Local structure and bonding in tin nanoparticles probed by temperature-dependent EXAFS

Tin nanoparticles (n-Sn) are expected to undergo a phase transition between<i>α</i>-Sn (a semiconductor with a diamond structure) and<i>β</i>-Sn (a metal with a tetragonal structure) as a function of particle size, similar to the temperature-induced transition that occurs near room temperature. X-ray absorption fine structure measurements were conducted to identify the local structure and extract detailed structural parameters of n-Sn with a diameter of 24.7 Å. The nearest-neighbor atomic distance suggests that the local configuration of n-Sn resembles the diamond-like structure of<i>α</i>-Sn. However, several experimental observations have indicated that n-Sn does not adopt a perfect diamond structure like that of<i>α</i>-Sn, but rather exhibits an amorphous character. These observations include a decrease in the coordination number, an increase in the static component of the mean-squared relative displacement of the first atomic correlation, and the disappearance of all atomic correlations beyond the nearest-neighbor. Furthermore, the covalent bonds in n-Sn elongate with increasing temperature, in contrast to those in<i>α</i>-Sn which exhibits no temperature dependence. The covalent bond strength of n-Sn was also found to be weaker than that of<i>α</i>-Sn.