We report a structural evolution analysis of the long-range and local order and disorder of lead scandium niobate (PSN) down to 10 K, using a combination of neutron and x-ray diffraction on powder and single crystals. The structure of PSN is discussed and compared with those of simple perovskite compounds and . In PSN the existence of a long-range but still disordered ferroelectric phase, different from those of simple perovskites, is evidenced. The disorder of lead and scandium/niobium atoms on short-, medium- and long-range scales is discussed in connection with the dielectric properties of these materials.

In aqueous solution, pure DNA forms multiple liquid crystalline and crystalline phases whose nature depends on the polymer concentration. The following phase sequence is observed when the DNA concentration increases : isotropic ↦ cholesteric ↦ columnar hexagonal ↦ crystalline phases. The aim of this work is to obtain structural information about the highly concentrated phases formed by 500 Å  long DNA molecules — in particular about the crystalline phases — by means of X-ray diffraction. We show that in the two-dimensional (2D) ordered hexagonal phase a longitudinal order progressively appears between neighbouring DNA helices leading continuously to a three-dimensional (3D) ordered hexagonal phase. For higher concentrations the specimens undergo a discontinuous transition towards an orthorhombic phase. The characteristic structural parameters of these different phases have been determined. An important result is that the number of nucleotides per helix turn decreases continuously, when the DNA concentration increases, from 10.3±0.1 at the cholesteric ↦ hexagonal transition down to 9±0.1 without any apparent change of the B conformation of the molecules.

Optical absorption spectra of MgO crystals irradiated with high energy (250 t o 450 keV) alkali ions (Li+, K+) present two bands of colloidal nature after thermal annealing of the crystal.Mie theory cannot represent all the features of this behaviour which are related to the very high dose (5 x 10l6 to lo1' ions/cm2) possible to get with such a technique (LIP to lo'" cm-3 implanted ions). The results are interpreted using Maxwell-Garnett theory applied to a granular metal film embedded in a dielectric host and it is possible to attribute the two observed absorption bands to plasma resonance and optical conduction absorption of the metal precipitate. Since the development' of nuclear reactors, the study of defects produced by irradiation damage became of great' interest and has been extensively developed in many kinds of materials. Insulating material like MgO is a good probe for this purpose.Among defects created by irradiation, and following the classification given by Sonder and Sibley [l], this paper is mai,ily interested in large defects like metal aggregates of implanted ions, leaving apart either electronic defects which involve changes in valence states or ionic defects which consists of displaced lattice ions.I n ion implanted crystals we are dealing with a spatial repartition of extra ions in a very thin layer close to t'he surface of the target crystal. The repartit'ion profile of the implanted ion can be deteriiiined by backscat'tering t'echniques [ 2 ] . Depending upon the dose, t'he concentration of foreign ions can be very high and under thermal annealing, these foreign ions can aggregate and form metal particles enibedded in the, host lattice. Optical absorption of such extrinsic "colloids" is generally analysed using Mie s theory [3]. Shape and/size of colloids can be studied by electron microscopy [4]. The Mie theory does not, represent the experimental result's in the case of very high doses. Therefore, direct use of the Maxwell-Garnett theory [5] is more appropriate to h a t ' the probletn optical properties of a discont'inuous metal film sandwiched wit'h a dielectric.The niainpurposesof this paper are to show that t'he Maxwell-Garnett theory is able to explain experimental absorption spectra of lithium and potassium precipitates in MgO crystals and to deduce some interesting aspects of the aggregat'ion processes in irradiated MgO.

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Small-angle neutron scattering measurements were performed on two very different proteins unfolded by guanidinium chloride. It is shown that the neutron spectra given by unfolded phosphoglycerate kinase and β-casein are similar to those of excluded volume homopolymer chains

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Nous présentons les résultats obtenus en diffraction de rayons X sur le nitrite de sodium : nous nous sommes principalement intéressés à la localisation des satellites et de la diffusion diffuse dans la phase incommensurable, ainsi qu'aux effets prétransitionnels dans la phase désordonnée. Ces travaux ont été complétés par des expériences de diffusion élastique de neutrons, qui conduisent à une détermination très précise de l'évolution avec la température de la période de modulation. Ces différentes expériences, auxquelles s'ajoutent des expériences préliminaires de diffusion inélastique des neutrons, permettent d'ébaucher une discussion sur la nature de la modulation dans la phase incommensurable

Direct observation of potassium colloids in additively‐coloured KCl crystals by transmission electron microscopy reveasl their appearance particularly in regions of high dislocation density and confirms a large distribution of sizes and their heterogeneous density. Observed precipitate contrasts, together with the absence of additional electron dirffraction inforamtion, suggest that the potassium colloids may have an f.c.c. structure coherent with the alkali halide matrix.