The model used is a rigid-ion model with an effective ionic charge including general interactions for nearest and next nearest neighbours and long range Coulomb interactions. It provides a good fit with recent neutron data for ZnTe and CdTe on condition that no hypothesis is made a priori on the interatomic forces and that the eleven parameters given by the model are used. E s wird ein Modell starrer Ionen mit einer effektiven Ionenladung benutzt, unter Einbeziehung allgemeiner Wechselwirkungen fur nachste und iibernachste Nachbarn und langreichweitiger Coulomb-Wechselwirkungen. Es liefert eine gute Anpassung an neuere Neutronendaten fur ZnTe und CdTe unter der Voraussetzung, dal3 a priori keine Hypothese iiber die interatomaren Krafte gemacht wird, und dal3 die elf Parameter, die das Modell liefert, benutzt werden.A rigid-ion model (RIM) with four parameters was used to calculate the phonon dispersion curves for ZnTe [5] and to study localized substitutional impurity modes in compounds having zincblende structure including CdTe and ZnTe [B]. A seven-parameter RIM was used later to calculate the CdTe and ZnTe phonon dispersion curves [7]. I n both cases the determination of the force constants was based only on optical data and on experimental values of the elastic constants Cll, C12, Cd4, and a noticeable lack of agreement was found between the phonon dispersion curves thus calculated and the neutron scattering data obtained afterwards [8,9].Kunc [lo, 111 gave a dynamic description of 15 compounds having the zincblende crystal structure; these include ZnS, ZnSe, CdTe, ZnTe but for the last two, in the absence of neutron scattering data, the description was incomplete. l) B.P. 2, 91 190 Gif-sur-Yvette. } 352 52 113-6. Discussion A first observation is the high sensitivity of the C branches to the values of the parameters El and E,. This is illustrated by the CdTe phonon dispersion curves calculated for El = E, = 0, using the values of Table 1 for the other parameters. The conclusion is that a priori hypotheses on short-range forces reducing the number of parameters (for instance the assumption of central

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A theoretical study of impurity modes in mixed cuprous halides (CuA, "8, with A,B = Cl, Br, and I) in the low-concentration limit (x&1) is reported using the Greens-function technique. The perturbation caused by the halogen impurity B on the phonon spectra of CuA is accounted for in terms of mass change at the impurity site as well as the change in the nearest-neighbor impurity-lattice interaction. The involved lattice Green's functions have been computed by incorporating the phonons generated by an 11-parameter rigid-ion-model fit to the recent neutron data. We find that different substitutional impurities in cuprous halides may give rise to well defined symmetry vibrations both in and outside the band-mode region. The possibility is pointed out for soine of the important cases to be observed by (ir or Raman) experimental techniques. In the cases of known experimental data, we have determined the force perturbation describing the impurity-host bonding in terms of our one-parameter model. The calculated force-constant changes, in particular for CuCl:Br and CuC1:I systems, are similar to some extent to the behavior noticed in alkali halides rather than in II-VI and III-V compounds. In view of the insufficient experimental data, the possibility of the size effect cannot be ruled out. However, more experimental data on isoelectronic substituents in cuprous halides are very much needed to arrive at a general conclusion regarding the trend of force-constant change with the size of the impurity atom.

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Green's fcnction calculations of force variation due t o charged impurities in elemental and compound seniiconductors are analysed on the basis of simple physical arguments. It is strongly suggested that the trends of force variation may not be related with the long range Coulomb interactions, however, the important criteria should be the effect of electric field of charged impurity on its nearest neighboiir bonds that mainly affects the short range couplings.Some new experimental results by i.r. method due to Si and CI impurities in CdTe are also included that lend additional justification t o t h e arguments.Rechnungen inittels Greenscher Funktion fur die Kraftanderung infolge geladener Storstellen in Element-und Verbindungshalbleitern werden auf der Grundlage von einfachen physikalischen Argumenten analysiert. Es wird angenommen, dall die Trends der Kraftanderungen nicht mit, langreichwcitigen Coulombwechselwirkungen verknupft zu sein brauchen, daB jedoch das wesentliche Kriteriiim der EinfluB des elektrischen Feldes der geladenen Storstelle auf die Bindung seiner nachsten Nachbarn sein sollte, der hauptsachlich die kurzreichweitige Kopplung beeinflullt. Einige neue experimentelle Ergebnisse durch IR-Methoden von Si-und C1-Storstellen in CdTe werden ebenfalls angegeben, die eine zusatzliche Rechtfertigung fur die Argumente liefern.

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An investigation is made of the lattice dynamics of low concentration of H g atoms (z < 0.3) in Cd-rich Cdl -zHg,Te mixed system. The vibrational mode frequency for the isolated Hg replacing Cd atoms in CdTe is observed at 131 cm-1. The impurity is described by a perturbation model that accounts for the change of mass a t the impurity site as well as the local variation of nearestneighbour forces. A calculation based on the Green's function theory suggests stiffening in nearestneighbour forces (Af/f = +0.25) due to isolated Hg in CdTe system.On Btudie la dynamique de reseau des alliages Cdl -,Hg,Te du cat6 riche en cadmium (z < 0,3).La frequence du mode vibrationnel du mercure en substitution isolee sur site Cd est trouvde Bgale B 131 cm-l. Ce cas est discute en connexion avec la variation locale des forces introduites par la presence du d6faut. Un calcul bas6 sup une technique de fonctions de Green r6vele que l'introduction d u mercure isole (z = 0) conduit B un net renforcement des forces entre l'impuretd e t ses premiers voisins (Af/f = +0,25).

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