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Seventy-five per cent of all papers on experimental Mossbauer spectroscopy are concerned with the first excited-state decay of 57Fe; indeed, because of this, 57Fe and Mossbauer spectroscopy are synonymous to many people. The sheer volume of published work makes a completely exhaustive survey impossible, but in the following chapters a series of critical reviews will be given of specific areas defined by chemical classification. In this way a comprehensive view of the field can be obtained. The nuclear parameters of the 57Fe γ-decay have also received more attention than usual. This chapter summarises the currently available data on the 57Fe nuclear parameters, and then applies the general theory of hyperfine interactions given in Chapter 3 to the specific case of this isotope.

Mossbauer spectroscopy of the rare-earth elements as a group has gained considerable attention from physicists. Successive addition of 4f-electrons along the series has little influence on many of the chemical properties, which are mainly those of a tripositive cation. However, such uniformity does not extend to properties derived from the electronic spin moment or from the atomic nucleus and therefore to the Mossbauer spectra. A resonance is known in at least one isotope of every rare-earth element except the first (cerium) and the last (lutetium). Furthermore, a resonance is often known for several isotopes, and in some instances for several transitions within a given isotope. It has therefore been possible to measure such properties as the nuclear magnetic moment for a large number of nuclear states within a small range of masses, and much of the work done has been directed to this end. The significance of the results to an understanding of the structure of the nucleus will not be considered here. Rather we shall describe the experimental phenomena and any derived nuclear parameters, together with the smaller proportion of data accrued which are chemically interesting. Numerical data for all transitions are summarised in Appendix 1.

A Mossbauer resonance is known in at least one isotope of fourteen transition metals in addition to iron. However, none has been extensively used up to the present time. Several of them present extreme difficulties in measurement, but as this chapter will show, sufficient background information has been collected to assess the feasibility of chemical application.

The facility with which 57Fe 14·41-keV Mossbauer spectra can be measured has resulted in considerable use of the technique for probing iron solid-state systems; so much so that it would be impossible to summarise the data adequately in a single chapter. The general field of iron chemistry has therefore been subdivided into a number of subsections and, although this division is to some extent arbitrary it simplifies the task of presenting the results considerably. The subsections are as follows, (i) iron(II) and iron(III) complexes with high-spin electronic configurations (Chapter 6); (ii) iron(II) and iron(III) complexes with low-spin electronic configurations (Chapter 7); (iii) ligand-field crossover situations and uncommon paramagnetic oxidation states (Chapter 8); (iv) covalent, diamagnetic iron complexes (Chapter 9); (v) iron oxides, chalcogenides, and minerals (Chapter 10); (vi) intermetallic systems and alloys (Chapter 11); (vii) decay after-effects and impurity studies, together with some miscellaneous topics (Chapter 12); (viii) biological systems (Chapter 13).

Having discussed at length the data for 57Fe and 119Sn which have undoubtedly dominated the first decade of chemical Mossbauer spectroscopy, we now turn to other elements. In this chapter we discuss the remaining Main Group elements in which the Mossbauer effect has been observed in order of increasing atomic number. The elements are potassium, germanium, krypton, antimony, tellurium, iodine, xenon, caesium, and barium, although not all of these have been used for chemical studies. Particular attention will be paid to the experimental difficulties involved, and the potential of each will be assessed with regard to further study in the near future. Where estimates have been given for the errors in the original data we quote these in the form 3·90(35), indicating 3·90 ± 0·35. This is to allow the significance of small differences in values to be easily assessed.

The extensive use of Mossbauer spectroscopy to study oxide phases has recently been fully reviewed1,2 and little point would be served in giving another general survey. Instead, I would like to discuss in some depth the contributions which Mossbauer spectroscopy has made to two key systems: (a) the defect structure of nonstoichiometric iron monoxide Fel-xO and (b) the magnetic properties of ternary and quaternary ruthenium oxides.

In Chapters 6 and 7 the data for both high-spin and low-spin iron(II) and iron(III) complexes were discussed. Although these are the common electronic configurations for these oxidation states, there is also a minority group of compounds which show either ligand field crossover from high-spin to low-spin configuration or intermediate spin states, and in addition there are some uncommon oxidation states.

It was seen in the preceding chapter that the differing electronic configurations of iron(II) and iron(III) in high-spin complexes caused substantial and characteristic differences in the chemical isomer shift, quadrupole splitting, and magnetic field parameters of their Mossbauer spectra. The corresponding strong-ligand-field or low-spin complexes have \( {}^1{A_1}_g{\rm{ and }}{}^2{T_2}_g \) configurations under octahedral symmetry. The low-spin iron(II) \( {}^1{A_1}_g{\rm{ (}}{t_2}{_g^6}{\rm{)}} \) configuration has no unpaired electrons and is diamagnetic, whereas low-spin iron(III) \( {}^2{T_2}_g\;({t_2}{_g^5}) \) embodies an ‘electron hole’ in the \( {t_2}_g \) manifold and is paramagnetic, although magnetic ordering is sometimes observed at very low temperatures.

Interest in the actinides stems from the importance of measuring the physical nuclear constants required for the study of the nuclear structure in this region of comparative instability. Few low-energy levels are conveniently populated by a radioactive parent, but the successful and detailed studies with 237Np using both 237U and 241Am parents have stimulated the use of more difficult techniques such as Coulomb excitation. Consequently the 232Th, 231Pa, 238U, and 243Am resonances have now also been detected. Full details of the known nuclear parameters of these nuclides are tabulated in Appendix I.

The study of recoilless nuclear resonant absorption or fluorescence is more commonly known as Mossbauer spectroscopy. From its first origins in 1957, it has grown rapidly to become one of the most important research methods in solid-state physics and chemistry.

There is a large class of iron organometallic compounds which can be described as molecular, diamagnetic, and highly covalent. They are derived in the main from the parent compounds iron pentacarbonyl, Fe(CO)5, and the cyclopentadienyl complex ferrocene, (π-C5H5)2Fe or Cp2Fe. In view of their large numbers, it is not surprising that considerable Mossbauer spectral data are available, but as we shall now see the interpretation of this data introduces considerable problems.

The magnetic properties of samples of rock, fines, and magnetic separate from the fines from Apollo 11 have been measured. Native iron, or possibly nickel-iron, of submicroscopic particle size is the most important constituent, with minor contributions from ilmenite, paramagnetic iron minerals, and other iron-titanium oxides. The remanent magnetization of a sample of the micro-breccia rapidly acquires a viscous magnetization and does not appear to have a significant stable remanence. The crystalline sample has a weak natural remanence showing some stability.