Chemistry Reference
In-Depth Information
2.1 Introduction
Shortly after the discovery of recoilless nuclear resonance fluorescence of gamma
radiation by Rudolf Mössbauer [
1
-
4
], physicists and chemists explored the pos-
sibility to use this effect as a basis for a new physical technique in materials
science. One of the first publications, by Kistner and Sunyar [
5
] reported the
magnetic hyperfine splitting of a-Fe
2
O
3
(Fig.
2.1
a). This was the first spectrum of
a material that reflected all three types of hyperfine interactions between nuclear
moments and electrons penetrating the nucleus that can be observed in a
Mössbauer spectrum: the electric monopole interaction resulting in the isomer
shift, the electric quadrupole interaction causing the quadrupole splitting and the
magnetic dipole interaction giving rise to magnetic splitting of degenerate nuclear
levels. These interactions are all taken into account in Fig.
2.1
b (middle) in the
case of Fe
2
O
3
, and are compared on the left side of Fig.
2.1
b with metallic iron
which shows a magnetic splitting only and on the right side with stainless steel that
displays an isomer shift only. This report demonstrated the usefulness of the
Mössbauer effect as a new spectroscopic technique—Mössbauer spectroscopy—
for the non destructive characterization of materials and initiated overwhelming
research activities in physics, chemistry, geo- and earth sciences, and even
industrial applications.
The purpose of this tutorial lecture is to demonstrate how Mössbauer spec-
troscopy can help solving chemical problems. There are several thousands of
excellent examples that have appeared in the literature since the discovery of the
Mössbauer effect [
6
-
17
]. Because of limited space we shall present a selection of
chemical applications of Mössbauer spectroscopy mainly from our own work
dealing with investigations of transition metal compounds featuring:
• Bonding and structural properties
• Valence state
• Solid state reactions
• Electron transfer reactions
• Mixed valency
• Spin crossover
• Magnetic properties
The major part of the chapter will be devoted to the phenomenon of thermally
induced spin crossover in iron complex compounds. This research topic has
recently gained increasing interest by chemists and physicists for the promising
potential of technical applications as devices. It will be demonstrated that
Mössbauer spectroscopy, together with magnetic measurements, is particularly
suited to follow the electronic structure dynamics of such materials under various
conditions.
