Lipson, Henry

If no external evidence is available, it is still possible to determine the unit cell dimensions of crystals of low symmetry from powder diffraction patterns, provided that sharp patterns with high resolution are avail able. Hesse (1948) and Lipson (1949) have used numerical methods successfully for orthorhombic crystals. (Sec also Henry, Lipson, and Wooster, 1951 Bunn 1955.) Ito (1950) has devised a method which in principle will lead to a possible unit cell for a crystal of any symmetry. It may not be the true unit cell appropriate to the crystal symmetry, but when a possible cell satisfying all the diffraction peaks on a powder pattern lias been obtained by Ito s method, the true unit cell can be obtained by a reduction process first devised by Delaunay (1933). Ito applies the reduction process to the reciprocal lattice (see p. 185), but International Tables (1952) recommend that the procedure should be applied to the direct space lattice. 

Lipson and Guillet (1982) provide Henry s Law constants for the chloroform -poly(ethylene-co-vinyl acetate) [P(E VAC)1 system at 338.15 K. 

Bragg following advice to do so from his father, W. H. Bragg/ The calculations involved were many and tedious, and therefore Henry Lipson, C. Arnold Beevers, A. Lindo Patterson, and George Tunell provided more convenient methods of computing the electron-density functions in the days before high-speed computers were available. Currently, the computation of an electron-density map is simple and fast because of the efficiency of available computers. 

Fig. 4-7 Effects produced by the passage of x-rays through matter, after Henry, Lipson, and Wooster [G.8].

These are the usual multiplicity factors. In some crystals, planes having these indices comprise two forms with the same spacing but different structure factor, and the multiplicity factor for each form is half the value given above. In the cubic system, for example, there are some crystals in which permutations of the indices (M/) produce planes which are not structurally equivalent in such crystals (AuBe, discussed in S. 2-7, is an example), the plane (123), for example, belongs to one form and has a certain structure factor, while the plane (321) belongs to another form and has a different structure factor. There are = 24 planes in the first form and 24 planes in the second. This question is discussed more fully by Henry, Lipson, and Wooster [G.8]. 

G.8 N. F. M. Henry H. Lipson and W. A. Wooster. The Interpretation of X-Ray Diffraction Photographs (London Macmillan, 1951). Rotating and oscillating crystal methods, as well as powder methods, are described. Good section on analytical methods of indexing powder photographs. 

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