond length, has yet to be studied at length. Another previous study showed that implanted copper ions are present as Cu(I) state and that a copper atom forms ca. two Cu–O bonds, the length of which is 188 pm in 2-MeV 1 × 1017 Cu+ ion/cm2implanted silica glass.10 Moreover, it was shown from the viewpoint of thermodynamics that Cu(I) is more stable than Cu(0) and Cu(II) under the condition that the transport process is restricted. Thus, we note that although the lowest valence of copper atoms in copper oxides is monovalent under conventional conditions, the
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 1, Jan 2001
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lowest valence of iron, cobalt, and nickel atoms is divalent under conventional conditions, and it is for this reason that a study of the valence and coordination state of Fe, Co, and Ni ions is of interest. In this study, we have examined the valence state and coordination state of implanted iron, cobalt, and nickel atoms in silica glass by x-ray absorption fine structure (XAFS) spectroscopy, both the extended x-ray absorption fine structure (EXAFS) and the x-ray absorption nearedge structure (XANES). II. EXPERIMENTAL PROCEDURE A. Sample preparation
Fe+, Co+, Ni+ ions were implanted in silica glass plates (
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