Other dopants, different structures

Boyle et al. [318] have used real-time diffraction at a synchrotron source for following the effect of heat treatment of various doped PANIs. The x-ray data are supplemented by mass spectroscopy, thennal gravimetry and differential thermal analysis data. The authors consider PANI-HCl, PANI-HCIO4 and PANl-H2SO4, as well as a sample free of dopant but containing water. Diffraction patterns are decomposed into Gaussian peaks in order to describe the evolution 

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For example, if we dope a semiconductor with an impurity by diffusion, there will be a higher concentration of dopant atoms near the surface, but we do not consider this a separate phase unless we exceed the solid solubility with the concentration of dopant atoms causing them to form a precipitate. The precipitate would be considered a second phase. Similarly, if we solidify a solid solution (isomorphous) system, the resulting polycrystalline alloy is considered a single phase because all of the grains have the same crystal structure, even though the first-to-freeze grains may have a different composition than the last-to-freeze. On the other hand, a liquid or gas may have the same composition as a solid, but clearly have different structures and therefore must be considered as separate phases. 

Other single-crystal x-ray diffraction studies of transition element dopants in jS-rh boron are based on the results of a refinement of the /3-rh boron structure that establishes the occurrence of four new low-occupancy (3.7, 6.6, 6.8 and 8.5%) B positions in addition to the earlier known ones. The dopant elements studied, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Hf and Ta, do not enter B positions in the framework, but they enter the Al, A2, D and E positions. In some cases the doping elements have been studied at several concentrations for each element and for different cooling rates. The percentage occupancies of certain positions are eorrelated with the atomie sizes of the dopants. The bond distances between the polyhedra are shorter than those within the polyhedra. The mechanism of doping for some cases is denoted displacive, rather than interstitial or substitutional, because of competing interactions between the six different partially occupied B positions and dopant atoms. 

It also became evident that a great variety of catalysts, potentially exhibiting a large flexibility, could be prepared via solid solutions. Three different degrees of freedom can be varied in a controlled fashion the chemical nature of the host matrix AO, the chemical nature of the guest cation M, and the dopant concentration x. Furthermore, solid solutions can be formed not only by cubic oxides but also by alumina, titania, zirconia, and others. Thus, another degree of freedom is added, namely, the different crystal structures. 

Even if unwanted chemical transformations of doped conjugated materials can be avoided by strict exclusion of air, the long-term stability is limited by a de-mixing of dopant and redox system. An alternative approach to persistent electrical conductors is therefore concerned with the synthesis of intrinsic conductors. Toward that end the lowering of the band gap is desirable which, on the other hand, is known to reduce the chemical stability. The synthetic approaches taken must therefore focus on a compromise between electronically attractive 7i-structure, stability and processability of the material. In conclusion, it is the flexible combination of different areas of research which provides new insights into the properties of monomeric, oligomeric and polymeric radical ions. 

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