EXAFS Study of Radiation-Induced Defects in Molybdenum and Tantalum Metals

Abstract

Radiation defects arising from the high-dose implantation of ⁵⁷Fe⁺ ions into matrices of refractory Mo and Ta metals are studied by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy in the region of the Fe K-edge Fe. The concentration of Fe atoms, which is calculated using the SRIM program, is 2.5 at % at a depth of ≈300 nm. The concentration of radiation-induced defects exceeds 100 dpa. The positions of Fe atoms are determined in the samples after implantation and additional annealing at 700°C. In Mo, Fe atoms are predominantly located in substitution positions in a body-centered cubic (bcc) crystal lattice. An analysis of the EXAFS spectra shows that, on average, 1.4 atomic vacancies are in the first coordination sphere of Fe, which corresponds to a vacancy concentration of about 24 at %. In the Ta matrix, Fe atoms are located in a more complex manner. To analyze the data, model EXAFS spectra are calculated for typical atomic configurations of Fe atoms in a Ta matrix, which form as a result of irradiation. Relaxation of the crystal environment of Fe atoms is taken into account by the density-functional-theory method. Comparison of the experimental spectra with the calculated ones shows that Fe atoms are located in several positions in Ta, including interstitial and substitution positions. Annealing leads to a change in the coordination and partial segregation of Fe atoms. The EXAFS data are consistent with previously published Mössbauer-spectroscopy data. It is demonstrated that the EXAFS method makes it possible to more accurately determine the configurations of defects arising from irradiation.