Polytypism in sodium 2-oxocaproate crystals

give a pattern of sharp and diffuse X-ray reflexions. It is seen from zero and higher layer Weissenberg photographs that the reflexions with h + k + l even and k + l even are sharp while those with h + k + l even and k + l odd, although sharp, appear above a background of diffuse maxima. Further, reflexions corresponding to an a axial length twice that of the normal cell appear on either side of the reflexions with h + k + l even and k + l odd; with respect to the double cell, these reflexions have the indices Hkl with H odd. Figs. from the equivalent points of A and B that if the B layer is displaced by 1⁄2(b + e), the unit cell becomes AA instead of AB. For such disorder, it can be shown (Pant, 1964) that as observed, reflexions with h + k + l even and k + l even are sharp while those with h + k + l even (i.e. HI2 + k +l even) and k + 1 odd are diffuse; the sharp reflexions appearing above the background of diffuse maxima (e.g. reflexions marked 30.1.0 and 30.3.0 in Fig.2(a) and (b) respectively) come from the ordered region in the crystal. The structure of the polytype is, as shown later, A A B B A A B B . . . The equivalent points for the unit cell of the polytype are as follows: