Some Observations on the Structure of TATB

The recent discovery of second-harmonic light generation (SHG) from 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) has renewed interest in both the crystallography and the identification of defect structures in this material. The accepted crystal structure is triclinic but centrosymmetric (P-1); SHG is not expected from materials of this symmetry. A wide variety of syntheses have been shown to produce SHG-positive materials, including an emulsion technique, two different recrystallization methods, and a variety of combined reaction - precipitation processes. In addition, two standard commercial powders have shown the property after annealing in the 300 C range. We have examined the structures of several of these powders using polarized light microscopy (PLM), scanning electron microscopy (SEM), and powder X-ray diffraction (XRD). We find that SHG-positive materials typically contain crystals both more transparent and more strongly faceted (i.e. ''more perfect'') than weak-SHG ones and that the positive samples show stronger texture in their powder diffraction patterns. These results are compared with predictions from Cady's structure

So close, yet so far away? the effects of city size, density, and growth on local civic participation

Recent studies in the U.S. context have suggested that political participation is a function of the size and concentration of a city’s population. Most of this research focuses on the idea that there is an optimal size and concentration of population that favors active political participation in terms of a higher propensity to vote in local elections, contact local officials, and attend community meetings. The conventional argument suggests a negative relationship between city size and political participation that is mitigated to some extent by the deeper social interactions generated by increased population density. We extend this research by also investigating the influence of population growth on the broader concept of civic participation. Civic participation is a multidimensional concept that requires the use of a broad set of indicators. We expand the number of measures to gauge civic participation at the local level by including data on the formation of volunteer associations, volunteer fire brigades and not-for-profit organizations as well as voter turnout. We test the hypotheses derived from extant research using aggregate data collected from Portuguese cities and discuss the implications of our findings for the literature on local civic participatio