Observation and analysis of lattice defects

Observation and analysis of lattice defects 5.1.1 Point defects... 

Another contribution to variations of intrinsic activity is the different number of defects and amount of disorder in the metallic Cu phase. This disorder can manifest itself in the form of lattice strain detectable, for example, by line profile analysis of X-ray diffraction (XRD) peaks [73], 63Cu nuclear magnetic resonance lines [74], or as an increased disorder parameter (Debye-Waller factor) derived from extended X-ray absorption fine structure spectroscopy [75], Strained copper has been shown theoretically [76] and experimentally [77] to have different adsorptive properties compared to unstrained surfaces. Strain (i.e. local variation in the lattice parameter) is known to shift the center of the d-band and alter the interactions of metal surface and absorbate [78]. The origin of strain and defects in Cu/ZnO is probably related to the crystallization of kinetically trapped nonideal Cu in close interfacial contact to the oxide during catalyst activation at mild conditions. A correlation of the concentration of planar defects in the Cu particles with the catalytic activity in methanol synthesis was observed in a series of industrial Cu/Zn0/Al203 catalysts by Kasatkin et al. [57]. Planar defects like stacking faults and twin boundaries can also be observed by HRTEM and are marked with arrows in Figure 5.3.8C [58],... 

Glusker JP, Traeblood KN (1985) Crystal Stmctnre Analysis. 2nd edition. Oxford Univ Press, Oxford, UK Gnutzmaim V, Vogel W (1990) Surface oxidation and reduction of small platinum particles observed by in situ X-ray diffraction. Z PhysikD (Atoms, Molecules, Clusters) 12 597-600 Greenwood NN (1970) Ionic Crystals Lattice Defects and Norrstoichiometry. Butterworths, London Guinier A (1963) X-ray Diffraction in Crystals, Imperfect Crystals, and Amorphous Bodies. W H Freeman, San Francisco... 

To avoid using arbitrary a priori selected) profile functions, whose parameters are not directly and uni vocally related to physically observable quantities, a different approach can be followed line profiles can be described directly in terms of physical models of the micro structure and lattice defects present in the studied material. This is the foundation of the WPPM method in this respect, modelling is opposed to fitting as the former involves the use of physical information at all stages of the analysis, whereas the latter uses an a priori selected (though flexible) form for the PD profiles. 

The direct method includes direct observation by electron microscope and field emission technique structural analysis using X-ray, neutron and electron diffractometry, or channelling technique and also resonance techniques such as ESR, NMR, and Mossbauer absorption. The techniques used in the indirect method include the measurement of a property sensitive to the nonstoichiometric composition, such as lattice constant, density, equilibrium partial pressure, and electric conductivity. The defect structure is estimated from the correspondence between the defect model assumed and the measured change of the property. With the indirect method, it is rather difficult to estimate defect structures more complex than the simple point defect. 

Furthermore, the presence of defects in a crystal lattice may also alter the diffraction pattern Depending on the type and the concentration of the defects, systematic peak broadening, peak shifts as well as peak splitting may be observed, and stress and strain may also influence the diffractograms [20, 21]. Thus the detailed analysis of measured peak positions, their widths and intensities can be used for the identification of the defects existing in a particular sample. 

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