147others may be allowed to vibrate independently. The T and e~ tensors of the several rigid units are refined directly, as in Pawley's method, along with the usual coordinates, occupancy factors, etc. Experience suggests that this onestage procedure may be highly advantageous, especially in rate of convergence, provided the molecules are sufficiently rigid and the f curves used (for X-ray data) represent the stationary-atom electron densities to the required accuracy. When these conditions are not met, the two-stage procedure, or a comparison of the two, may help to reveal inadequacies of the model, due to internal vibrations, charge polarization, altered hybridization, etc., more readily than extensive difference syntheses.The tensor formulation is also useful for evaluation of the libration corrections to the atomic coordinates. Assuming for simplicity that our refinement procedure has located the centroids of the atomic peaks rather than their maxima (either assumption is an approximation requiring justification in particular circumstances), we may, for the present argument, disregard the factor D(a(o) in Cruickshank's (1961) equation (6) and obtain from his equations (10) Acta Cryst. (1966). 20, 147The anisotropie temperature factor of atoms in special positions. In this paper a table of symmetry restrictions on anisotropic temperature factor parameters for all special positions of the 230 space groups is presented. The text explains the table and describes the computer program which was used to derive it. The relationships between the various restricted forms are displayed diagrammatically. No recommendations for the method of programming these restrictions in least-squares refinement are included. M3M ]aM 432

Limitations to the application of the electronbeaminduced current in hydrogenpassivated silicon grain boundaries J. Appl. Phys. 56, 3011 (1984); 10.1063/1.333777Determination of surface recombination velocity at a grain boundary using electronbeaminduced current

The article presents a study of the interaction of iron with a grain boundary in boron-doped multicrystalline silicon. The sample was intentionally contaminated with iron to a few 1014 cm−3 and investigated by the electron-beam-induced-current technique (measurement of minority-carrier diffusion length, quantitative imaging) in the temperature range 80–300 K. The measurements were carried out for two different states of iron in the sample: (i) iron paired with boron, i.e., as FeB, and (ii) iron as interstitial iron Fei. The differences between diffusion lengths for these two states were used to estimate the iron concentration. The analysis of the data revealed a pronounced iron profile around the grain boundary, indicating gettering of about 4×1011 iron atoms per cm2. The recombination velocity of the grain boundary is about 5×105 cm s−1 at 300 K and is not changed by the FeB destruction treatment. The temperature dependence of the iron-related diffusion length components is discussed and found to be in satisfactory agreement with what is expected from Shockley–Read–Hall theory. Further, the diffusion length analysis revealed also a strong recombination channel of unidentified origin.

Homologous series of syn‐bridged annulenes are of interest for a deeper understanding of the Hückel rule. Dialdehydes like (3) could be key compounds for syntheses of such annulenes. The synthesis of (3) from the dialdehyde (1) was accomplished in three steps: reaction with (2) to give the diester, reduction to the diol, and oxidation to (3). This three‐step sequence is also suitable for synthesis of the two next higher dialdehydes with three and four CH2 groups, respectively.

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