Mössbauer-Active Transition Metals Other than Iron

The previous chapters are exclusively devoted to the measurements and interpretation of ⁵⁷Fe spectra of various iron-containing systems. Iron is, by far, the most extensively explored element in the field of chemistry compared with all other Mössbauer-active elements because the Mössbauer effect of ⁵⁷Fe is very easy to observe and the spectra are, in general, well resolved and they reflect important information about bonding and structural properties. Besides iron, there are a good number of other transition metals suitable for Mössbauer spectroscopy which is, however, less extensively studied because of technical and/or spectral resolution problems. In recent years, many of these difficulties have been overcome, and we shall see in the following sections a good deal of successful Mössbauer spectroscopy that has been performed on compounds of nickel (⁶¹Ni), zinc (⁶⁷Zn), ruthenium (mainly ⁹⁹Ru), tantalum (¹⁸¹Ta), tungsten (mainly ¹⁸²W, ¹⁸³W), osmium (mainly ¹⁸⁹Os), iridium (¹⁹¹Ir, ¹⁹³Ir), platinum (¹⁹⁵Pt), and gold (¹⁹⁷Au). The nuclear γ-resonance effect in the case of technetium (⁹⁹Tc), silver (¹⁰⁷Ag), hafnium (¹⁷⁶Hf, ¹⁷⁷Hf, ¹⁷⁸Hf, ¹⁸⁰Hf), rhenium (¹⁸⁷Re), and mercury (¹⁹⁹Hg, ²⁰¹Hg) has been of relatively little use to the chemists, so far. There are various reasons responsible for this, viz., (1) extraordinary difficulties in measuring the resonance effect because of the long lifetime of the excited Mössbauer level and hence the extremely small transition line width (e.g., in ⁶⁷Zn), (2) poor resolution of the resonance lines due to either very small nuclear moments or the very short lifetime of the excited Mössbauer level resulting in very broad resonance lines, (3) insufficient resonance effects due to unusually high transition energies between the excited and the ground nuclear levels, which in turn increase the recoil energy and thus reduces the recoilless fraction of emitted and observed γ-rays.