Accurate X-ray absorption correction factors A* for cylindrical samples were calculated in the range of the product of linear absorption coefficient/t and cylinder radius R of 0 < gR < 2.5 and in the range of Bragg angle 0 of 0 ° < 0 < 90 °. A method of double numerical integration that is different from the method usually used resulted in a maximum error of 0"1% with the typical error being much less. A table of values of A* was prepared, and simple equations were obtained for interpolation between values in the table to an accuracy of 0.1% or better. A full-range curve-fitting procedure that is accurate to 0.1% also was developed for routine use when many values of A* must be calculated for a single sample.

The crystal structures of the isomorphous garnet-red copper salts, KCuCl3 and NH4CuCl3, have been determined. Each compound contains discrete, planar, Cu2Cl6= dimers. These dimers are stacked above each other along the crystallographic a axis. The cation exhibits a ninefold coordination. KCuCl3 is antiferromagnetic below 30°K. Both KCuCl3 and NH4CuCl3 are pleochroic with maximum visible absorption when the electric vector is parallel with the Cu–Cu vector of the dimer.

sequence creates 321123 out of (22)3. It is to be noted that a scheme similar to ours has been put forward by Mardix, Kalman & Steinberger (1969) to explain the growth of ZnS polytypes. However, their work relates to transformation of polytypes in the same crystal and not to the growth of different polytypes. Also, their scheme always envisages stacking faults at equal intervals and up to a certain step the layers slip in a clockwise direction and for the rest of the steps in an anticlockwise direction, and this sequence of slip repeats periodically. The present scheme assumes only periodic stacking faults; the faults may not be at equal intervals as for example in the case of 32H. Moreover, the slip of layers is always alternately in clockwise and anticlockwise directions. Such a scheme of layer transposition is possible only when suitable partial dislocations, which nucleate stacking faults, sweep the basal plane; the sweeping occurs in the sequence after certain regular period in the parent structure. In order to find out whether chains of basal stacking faults occur in cadmium iodide crystals, we tried to observe the crystals directly in the electron microscope. Since cadmium iodide crystals are extremely susceptible to electron beam damage, it is not usually possible to observe the dislocation pattern in these crystals. However, by improving the thermal contact of the crystal with the specimen grid and employing full excitation of condenser lenses, we were able to observe the dislocation patterns in nearly 60% of the crystals. It is, however, not always possible to study completely each individual crystal. We found that dissociation of basal dislocations producing twofold, threefold and fourfold ribbons usually took place in all the crystals observed. Fig. 5 represents a typical example of twofold, threefold and fourfold partial ribbons. Thus it seems reasonable to believe that sequences of partial ribbons producing stacking faults occur in the cadmium iodide crystals and a regular sequence of stacking faults creating a polytype can easily occur. This lends support to the scheme of the growth of the polytypes that we have presented. Finally the determination of probable structure of a polytype based on calculation of theoretical stacking fault energy seems to be a new approach which needs to be explored further. Also the minimum stacking fault energy criterion lends credence to the above explanation of growth of polytypes in terms of creation and ordering of stacking faults. Equations that require numerical integration over only one variable were derived for calculating the intensity of secondary scattering of X-rays for non-crystalline samples of finite thickness. Both the reflection and transmission geometry cases were considered. Tables are given that allow the intensity ratio of secondary to primary scattering to be determined without making lengthy calculations. Modification of the normalization procedure when secondary scattering is important is discussed.

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