Material Science and Engineering

Biography

Biography: Georges Denes

Abstract

Crystallography is unquestionably the most powerful method for obtaining structural data about crystalline solids. However, there are some cases where even the most powerful method can benefit from help from techniques that are not used for structural determination. In the current work, ¹¹⁹Sn Mossbauer spectroscopy was used to assist crystallography, for finding the tin (II) positions in the unit cell and determine a tin (II) coordination in agreement with both the diffraction data and the tin electronic structure. The first case will show that even high-quality single crystal data do not always guarantee that the right solution will be obtained. A first attempt at the structure of α−SnF2 yielded the tin positions with very reasonable R and R_w residuals, 0.23-0.25. However, the fluorine positions could not be found. After many other attempts, the full crystal structure was finally solved 14 years later. The difference in the tin position between the two solutions was that, in the latter, half of the tin atoms were on special sites; however, the tin sublattice was identical. Because the tin sites in the initial solution gave very reasonable residuals, 14 years of efforts were wasted. The presentation will show that this could have been avoided using ¹¹⁹Sn Mossbauer spectroscopy. This was possible since the spectrum had already been recorded. It will also be shown how Mossbauer spectroscopy can help determine the tin coordination, when combined with powder diffraction data, in the case of disordered structures. The presence of tin(II), disordered with a metal ion in cubic coordination, when diffraction shows there is no lattice distortion and no superstructure, suggests that tin has also a cubic coordination. This would require the tin lone pair to be non-stereo active; however, Mossbauer spectroscopy shows it is stereo active. The same technique helps to suggest an alternate disordered structure in agreement with the X-ray powder diffraction data. Furthermore, ¹¹⁹Sn Mossbauer spectroscopy was also used to assist diffraction for solving the crystal structure of a compound suffering from an extreme case of preferred orientation. The presentation will show the hurdles faced by diffraction methods alone, and how we designed the use of Mossbauer spectroscopy in order to rescue crystallography