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Exact changes in force constant due to H-substitution (U centres) in alkali iodides are systematically analysed using Green's function technique. NaI :H-, KI:H-, Rb1:H-, and Cs1:H-systems are considered. There is a weakening of the force constant varying from 50% to 63% depending on the mass of the neighbour in the host lattice.Die genauen Anderungen der Kraftkonstante in Alkalijodid, die durch H--Substitution (UZentren) verursacht werden, werden systematisch mit der Technik Greenscher Funktionen analysiert. Dabei werden die Systeme NaJ:H-, KJ:H-, RbJ:H-und CsJ:H-beriicksichtigt. E s nird eine Erniedrigung der Kraftkonstante zwisrhen 50% und 63% in Abhangigkeit von der Masse des Nachbarn im Wirtsgitter gefunden.

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A Green function formalism, as first used by Govindarajan et al. (1979) for Schottky defects in alkali halides, has been reformulated on a more rigorous basis and then applied to the calculation of the entropy of formation of an interstitial and of an anion vacancy in the alkaline earth fluoride, CaF2. The relevant lattice Green functions are calculated using a shell model. The force constant changes are obtained from the coordinates of the relaxed ions which are computed using the Hades program due to Norgett (1974) and the potential parameters of Catlow, Norgett and Ross (1977). This calculation of the entropy of formation of a Frenkel defect in CaF2 is a quasiharmonic one at the temperatures 300, 1000 and 1500K. The calculated values are significantly lower than experimental results. The principal reason for this discrepancy is that the calculations refer to the entropy of formation at constant volume (sv) whereas the experiments measure the entropy of formation at constant pressure (sp). A calculation of sp-sv is presented and it is demonstrated that this term is an important component of sp.

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