Patterns, interpretation

Before this work was undertaken, only Pauly s data and Pearson s structure report on Li2AlAg were available [40, 41]. The original paper of Pauly, based on X-ray powder pattern interpretations, reports Li2AlAg to crystallize as two cubic polymorphs (F43m and Fd3m, a=6.350 A). We have recently discovered two further compounds in this system LiAlAg2 and LiyAl4Ag [42]. 

Figure 14.8 (a) THEED pattern of a fi-W2N film, two crystal lattices are present the metal lattice and the ordered nitrogen lattice (b) pattern interpretation. 

An electron reflected from a surface carries diffraction information if it has experienced elastic scattering or information on the excitation of phonons, plasmons, and electronic or vibrational transitions as the result of inelastic events. The major effort in diffraction studies has concentrated on the use of low energies ( 200 eV) but reflection high-energy electron diffraction (RHEED 20—40keV) is suitable also for surface work (see ref. 2 for a description of pattern interpretation). RHEED... 

One does, however, have other aids to pattern interpretation. When... 

The combined symmetry of the top few planes determines the symmetry of the pattern. Because of this, attention has been directed to the diperiodic groups in two dimensions which are 80 in number (142) rather than to the 17 strictly plane groups (128) which do not admit symmetry operations involving the third dimension. At the present unsatisfactory stage of understanding of LEED intensities, application of these diperiodic groups in pattern interpretation is risky and is not recommended. 

The utility of the reciprocal space construction is evident for a postacceleration type LEED tube (Fig. 7). There is a one-to-one correspondence, of the intersections of rods with the Ewald sphere, and LEED beams as observed on a spherical screen in a display-type apparatus. In other words, the geometric spot arrangement in a photograph of a LEED pattern is a representation of the reciprocal mesh of a surface structure. This important analogy makes the reciprocal construction helpful in pattern interpretation. 

Fig. 14. Example of ambiguity that can arise in pattern interpretation. At left, a (2 X 2) structure on a (111) plane of an fee lattice, with corresponding schematic LEED pattern drawn above it. Half-orders are shown as crosses. At right, a (2 x 1) adlayer having three rotational orientations. Pattern that arises from separately scattering patches will be a superposition whose synthesis gives the same spot pattern as at left.

Thus, in the approach of current use, refinement of the X-ray diffraction pattern interpretation results in an increase in the number and variety of fitting parameters in the theoretical model of a structure studied. Besides that, most of these parameters are introduced ad hoc and have an unclear physical meaning outside the eontext of the interpretation of diffraction patterns. 

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