Several salt forms of 4-aminopyridine and 3,4-diaminopyridine with nitric, sulfuric, and phosphoric acids, comprising anhydrates and some hydrates, have been prepared and structurally characterized, and the role of water assessed in the latter cases. Our study has confirmed that anhydrates can be obtained even when water is present in the crystallizing solution. Protonation of the aminopyridines is consistent with ΔpK a differences. 4-Aminopyridine uniquely forms a mono cation only, with protonation at the pyridine nitrogen, whilst 3,4-diaminopyridine forms both a mono cation, again with protonation at the pyridine nitrogen, and a dication, with the second protonation at the 3-amino position. Thus, 4-aminopyridine forms a 1 : 1 nitrate anhydrate, a 1 : 1 bisulphate anhydrate, a 2 : 1 sulfate hydrate and a 1 : 1 dihydrogen phosphate hydrate. 3,4-Diaminopyridine forms a 1 : 1 nitrate anhydrate and 1 : 2 nitrate anhydrate and hydrate, 2 : 1 and 1 : 1 sulfate hydrates and a 1 : 1 dihydrogen phosphate anhydrate. Analysis of the structures found suggests that the H-bonding capability of the water O-H donors and O acceptor components have similar tendencies to N-H donors and other O acceptors. At the same time, we recognise that, whilst water molecules may occasionally be structure forming, they also act as spacers or fillers in the development of the primary H-bonded assemblies. These will mainly be controlled by the stoichiometries and H-bonding possibilities of the anion/cation components. It is also possible that, in some circumstances, the inclusion or otherwise of water in structures may be competitive with supplementary weak interactions such as C-H⋯O hydrogen bonding. pyridine with other, generally pharmaceutically acceptable acids, including the inorganic acids hydrochloric, hydrobromic, nitric, sulfuric, and phosphoric, and characterized the products, and their behaviour under humid conditions. The halide salts proved to be a complex group of compounds, particularly in the formation of hydrates, and the structures adopted. We will describe these results, still undergoing analysis, in separate papers, 2,3 but report here on the particular forms obtained with the three oxyacids. In this work, we were interested in assessing, on the one hand, the interplay between the acidities and basicities of the components, yielding, where relevant, mono-and/or di-acid or base salt forms, and on the other, the formation, or not, of hydrates.

The particle size and nickel-doping effect on pure nanocrystalline WO 3 powders are addressed through X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. A brief review of different structure types of tungsten oxides is also given. Stable and metastable crystallographic structures, resulting from oxygen deficiency, metal doping, or low-temperature synthesis, are discussed. The focus is put on the topology of the structures and notably on the structural features allowing ion intercalation. Small particle size WO 3 powders were synthesized by two different wet chemical methods to determine the impact of particle size on the crystal symmetry: in the first method, a freeze-drying technique was utilized, whereas the second technique was based on a reverse micelle method. Both methods yielded similar powders with an average size of approximately 10 nm. However, the first method yielded single-phase rhenium oxide structured particles, whereas the latter method produced a mixture of hexagonal tungsten bronze and rhenium oxide structures. In the case of single-phase rhenium oxide structure powders, the crystal symmetry was found to increase from monoclinic P2 1 /n to orthorhombic Pbcn when particle size decreased below 20 nm. The effect of nickel doping (≈1 wt %) and synthesis conditions on WO 3 powders were studied. Ni-doped WO 3 was spatially inhomogeneous: the most abundant phase was monoclinic WO 3 , whereas the minority phase was either perovskite tungsten bronze (annealing temperature below 500°C) or wolframite (annealing temperature 500°C or higher) showing that annealing conditions are a way to selectively produce different crystal structures. The wolframite and tungsten bronze structures are very different with different applications. The results are discussed in the context of thin film synthesis and sensor applications. ■ INTRODUCTIONThe crystal symmetry of tungsten trioxide (WO 3 ) is very prone to temperature, stoichiometry, and particle size. The changes are accompanied by corresponding changes in physical properties, such as electrical conductivity and color, which form a basis for many industrially important applications, including gas sensors and smart windows. For applications in mind, one practical approach is to consider how open the structure is. Structures formed at low temperatures tend to have channels and flexibility to host intercalating ions, whereas structures formed at high pressure and temperature are characteristically densely packed. The synthetization method, such as thin film deposition, can be optimized to yield the desired crystal structure for the need.Many applications require small particle size powders or thin films, which are typically achieved through low-temperature synthesis. This in turn often results in a formation of metastable phases. Besides various distorted WO 3 structures, the changes in composition or stoichiometry may result in larger structural changes, such as the formation of tungsten bronze structures or structures derived from WO 3 by crys...

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