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A consistent set of shell parameters for alkali halide crystals of both rocksalt and CsCl structures is reported. The fitting procedure, values of the parameters and their sensitivity are discussed. Short range interionic potentials are determined and are shown to give a good description of both static and dynamic properties of the crystals.

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Transition metal ions in MgO and CaO are observed in several charge states.We investigate the conditions for stability of the observed ions of manganese, iron, cobalt and nickel.First, the stability of so many states results from a balance between the variation of ionization potential with charge state and the dependence on excess charge of the polarization and distortion of the host. Covalency is unimportant, contrary to mechanisms discussed for gold in germanium. Secondly, the detailed quantitative calculations indicate that the full set of experimental data require that these oxides have a negative electron affinity, probably more negative than -1.3 eV in MgO and more than -0.9 eV in CaO.These lead to two main conclusions. $ 1 . INTRODUCTION Some years ago Haldane and Anderson (1976) addressed the problem of multiple charge-states in semiconductors. They considered, for example, the states Au3-and Au+, which both appear to occur in Ge, despite the mere 0.8 eV band gap of the host. They concluded that the data could be understood within Anderson's model for magnetic impurities in metals, provided that there is large enough hybridization of the impurity d orbitals with the delocalized host orbitals.The attractive model of Haldane and Anderson would seem to solve the problem were it not for the fact that multiple charge-states are well known in highly ionic hosts like MgO and CaO. A list of such cases is given in table 1. Moreover, the analogous hybridization can be seen to be very small from the known spin resonance parameters (see, for example, Owen and Thornley 1966). Certainly the band gap is larger than in Ge-around 7.7 eV for MgO-yet the important Madelung energy terms and ionization potentials can be much larger than the gap itself. We consider the multiplicity quantitatively in this paper, and we conclude a different mechanism operates. It remains an open question what mechanism is appropriate in the polar semiconductors like ZnSe.We shall ask whether a particular charge state MN+ of a transition metal ion M is stable against three types of transition. The two most important are the loss of an electron to the conduction band, 0, (1) MN++M(N+l) + e and the loss of a hole to the valence band, i.e. the capture of an electron from the valence band, MN++M(N-l)+ + h,.(2) t Permanent address : J. J.

An existing consistent set of model potentials for alkali halides formulated for convenient use in defect applications is extended to incorporate hydrides. The additional parameters required are fitted to the limited available experimental data for perfect alkali hydride crystals. Calculations of local-mode frequencies for isolated H-substitutional (Ucentre) and interstitial impurities, H-H-, H-D-, H-cation-impurity and H-anionimpurity pairs in alkali halides are reported. These calculations, which are based on large periodic defect cells, involve no further adjustable parameters and are in good agreement with results of infrared absorption measurements found in the literature.