No abstract

No abstract

No abstract

The FOCUS method, in which both crystal chemical information and powder diffraction data are included in the structure determination process, is presented. FOCUS combines automatic Fourier recycling with a specialized topology search specific to zeolites, which can be described as having three‐dimensional four‐connected framework structures. The capabilities of FOCUS have been tested with seven examples of medium to high complexity. The method was then applied to three novel zeolite structures and a promising model could be obtained in each case. Experience shows that the approach of using chemical and geometric knowledge can compensate for some of the information that is lost as a result of the overlap problem. At the same time, there is an intrinsic disadvantage: any method based on assumptions of certain structural properties is also limited to materials which conform to these assumptions. Examples which show the consequences of relaxing the structural assumptions are also given.

No abstract

The Rietveld full-profile refinement technique (J. Appl. Cryst. 2, 1969, 65-71)2has been used to determine cation distributions among Me .,. ions in numerous solid solutions. These investigations have been based on neutron powder diffraction data or photographic X-ray Guinier-

A new method for unravelling the intensities of severely or exactly overlapping reflections in a powder diffraction pattern has been developed. The fast iterafive Patterson squaring (FIPS) method involves an iterative procedure in which (1) a Patterson function is calculated using equipartitioned data (intensity ratio for overlapping reflections set to 1.0), (2) each point in the map is squared, (3) the new map is back-transformed to obtain new Fourier coefficients, and (4) these coefficients are then extrapo-~W 2 ow.,rl~tp lated to give a new set of ~ hkl values and a new intensity distribution for the overlapping reflections (nonoverlapping ones remain unaffected). The cycle is repeated until the intensity statistics of the overlapping reflections approximate those of the non-overlapping ones. Only after a redistribution of the intensities by the FIPS method was the ab initio structure solution possible for the molecular sieve SAPO-40 (structure-type code AFR), in which 65% of the reflections severely overlap.

No abstract