Structural study of an amorphous NiZr2 alloy by anomalous wide angle X-ray scattering and Reverse Monte Carlo simulations

The local atomic structure of an amorphous NiZr2 alloy was investigated using the anomalous wide-angle x-ray scattering (AWAXS), differential anomalous scattering (DAS) and reverse Monte Carlo (RMC) simulations techniques. The AWAXS measurements were performed at eight different incident photon energies, including some close to the Ni and Zr K edges. From the measurements eight total structure factor S(K,E) were derived. Using the AWAXS data four differential structure factors DSFi(K,Em,En) were derived, two about the Ni and Zr edges. The partial structure factors SNi-Ni(K), SNi-Zr(K) and SZr-Zr(K) were estimated by using two different methods. First, the S(K,E) and DSFi(K,Em,En) factors were combined and used in a matrix inversion process. Second, three S(K,E) factors were used as input data in the RMC technique. The coordination numbers and interatomic distances for the first neighbors extracted from the partial structure factors obtained by these two methods show a good agreement. By using the three-dimensional structure derived from the RMC simulations, the bond-angle distributions were calculated and they suggest the presence of distorted triangular-faced polyhedral units in the amorphous NiZr2 structure. We have used the Warren chemical short-range order parameter to evaluate the chemical short-range order for the amorphous NiZr2 alloy and for the NiZr2 compound. The calculated values show that the chemical short-range order found in these two materials is similar to that found in a solid solution.Comment: Submitted to Phys. Rev. B, 8 figure

A New Structural Study for an Old Quasicrystal

In this paper, we have described the experimental evidence for the polyhedral order in metastable Al-Mn alloy (icosahedral phase). We have used the anomalous diffractometry technique to identify the X-ray patterns of this alloy. For the first time, the radial distribution function of the diffuse rings scattered by this alloy is obtained by applying this new technique. Unlike previous methods used for interpreting the X-ray patterns of this phase, we have introduced a new method of separation for interpreting X-ray patterns. The X-ray diffraction pattern has been numerically resolved into a broad diffuse halo and sharp lines. Using this procedure the correlation position was deduced from a high quality interference function. We have not been able to explain either the origin of the sharp lines or the relationship of the two parts responsible for the lines and rings. In order to explain the above we need an appropriate geometrical model and some structural information on the medium range order of the atoms causing scattered rings. This information is contained in the partial pair-pair correlation function of the coherent background. The result of this work may be of aid in order to comprehend the “genetic” relationship between the structure of the crystalline, amorphous FK phases and this new metastable alloy or icosahedral phase

Evolution structurale d'alliages Pd-Si amorphes en fonction de la température

We have studied the thermal behaviour of two kinds of amorphous Pd 80Si20 alloys prepared by sputtering and by rapid quenching from the melt. Interference functions were determined by the X-ray non-dispersive method. This method allows the diffraction study of a sample held in a furnace at a stable temperature. We do not observe any difference between the two kinds of samples. Interference and radial distribution functions evolve with increasing temperature (20° → 340°C). Nevertheless no sensitive evolution of the functions I(K) and W(R ) at 300 °C is observed during an isothermal annealing of 24 h. If the isothermal annealing at high temperature (∼ 300 °C) is short, there is a quasi-reversible behaviour ; but if the annealing is long (24 h) the functions I(K) and W(R ) obtained after return to 20°C are modified relative to the initial functions. These results are analysed in terms of an increase of the tetrahedral perfection.Nous avons étudié sur deux types d'alliages Pd80-Si20 préparés par pulvérisation cathodique et par trempe ultra-rapide l'influence de la température sur la structure. Les fonctions d'interférence ont été obtenues par la méthode de diffraction des rayons X avec analyse d'énergie à angle fixe (méthode non dispersive). Cette méthode permet l'étude de la diffraction d'un échantillon maintenu dans un four à température bien déterminée. Nous n'observons pas de différence fondamentale dans l'évolution des deux types d'échantillon. Les fonctions d'interférence I(K) et de distribution radiales W(R) évoluent avec un accroissement de la température (20 → 340 °C). Par contre aucune évolution sensible des fonctions I(K ) et W(R ) à 300 °C n'est observée lors d'un maintien 24 h à cette température. Si la durée de maintien au-delà de 300 °C est courte (quelques heures), on observe un comportement quasi-réversible. Par contre, si le maintien à haute température (∼ 300 °C) est long (∼ 24 h), les fonctions I( K) et W(R ) obtenues après retour à 20 °C ont évolué par rapport aux fonctions initiales. Ces résultats peuvent être interprétés par l'accroissement de la symétrie locale (sites tétraédriques plus parfaits) avec l'accroissement de la température

Edge of disorder

Polyanilines belong to a family of electronic polymers. Due to a wide range of variable parameters in the synthesis of polyanalines, a variety of samples with different physical properties can be achieved. Different investigators have identified the degree of crystallinity for different varieties of polyanilines. They have classified them into two classes of emeraldine forms. In these structural investigations, the traditional "concept of the two phase system" was used for differentiating the crystalline phase. Recently, it has been shown that washing the emeraldine base (EB) by NMP or THF will establish two different X-ray patterns. The X-ray pattern of the sample washed with THF shows a limited number of Bragg lines superimposed on the broad diffuse halos. Other patterns only indicated broad diffuse halos. These experimental facts do not advocate the "two phase system model" , especially with scarce Bragg reflections. We have set-up an elegant prototype for X-ray diffractometry which allows us to obtain a high resolution X-ray pattern from polymers with coherent intrinsic backgrounds. Formally, this coherent background was excluded for polymer structural identification. Nevertheless, we have introduced a new method of separating the reflected lines from a coherent background. The interference functions F(K) of the coherent background were obtained applying this novel technique. The advantage of our method of separation is verified by comparing the background of F(K) with the F(K) of the sample washed with NMP. Fourier inversion is used to identify the short range order of the atoms responsible for the diffuse scattering. This result enabled us to choose a standard state of disorder in polymers. This standard state of randomness can be considered as the edge of disorder. By defining this reference state of randomness, it can be seen that the atomic arrangements of long and fast changing structural chains correspond best to the random array model rather than the two phase system model. The novel experiment and analysis technique presented here offer a possibility of application of structural studies to many common polymers, all of which have the characteristic of the diffuse halo underlying the diffraction lines

A metastable phase Al<SUB>3</SUB>Cu<SUB>2</SUB>

A new metastable intermediate phase having a trigonal unit cell belonging to the space group P3&#x0305;ml has been detected in a rapidly solidified aluminium-45 at. % copper alloy. The unit cell dimensions are a=4.106 and c=5.094 &#197;. With five atoms per unit cell, the observed structure can be regarded as an isotype of Al3Ni2

EFFECT OF ISOTHERMAL ANNEALING ON THE STRUCTURE OF AN AMORPHOUS Pd-Si ALLOY

L'effet d'un chauffage sur la structure d'alliages Pd-Si amorphe est présenté. Des effets quasi-reversibles sont observés avant qu'apparaissent les effets de relaxation structurale.The effect of heating on the structure of amorphous Pd-Si alloys presented. Quasi-reversible effects are observed prior to structural relaxation

Relationship between polytetrahedral crystals and glasses by using anomalous X-ray diffraction technique and geometrical modelling

Partial interference functions and radial distribution functions are determined using the anomalous X-ray diffraction technique for the amorphous alloy Cu 2Y. These results are generalized for the amorphous family of A 2B amorphous alloys where A is Co, Fe, Cu, or Ni and B is Zr or Y. By comparing the coordination number and interatomic distances obtained experimentally with those of geometrical models, the icosahedral order and the occurrence of disclination networks in these Frank and Kasper crystalline and glassy phases are analysed.Les fonctions d'interférence et de distributions radiales sont déterminées en utilisant la technique de diffraction anomale des rayons X pour l'alliage Cu2Y. Ces résultats permettent d'inclure cette structure dans une large famille A2B où A est du Co, Fe, Cu ou Ni et B est du Zr ou Y. Par comparaison des nombres de coordination et des distances interatomiques obtenus expérimentalement avec ceux de modèles géométriques, nous analysons l'ordre icosaédrique et la présence de réseaux de disinclinaison dans ces phases de Frank et Kasper, vitreuses ou cristallines

THE APPLICATIONS OF ANOMALOUS DIFFRACTION IN POWDER DIFFRACTOMETRY

An atom scatters X-ray with an amplitude and a phase that can be represented by the complex number ; ftof (K,E) = fo (K) + f' (K,E)+ i f" (K, E) . fo (K) represents the atomic scattering factor for the radiations with frequencies smaller than the absorption edge frequencies. The anomalous scattering terms f' and f", which are functions of the wavelength, describe the in-phase an out phase components of the change due to finite binding energies of the electrons in the atom. Near an absorption edge of an atom, where the x-ray energy is close to the binding energy of an inner shell electron both f' and f" vary rapidly with wavelength, f" jumps rapidly up to a maximum while f' dips and then rises again as the x-ray energy goes from below the edge to above it. The f" term results in breaking of Friedel's law, this effect has long been used to find the solution of the crystallographic phase problem. Very few applications were performed to show the useful aspect of anomalous dispersion on the powder diffractometry. Even though the principle is simple and straight forword the implementation was not a trivia tasK. The main difficulty was to obtain reliable data. As the energy dependence of diffraction intensity is usually small then a very high accurancy of measurments were required. The recent availability of synchrotron radiation as a tunable source of x-ray, with very high intensity made it possible to use this effect to obtain structural information from a powder diffractometry. For this purpose a powder automated diffractometer has been designed to be adapted to such new sources (L. U. R. E. DCI) . This diffractometer consistes of three main part