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The results of reflection spectra at 300 °K in the far-infrared region (100–400 cm−1) are reported due to low concentration of Hg atoms in the Cd-rich CdHgTe mixed crystals. A gap mode is observed at 131 cm−1 due to isolated Hg occupying Cd in CdTe. The experimental results are analyzed theoretically using the well-know Green’s function technique. The perturbation caused by low concentration of Cd(Hg) is described by the mass change at the impurity site as well as local variation of impurity-host interactions. In the nearest neighbor approximation, all the involved Green’s function matrix elements have been computed numerically by incorporating the lattice phonons from an eleven parameter rigid-ion model fitted to recent neutron data of HgTe and CdTe. We find that different substitutional defects in Hg-Cd tellurides can give rise to well-defined symmetry vibrations both within the gap and outside the maximum phonon frequency regions. Our calculations suggest that a gap mode of Hg substituted to Te is likely to be the possible explanation for the origin of the observed Ω2 mode in HgTe. In the cases of known experimental data (viz., CdTe:Hg and HgTe:Cd) we have estimated the nature of force perturbations and the results are compared and discussed with those obtained from a modified-random-element isodisplacement model.

International audiencePhthalocyanines substituted by paraffinic chains are known to form liquid crystals with the macrocycle stacked in columns. The Langmuir-Blodgett (LB) technique is used to prepare thin films of three types of tetrasubstituted phthalocyanines that differ by the nature of the substituent. The LB films are investigated by linear dichroism in the infrared range, which reveals a moderate in-plane anisotropy that strongly improves upon annealing a few hours at 100 C or higher. The highest in-plane anisotropy is obtained when using an aliphatic substituent (C6) linked to the macrocycle through an ester link. This substituent seems to be a good compromise between a purely aromatic one and an aliphatic-rich one (with two C8 chains), which ensures fluid Langmuir films and allows the high in-plane anisotropy after transfer. The texture of these high in-plane anisotropic films is further investigated by atomic force microscopy in tapping mode. We find that the films are lamellar and have defects one monolayer or a few monolayers deep. It is shown that the variations of the texture of the film can be explained assuming a transfer ratio slightly below 1. Investigations of the film after annealing reveal that the number and size of the defects of the layers are greatly reduced. Their shape becomes elongated, and they become aligned with the dipping directio

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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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