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The concept of bond valence and its correlation with bond length are reviewed. It is shown how to augment bond valence sums at an atom with other constraints so that bond lengths can be predicted for a given topology. These constraints can be read directly from the connectivity matrix for the structure. The physical reasons for the constraints are analysed, and this analysis leads in turn to a reformulation of Pauling's electrostatic valence sum rule. The apparent valences of atoms, calculated as a sum of bond valences derived from bond lengths, are often significantly different from the actual valences. It is demonstrated that such observations are often diagnostic of non-bonded repulsions. Limitations and possible extensions of the method are outlined. [ABSTRACT FROM AUTHOR]

Mixed valence has been considered with reference to three model systems: Fe3O4 (magnetite) is a representative of the ferrospinels and the iron oxides with spinel-related structures, Fe1−δO (wüstite) illustrates a system with random defects and clustering, CaFeO3 (a perovskite) illustrates the consequences of strong Fe4+-O-Fe4+ interactions. The influence of counter cations is particularly marked in systems like the spinels Fe2SnO4 and Fe2MoO4; more subtle effects are illustrated by a comparison of CaFeO3 with SrFeO3. The time τh for an electron transfer between cations of different valence has been considered relative to two times: the period ωR−1⋍10−12s of an optical-mode lattice vibration that traps a mobile electron and the time τn for a Mössbauer nuclear excited state to decay to its ground state. The full range of electron-transfer times τh<10−12s to τh>10−8s has been found for both Fe-Fe interactions across shared site edges or faces and Fe-O-Fe interactions across shared site corners. Perturbations of the periodic potential of an iron array by lattice defects or substitutional ions is shown to increase τh. Moreover, electron-lattice interactions were found to induce cooperative Jahn-Teller distortions in some compounds and charge-density waves (CDW) in others. In the mixed-valent compound Fe3O4, electrostatic interactions between mobile ions combine with electron-lattice interactions to produce, with increasing temperature, transitions from ionic ordering (or a static CDW) in the temperature interval 120

Sulphur containing amino acid residues are found among the structural and functional constituents of many proteins. The ease of oxidising sulphur in these biologically important polymers is well known. At present, there is no simple and fast technique available to monitor the respective oxidation states using chemical reactions. X-ray photoelectron spectroscopy has proven to be both convenient and efficient to follow these electron transport reactions. In the present survey a choice of some sulphur and selenium bearing proteins and metalloproteins will be examined upon excessive irradiation, treatment with hydrogen peroxide and cleavage of metal sulphur bonding. It will also be shown that the use of this technique in general will allow a quick decision on the rate of deterioration of a protein. The limitations of this spectrochemical approach will be debated. [ABSTRACT FROM AUTHOR]

The alternative approach to structures is an inversion of the traditional one: we concentrate on a packed array of cations, into the interstices of which the anions are inserted, rather than putting cations into a packed anion array. Even in conventional terms the logic of the new approach can be justified, since many structures have their own antitypes (and even more show a partial structure/antistructure relation). In this way we find that, in oxides for example, many cation arrangements are identical to the arrangements of the atoms in known (or plausible) alloy structures. (In some cases the cations and atoms are identical in the two cases.) This is particularly helpful in those oxide examples where it has not previously been possible to describe the structure in any simple terms, often because the anion array is not regular in any simple way, for instance in many metal sulphates. But it is also revealing in some cases that are describable in conventional terms, e.g. the humites. A long (but still incomplete) list of examples is given in Table 3. Sufficient specific cases are described and discussed in detail in the text in order to expose the principles and some of the advantages of this unfamiliar approach. One of these advantages is that, in favourable cases, it leads at once to complete quantitative descriptions of the structures (i.e. until cell dimensions and anion position parameters), e.g. for spinel, garnet, bixbyite etc. Another is that it throws a new light on possible martensitic mechanisms for phase transformations (e.g. β- → γ-Ca2SiO4, low- → high-temperature K2SO4), and on the possibility of anomalous substitution (e.g. of CO32− for PO43− in apatite). It also reveals an explanation for those phase transitions, due to high pressure, that are not attended by an increase in primary coordination number, e.g. olivine → spinel, zircon → scheelite. [ABSTRACT FROM AUTHOR]