G. Dolino

X-ray diffraction studies of porous silicon

Abstract We review previous X-ray diffraction experiments on porous silicon, giving a determination of porous layer strain. The results of our X-ray measurements of the strain of as-formed and anodically oxidized samples are presented; the diffraction curves of thin layers and of superlattices and the strain induced by liquid wetting and vapour adsorption have been measured. The size and shape of silicon crystallites can be determined from the X-ray diffraction pattern, in particular by using reciprocal space maps.

Inelastic neutron scattering study of quartz near the incommensurate phase transition

Abstract Inelastic neutron spectra of B quartz were recorded at various temperatures, using triple axis neutron spectrometry. Our observations confirm the existence of a very low-lying phonon branch along the ⟨1,0,0⟩ directions, which exhibits a marked temperature dependence, in particular near the Brillouin zone center. This branch is very anisotropic in the (0,0,1) plane. Within the B phase a maximum of quasi elastic scattering is observed for q ≃ 0.03a*, premonitory to the appearance of satellite reflexions in the modulated phase. In addition, a transverse acoustic mode exhibits an anisotropic behaviour which is attributed to a coupling with the soft-mode. The results are consistent with a phenomenological model of the incommensurate phase transition and with a lattice dynamical model involving rigid SiO4 tetrahedra motions.

Optical observations of coexistence states during 1st order transition in KD2PO4, quartz and NH4C1.

Abstract At a 1st order transition the two phases are in equilibrium. In a solid, transition strain discontinuities induce elastic stresses which are of long range nature. In order to minimize elastic energy the interface takes definite orientations. The interface shapes, nucleation and motion have been optically observed in 3 materials : NH4C1 around the pressure induced tricritical point, quartz and deuterated KDP, with application of an electric field. In these 3 cases, observed interface orientations are in agreement with the predictions of the theory of coherent inclusions in anisotropic elastic materials.

Variations in the lattice parameter of porous silicon produced by wetting and vapour adsorption

Abstract High resolution X-ray measurements of the variations in the lattice parameter of p-type porous silicon produced by wetting or vapour adsorption are reported. While pentane wetting produces only a lattice expansion, pentane vapour adsorption gives a more complex behaviour: with increasing vapour pressure there is first a lattice contraction, followed by an expansion. These results are discussed in relation to vapour adsorption mechanisms.

Study of thermal expansion of quartz near the α-β phase transition under uniaxial stresses

Abstract We report measurements of the thermal expansion coefficients in the x and z directions of crystalline quartz under uniaxial static stresses (0 to 0,2 kbar) in the vicinity of the α-β transition (Tt ∼ 573°C).The thermal expansion coefficients and the isothermal compliances are measured close to the transition temperature. As there is hysteresis both α → β and β→α transitions are investigated. The shape of the dilatometric curves are found to be nearly invariant under different stresses. The displacements of the transition temperatures are determined and the experimental results agree with the predictions of PIPPARD relations in a phase.

Phenomenological interpretation of the critical behaviour of KDP and NH4C1 near their tricritical point

Abstract Recent order parameter measurements in KDP and NH4C1 near their pressure-induced tricritical point are interpreted using a Landau type free energy expansion of the following form: Good fits are obtained with the simple assumption that the coefficients a and b vary linearly with P and T near the tricritical point where they vanish simultaneously.

Study of α - Incommensurate - β transitions in quartz by white beam synchrotron radiation laue method

Abstract The transitions between β, incommensurate (inc) and α phases of quartz have been investigated with the white beam Laue method, using synchrotron radiation. During the 1rst order inc - α transition, inc satellites are observed to shift closer to the Bragg peak position. On the high temperature side, an intermediate phase with a 1q modulation exists in a small temperature interval of a few 0.01 K, between the usual 3q inc phase and the β phase. Several 1q domains have been directly observed by X-ray topography.

Light and X-Ray Scattering at the Incommensurate Transition of Quartz

The opalescence observed at the f -inc transitions of quartz is an example of the light scattering produced by complex microstructures occurring at the phase interface of first order phase transitions. Recently three models, related to the existence of the incommensurate phase of quartz, have been proposed to explain this light scattering. A new investigation of the origin of the quartz opalescence combining optical and high energy x-ray diffraction measurements has been performed. A diffuse x-ray streak observed between f and inc phases is probably associated to the structure of elongated triangles observed by electron microscopy.