Application of the pair distribution function analysis for the study of cultural heritage materials

The study of cultural heritage objects represents a challenge for materials sciences, because of their intrinsic complexity as well as because of their rare and precious nature, which requires the use of non-destructive methods. One of the major difficulties encountered by the methods of structural analysis of these materials is the presence of mixtures of crystallized and amorphous phases, the conventional methods of crystallography being poorly adapted to the latter. Here we present the Pair Distribution Function analysis method, which based on diffraction data, allows access to the identification, microstructural characterization and quantification of phases constituting a complex mixture, independently of their amorphous or crystalline character. Two recent examples will be presented to illustrate the use of this method in studies of cultural heritage materials.     

Quasicrystals and atomic clusters

Historically, two principal approaches exist to determine the quasicrystal atomic structures: one is derived directly from diffraction experiments, using cut and projection of higher dimensional space-representation techniques; the other one takes use of the similarity between so-called approximant crystalline phases, observed to co-exist with quasicrystals. The known structure of these phases is shown to be the starting point of building a quasicrystal model in terms of constitutive polyatomic clusters of icosahedral symmetry. It requires to apply inflation properties of quasiperiodicity, and decoration of elementary building tiles. One example is detailed and discussed, in terms of a two-cluster model, in the case of AlLiCu.     

Frustration and defects in non-periodic solids

Geometrical frustration arises whenever a local preferred configuration (lower energy for atomic systems, or best packing for hard spheres) cannot be propagated throughout space without defects. A general approach, using unfrustrated templates defined in curved space, have been previously applied to analyse a large number of cases like complex crystals, amorphous materials, liquid crystals, foams, and even biological organizations, with scales ranging from the atomic level up to macroscopic scales. In this paper, we discuss the close sphere packing problem, which has some relevance to the structural problem in amorphous metals, quasicrystals and some periodic complex metallic structures. The role of sets of disclination line defects is addressed, in particular with comparison with the major skeleton occurring in complex large-cell metals (Frank–Kasper phases). An interesting example of 12-fold symmetric quasiperiodic Frank–Kasper phase, and its disclination network, is also described.     

Diffuse scattering data acquisition techniques

Techniques are presented for acquiring and reducing X-ray diffuse scattering data from disordered crystalline materials. These methods are part of a comprehensive approach to study disorder in novel quasicrystalline phases as a function of temperature, but can be applied without further adaptation to periodic crystalline systems. By fully exploiting the possibilities of modern two-dimensional X-ray detector systems - using imaging plates or charge coupled devices (CCD) - large volumes of reciprocal space can be measured in a quantitative and rapid way. For this purpose, the classical rotation method for collecting integrated Bragg intensities is extended for acquiring quasi-continuous diffuse diffraction data. A new high-temperature furnace and helium beam path, designed for the diffraction geometry of the rotation method, are integral parts of the diffraction system. New methods are presented for handling the reduction of diffuse diffraction data from area detectors. One of the key techniques is the reconstruction of arbitrary slices and volumes in reciprocal space from a single series of rotation images taken from an arbitrarily oriented single-crystal (reciprocal space mapping).     

Applications of synchrotron X-ray nano-probes in the field of cultural heritage

Synchrotron-based techniques are increasingly used in the field of cultural heritage, and this review focuses notably on the application of nano-beams to access high-spatial-resolution information on fragments sampled in historical or model artworks. Depending on the targeted information, various nano-analytical techniques can be applied, providing both identification and localization of the various components. More precisely, nano-X-ray fluorescence probes elements, nano-X-ray diffraction identify crystalline phases, and nano X-ray absorption spectroscopy is sensitive to speciation. Furthermore, computed tomography-based techniques can provide useful information about the morphology and in particular the porosity of materials.     

Review on applications of synchrotron-based X-ray techniques in materials characterization

Synchrotron radiation (SR), as a result of its high-intensity, brilliant, monochromatic, and collimated beams, is becoming one of the most crucial components of research in various fields of materials science such as nanomaterials, biomaterials, and energy materials. SR-based characterization methods can be employed to analyze different systems such as powders, thin films, and bulk forms having complex crystalline or amorphous structures. In this review, peculiarities of SR are briefly explained. Moreover, various techniques carried out utilizing this instrument for material characterization such as X-ray powder diffraction, grazing-incidence X-ray diffraction, small/wide-angle X-ray scattering, X-ray absorption spectroscopy, different techniques of X-ray imaging, X-ray photoelectron spectroscopy, and X-ray microprobes/nanoprobes are presented. As a result, by shedding light on the advantages of SR and its superiority to the equivalent laboratory experiments, researchers are recommended to exploit the capabilities of this invaluable tool in their materials characterization.     

Birefringence and phase transitions in liquid crystals

The birefringence of liquid-crystalline phases is the result of the parallel order of molecules exhibiting a polarizability anisotropy. The magnitude and sign of the birefringence are determined by the structure and order of the liquid-crystalline phase types as well as by the polarizability properties of the constituent molecules. The characteristic change of the birefringence at phase transitions between liquid-crystalline phases indicates more or less pronounced structural changes. The temperature dependence of the birefringence is due to the temperature change of the molecular order.

It is shown that the structural variety of the liquid crystalline state is reflected by a big variety of their optical anisotropy properties.     

X-ray diffraction study of liquid Cs up to 9.8 GPa

We describe an x-ray diffraction study of liquid Cs at high pressure and temperature conducted in order to characterize the structural changes associated with the complex melting curve and phase transitions observed in the solid phases. At 3.9 GPa we observe a discontinuity in the density of the liquid accompanied by a decrease in the coordination number from about 12 to 8, which marks a change to a nonsimple liquid. The specific volume of liquid Cs, combined with structural analysis of the diffraction data, strongly suggest the existence of dsp(3) electronic hybridization above 3.9 GPa, similar to that reported on compression in the crystalline phase.     

Real-space visualization of intercalated water phases at the hydrophobic graphene interface with atomic force microscopy

The phase behavior of water is a topic of perpetual interest due to its reinai kable anomalous properties and importance to biology,material science,geoscience,nanoscience,etc.It is predicted confined water at interface can exist in large amounts of crystalline or amorphous states.However,the experimental evidence of coexistence of liquid water phases at interface is still insufficient.Here,a special folding few-layers graphene film was elaborate prepared to form a hydrophobic/hydrophobic interface,which can provide a suited platform to study the structure and properties of confined liquid water.The real-space visualization of intercalated water layers phases at the folding interface is obtained using advanced atomic force microscopy(AFM).The folding graphene interface displays complicated internal interfacial characteristics.The intercalated water molecules present themselves as two phases,low-density liquid(LDL,solid-like)and high-density liquid(HDL,liquid-like),according to their specific mechanical properties taken in two multifrequency-AFM(MF-AFM)modes.Furthermore,the water molecules structural evolution is demonstrated in a series of continuous MF-AFM measurements.The work preliminary confirms the existence of two liquid phases of water in real space and will inspire further experimental work to deeply understanding their liquid dynamics behavior.     

Crystallographic texture analysis of archaeological metals: interpretation of manufacturing techniques

Neutron probes and high energy X-rays are sources of primary importance for the non-invasive characterization of materials related to cultural heritage. Their employment in the characterization of archaeological metal objects, combined with the recent instrumental and computational developments in the field of crystallographic texture analysis (CTA) from diffraction data proves to be a powerful tool for the interpretation of ancient metal working techniques. Diffraction based CTA, when performed using penetrating probes and adequate detector coverage of reciprocal space, for example using large detector arrays and/or ToF mode, allows simultaneous identification and quantification of crystalline phases, besides the microstructural and textural characterization of the object, and it can be effectively used as a totally non-invasive tool for metallographic analysis. Furthermore, the chemical composition of the object may also be obtained by the simultaneous detection of prompt gamma rays induced by neutron activation, or by the fluorescence signal from high energy X-rays, in order to obtain a large amount of complementary information in a single experiment. The specific application of neutron CTA to the characterization of the manufacturing processes of prehistoric copper axes is discussed in detail. PACS 61.12.-q; 81.05.Bx; 81.40.Ef; 81.70.-q     

Disorder diffuse scattering from quasicrystals

Very sharp Bragg reflections accompanied by diffuse scattering phenomena are typical for most stable quasicrystals. The correlation length of the quasiperiodic average structure can reach several micrometers as proved by high-resolution X-ray diffraction experiments. This corresponds to a structural perfection of some quasicrystals similar to that of silicon. Nevertheless, the omnipresent diffuse scattering indicates significant deviations from a strictly ordered quasiperiodic structure especially in the case of decagonal phases. These structural deviations may be caused by phason fluctuations, by disorder in the packing of the basic atomic clusters, by the formation of nanodomains, by chemical disorder, or by superstructure formation on a short-range scale. Characteristic examples of different types of structural disorder present in icosahedral and decagonal quasicrystals are reported. The diffuse scattering phenomena in decagonal Al-Co-Ni as a function of composition and temperature are discussed in more detail.     

Effect of the chiral chain length on structural and phase properties of ferroelectric liquid crystals

Studies of structural and phase properties obtained on several ferroelectric liquid crystalline materials with 2-alkoxypropionate group used as a chiral centre and without any lateral substitution are presented. In dependence on the chiral chain length these compounds exhibit the cholesteric N* phase, the ferroelectric smectic C* and a low-temperature SmX phase. Values of the spontaneous polarization and spontaneous tilt angle have been determined within the whole range of the SmC* phase. A low-temperature SmX phase has been identified as the orthogonal hexatic SmB* phase. The molecular parameters, namely the layer spacing in the SmC* and SmB* phases and the average intermolecular distances (D) between neighbouring parallel molecules in all investigated phases have been determined using the results of the X-ray diffraction obtained on non-oriented samples. The effect of the chiral chain length on mesomorphic, structural and physical properties of the studied ferroelectric liquid crystalline materials is discussed.     

Reactivity of chromium-based pigments in a porcelain glaze

This paper presents a comprehensive investigation at the microscopic scale of various pigments composed of chromium from the French ‘Manufacture de Sèvres’ to establish the origin of color in glazes. Electron microscopy coupled with X-ray diffraction allows the determination of the microstructure and composition of the crystalline phases after firing. X-ray absorption spectroscopy reveals subtle changes in the medium-range ordering around Cr with high spatial resolution, in the pigment grain or at the pigment/glass interface. Principal results indicate systematic and common changes whatever the pigment types: (i) Cr-enrichment for the final crystals, that controls the coloration of the glaze, (ii) migration of specific elements such as Al or Zn from the pigments to the amorphous part of the glaze, and (iii) crystallization of anorthite in the near proximity of the altered Cr-bearing crystalline pigments.     

A new interpretation of the CO state in half-doped manganites: new results from neutron diffraction and synchrotron radiation experiments

In the R_1−xD_xMnO₃ (x0.5) manganites, the structural phase transition at T_CO is commonly interpreted as a concomitant charge and orbital ordering (CO/OO) process driven by a co-operative Jahn–Teller effect and Coulomb repulsion forces. The low-temperature phase is supposed to contain well-separated and ordered Mn³⁺ and Mn⁴⁺ ionic species in an NaCl-like pattern. Structure refinement, from a neutron diffraction experiment below T_CO on a Pr_0.6Ca_0.4MnO₃ single crystal, gives us a model for the displacement of atoms with respect to the high-temperature phase that invalidates the standard model based in the CO/OO picture. Our result is a non-centrosymmetric crystal structure with two non-equivalent MnO₆ octahedra, both being slightly elongated but displaying very similar average Mn–O distances (1.96 and 1.95 Å, respectively) and having off-centered Mn atoms. We argue that this is a proof of the absence of charge ordering in half-doped manganites in the sense of formation of separated Mn³⁺ and Mn⁴⁺ ionic species. A new qualitative interpretation of the CE-type spin ordering (SO) is proposed. The so-called CO transition is, in fact, a structural transition induced by the change in the mean free path of electrons that continue to be thermally activated below T_CO by forming ferromagnetic Mn–Mn pairs stabilized by a local double-exchange process. The CE SO pattern results from the ordering of these pairs formed at T_CO. High-resolution synchrotron powder diffraction shows a complex anisotropic/asymmetric strains appearing at the transition that can be phenomenologically fitted by additional phases. Complementary electron diffraction and microscopy have shown no trace of macroscopic phase separation.     

一种新的氮化碳相的冲击波合成

 为了合成理论预测存在的高密度氮化碳相,以富氮的C-N-O非晶材料和晶态的双氰胺为前体,在低于50 GPa的冲击压力范围内进行了一系列冲击回收实验。回收产物的XRD衍射表明,形成了由β-C₃N₄和一种新的氮化碳相组成的复合相。该新相的衍射峰可以完全指标化为一个单斜晶胞,晶胞常数为a=0.981 nm,b=0.723 nm,c=0.561 nm,β=95.2°,晶胞体积V_cell=0.396 6 nm³。根据实验结果可以认为,氮化碳复合相的形成是前体有机分子在瞬态的冲击波化学反应后,经历了极高速的冲击淬火过程（约10⁹ K/s）,作为一种高压亚稳相而被保存下来。冲击压缩富氮的有机物前体,是合成氮化碳相的一种新方法。     

Investigation of structural changes of paint-and-varnish copolymers under influence of ageing

Paintwork materials applied as coatings protecting from rust and corrosion suffer with time structural changes leading to worsening of their physical and mechanical characteristics. We study by X-ray diffraction technique the inner structure of a new modified silicate coating depending on its ageing. It is shown that new crystalline formations appear with ageing in samples worsening significantly their service properties.     

Self-assembling properties of lactic acid derivative with several ester linkages in the molecular core

The effect of several polar ester linkage groups incorporated in the molecular core of a chiral lactic acid derivative on self-assembling properties has been investigated by polarizing optical microscopy, small angle X-ray diffraction, differential scanning calorimetry, optical and electro--optical studies. The compound possesses the paraelectric smectic A* (SmA*) and ferroelectric smectic C* (SmC*) phases over a broad temperature range. Mesomorphic behaviour, spontaneous polarization, birefringence, optical transmission, dielectric anisotropy and structural properties of the self-assembled chiral material have been determined. The obtained results are discussed and compared with that of other liquid crystalline materials. Experimentally determined spontaneous polarization and tilt angle values are also used to elucidate the nature of SmC* to SmA* phase transition. The effect of polar ester linkages in the molecular core has also been discussed.     

Refinement of twinned crystal data

The refinement of structure from diffraction intensities measured from a twinned crystal involves the evaluation of intensity contributions from the various twin domains in the crystal and the corresponding structure factor derivatives. The modifications in the standard least-squares refinement program required for analysis of data from a twinned crystal sample are discussed. The procedure can be extended to cases involving more than two types of twin domains or to samples having mixed phases. The results and limitations of structure refinement using data obtained from a twinned LiKSO₄ crystal in various low-temperature phases are illustrated.     

Nucleation of quasi-crystalline and amorphous structures

Summary The structural characteristics of quasi-crystalline phases obtained by ion irradiation are compared with those of the corresponding amorphous structures. Chemical forces drive the formation of atomic clusters and their spatial organization under specific geometric constraints. The same elementary structural units exist in quasi-crystalline and glassy phases which display identical local order, their main difference being the degree of defect organization at the interface between locally ordered atomic groups. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.