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The morphology of fluorapatite-gelatine nanocomposites follows a fractal growth mechanism, in which elongated hexagonal seed units first develop into dumb-bell shaped aggregates and ending in notched spheres. In order to investigate the dependence of the morphology of the aggregates on the initial fluoride amount, a series of experiments was performed where the fluoride concentration was varied and calcium / phosphate concentrations were kept constant (Ca:P:F = 5:3:x, 0.46

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Brillouin scattering is used to determine the refractive index N=n+iκ of Ga0.52In0.48P layers, lattice matched to GaAs, in a wavelength range never investigated before (400 nm<λ<800 nm). It is shown that the variations of n are given by two different analytical expressions, depending on whether the corresponding photon energies E are lower or higher than the band gap Eg. Optical absorption measurements are deduced from the Brillouin linewidth. The whole set of elastic constant is also determined through the Brillouin shifts. For E∼Eg, i.e., in resonance conditions, n displays a small but sharp peak closely related to the abrupt increase of the optical absorption. Besides the Brillouin investigations, complementary results obtained by ellipsometry and Raman spectroscopy confirm our optical properties measurements in Ga0.52In0.48P and the electronic resonance behavior, respectively.

Patterns of alternating bright and dim stripes, a few micrometers wide and a few tens of micrometers long, parallel to ͓110͔, appear at the surface of ͑001͒ Ga 1Ϫx In x P epitaxial layers displaying atomic ordering, alongside cleavage planes, when observed between crossed polarizers under a microscope. It is shown that these patterns associated to twinned regions of the sample ͑each stripe corresponding to a single variant large domain͒ are a consequence of the spontaneous trigonal strains resulting from atomic ordering. The strains being differently oriented in the two variants of the CuPt B structure give rise to two index ellipsoids, distinctly oriented in each variant domain. The boundaries between two adjacent variants are (1 10) planes in order to maintain a strain compatibility between them. Under the polarized microscope these boundary planes appear as ͓110͔ directions at the surface of the sample. Obviously the same (1 10) planes separate large adjacent CuPt B domains produced by Chen and Stringfellow, starting from substrates patterned with ͓110͔-oriented groves. The ͑001͒ planes which also fulfill the strain compatibility between two CuPt B variants have been observed as boundaries between two variant domains by transmission-electron microscopy. The strong analogy between atomic ordering and ferroelasticity is pointed out and is exploited here, to give a straightforward solution to problems related to variant domain structure in ''ordered'' alloys. ͓S0163-1829͑97͒50436-X͔