Present-day microcomputers offer a wealth of computing power at very liBited costs. In fact, it is now well possible, as we showed some years ago, to have a simple least-squares structure refinement progran running on a $ lOOO.-computer.Another, and possibly more important, field in which microcomputers can be applied is teaching. As a rule, crystallographic methods depend heavily on computing, and the associated programs are nearly always black boxes which do not elucidate anything of uhat is g0ing on. To explain these methods to students, simulations can be used. As an example, a direct ~ethod teaching

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The crystal structures of CusZn8, CusCds and FeaZna0 have been studied using single-crystal X-ray diffraction techniques. All three alloys have the body-centred cubic space group 1743m with four formula units per unit cell. Lattice parameters are 8.878 (4), 9.615 (10) and 9.018 (3) A respectively for CusZn8, CusCd8 and the pseudocell of FeaZn~0. Evidence shows a new superstructure of 36 A at the composition FeaZn~0. Values of lattice parameter as a function of composition are given for the CusCd8 phase. New data are presented on the three structures, and the ordering of the alomic species is compared with that reported by previous workers. Significant differences in our ordering for FeaZn~0 are attributed to our different composition within the 7 phase. X-ray diffraction intensity data were collected using a 3-circle automatic single-crystal diffractometer and Mo Ke X-radiation. The crystal structures were refined using full-matrix least-squares methods to final R values of 6.1%, 7-5 %, and 5.5 % respectively. A discussion of possible reinterpretations for these and other ~, brass structures is presented.

A new classification of the crystal structures of normal valence compounds with simple atomic ratios is discussed. This classification is based on two atomic parameters which gauge the directional properties of the bonds in a compound. The parameters are (1) the average principal quantum number, ~, of the valence shell of the component atoms and (2) the difference,/lx, between the electronegativities of anion and cation. It is shown that the most common structures of compounds of composition AiX 1 (i, j = 1, 2, 3) occur only in certain well defined regions of a ~ versus ~Ix diagram, and the proposed classification, therefore, is of considerable help in understanding and predicting the structures of normal valence compounds.

Constituents in the public mental health system should seriously consider that health services utilization is socially patterned and not just an individual behavior.

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