XRD techniques

Table 15.1 Comparison of the mean diameters of iridium nanoparticles as determined by TEM, SAXS and XRD techniques [25],...

Quantitative determination of the major and minor minerals In geological materials Is commonly attempted by x-ray diffraction (XRD) techniques. Mineralogists use a variety of sophisticated and often tedious procedures to obtain semlquantltatlve estimates of the minerals In a solid sample. The mineralogist knows that XRD Intensities depend on the quantity of each mineral component In the sample even through expressions for conversion of signal Intensity to quantitative analysis often are unknown or difficult to determine. Serious difficulties caused by variables such as particle size, crystallinity, and orientation make quantification of many sample types Impractical. Because of a lack of suitable standards, these difficulties are particularly manifest for clay minerals. Nevertheless, XRD remains the most generally used method for quan-... 

Detailed analysis of different XRD techniques led to the conclusion that CdS used in CdS/CdTe PV cells was polytype, with essentially random stacking of cubic and hexagonal structures in individual crystals [18], This study goes a long way to explaining the wide variation in apparent crystal structure. 

To date, mainly two groups have been responsible for the development of the combined EXAFS/XRD methods, namely the group of Clausen, Topspe, Niemann, and co-workers, who developed the combined EXAFS/ XRD and QEXAFS/XRD techniques, and the group of Thomas, Greaves, and co-workers, who developed the DEXAFS/XRD method. In the following, some of the developments are discussed briefly. 

Although combined EXAFS/XRD has been found to be very useful in studies of dynamic phenomena in catalysts, only a limited number of experiments have been reported so far. The difficulty in designing and constructing in situ cells that can be used both in XRD and in X-ray absorption spectroscopy is probably the most important limitation hindering more widespread use. Here, we briefly review some of the work performed with the combined EXAFS/XRD techniques. 

At the present time much effort is being devoted to tailor-making of new nanomaterials with specific catalytic properties. In this quest for constantly decreasing the dimensions of the catalytically active components, one will unavoidably encounter materials that will be partly or completely X-ray amorphous. The present review has shown that the combined EXAFS/ XRD techniques are uniquely well suited for providing the necessary structural understanding. Thus, in view of the trend in catalyst technologies and advances in technique developments, the application of the combined techniques will no doubt play an increasing role in future catalyst characterization efforts. We now briefly discuss some likely applications and technique developments which involve the X-ray techniques discussed presently. 

The chemical compositions were determined by atomic absorption. The crystallinity of the samples was checked by XRD technique. Size and morphology of the crystals were examined by scanning electron microscopy. 

The most obvious choice to determine phases that may be present in the molybdena catalyst is XRD. Matching of diffraction lines obtained for the catalyst with those of pure bulk compounds gives unequivocal identification of phases present. This is one of the few techniques that yields positive results. The absence of matching diffraction lines, however, is not proof that the phase in question is not present in the catalyst. The XRD technique is limited to particle sizes of above approximately 40 A for oxides or sulfides, lower sized particles giving no discernible pattern over that of the broad alumina pattern. Thus, the presence of a highly dispersed phase, either as small crystallites or as a surface compound of several layers thickness will not be detected. Also, if the phase is highly disordered (amorphous), a sharp pattern will not be obtained, although some broad structure above that of the alumina may be detected. It is a moot point as to whether such a case is considered as a separate phase or a perturbation of the alumina structure. Ratnasamy et al. (11) have examined their CoMo/Al catalyst from the latter point of view, with particular emphasis on the effect of calcination temperature. 

In this chapter, we provide an overview of our recent efforts to develop a fundamental science base for the design and preparation of optimal lipid-based carriers of DNA and siRNA for gene therapy and gene silencing. We employ synthesis of custom multivalent lipids, synchrotron X-ray diffraction (XRD) techniques, optical and cryo-electron microscopy, as well as biological assays in order to correlate the structures, chemical, and biophysical properties of cationic liposome (CL)-NA complexes to their biological activity and to clarify the interactions between CL-NA complexes and cellular components. Earlier work has been reviewed elsewhere [1-7] and will not be covered exhaustively here. 

Cavatur and Suryanarayanan [1.164] have developed a low-temperature X-ray powder diffractometer (XRD) technique to study the solid states of solutes in frozen aqueous solutions. In frozen naftillin sodium solution (22% w/w), no eutectic crystallization was observed. Annealing at -4°C caused solute crystallization, which increased with annealing time. Two other products studied showed that XRD provides information about the degree of crystallinity without the interference of other events. 

The present work summarizes opportunities of using high-resolution synchrotron and standard xrd techniques for structural characterization as well as for investigations of structure-property-relationships. xrd will be used to determine quantitatively the phase content of morphotropic pzt. Temperature dependent measurements provide information about the phase transformation of morphotropic donor doped pzt ceramics and high-resolution synchrotron X-ray diffraction gives information about the extrinsic and intrinsic contributions to the electric field induced strain, xrd results are finally compared with electrical measurements to analyze the interactions among microstructure, phase content and properties. 

Table 1 is a list of textbooks and their characteristics in which the reader can find the physical background about the XRD technique. The following section includes a brief introduction to the physics (Langford and Louer, 1996 Alexander and Klug, 1974) of the diffraction experiment, intended to familiarize the reader with basic facts of the technique from a practical viewpoint that must be observed when planning experiments and interpreting data obtained under catalytic reaction conditions. 

At this time, the locations of cations in zeolites have been determined primarily by X-ray diffraction (XRD) techniques. Unfortunately, this method has the drawback of being able to locate only the most stationary cations in zeolites. In some studies of hydrated zeolites, less than 50% of the total cation population can be accounted for. A higher percentage of the cations can be located in dehydrated samples, but the effect of the dehydration step on the location of the cations is generally not well known. NMR measurements, on the other hand, are most sensitive to mobile cations and cations in high symmetry sites. 

The decomposition of Mg(BH4)2 has been extensively studied using in situ XRD techniques coupled with residual gas analysis (RGA) measurements of the gas release. They report that the borohydride decomposed starting at 300°C, releasing 9 wt% H2 by 350°C. No ordered phase was detected by the XRD between these two temperatures, indicative of an amorphous phase or phases. Above 350°C, MgH2 apparently recrystallized and was detected by the XRD. This phase subsequently released the additional hydrogen as the temperature was increased further. Initial attempts to recharge the spent material indicated that only the Mg rehydrided to form MgH2. 

XAS is in general less accurate than XRD techniques. The accuracy depends on the edge energy, the resolution of the monochromator and, last but not least, the quality of model building. As a general mle distances can be determined up to 5 X 10 A and coordination numbers at maximum to 10%. This is clearly inferior to XRD. 

So far, various studies focused on developing catalyst materials with improved ORR activity, but only few reported the stability and durability of ORR catalysts. The study of accelerated durability tests (ADT) in conjunction with electron microprobe analysis (BMPA), LEED, and XRD techniques on Pt-based al-loys ° observed hd metal dissolution, diffusion of 3bulk oxides on the surface, and migration and agglomeration of Pt. Yu et al. compared the durability and activity of PtCo/C with Pt/C catalysts. Throngh determination of the electrochemically active sniiace area, mass, and specific activities with respeet to the potential cycles, they found the overall cell performance of PtCo/C is higher than that of Pt/C. They also concluded that the observed dissolution of Co has no severe impact on the cell performanee or membrane conductance. Additionally, Popov et al studied the stabihty of Pt M/C for X = 1,3 and M = V, Fe, Ni, Co. ADT analyses revealed that Pt/C has the lowest activity when eompared to Pt-alloy catalysts, and that the metal dissolntion is lower for a Pt M ratio of 3 1 than compared to a 1 1 ratio. Also, Pt-Ni showed a lower dissolution rate than the other considered Pt-M alloys. 

For the accurate characterization of the adsorption phenomena, it is necessary to obtain accurate information on pore stmctures. However, most of ordinary microporous carbons and mesoporous carbons are obtained with amorphous stmctures that are characterized by irregular arrangements of non-uniform pores. X-ray (or electron) diffraction (XRD) techniques are not useful for such carbons because there are no well-defined stmctural factors to correlate with the adsorption behavior. Moreover, porous carbons exhibit wide varieties of the surface functional groups and the thickness of the pore walls, depending on the details of the synthesis conditions. The lack of distinct XRD lines makes it difficult to distinguish stmctural differences between samples which causes many works to depend empirically on specific samples. 

The XRD method can be successfully apply to detect the /-/" phase boundary by analysing the (112) and (400)/(004) reflections respectively at about 42 and 61-64 20 (Fig. 6.3.) [22,23], even though, due to the low intensity of these peaks, confirmation by using a synchrotron radiation or even neutron scattering source may be desirable [45,46]. Due to the low intensity of the conventional XRD technique to the oxygen atoms, the / -c phase transition is best detected by using... 

---