RESUMEN
The structural and physical properties of the ß polymorph of iron tungstate Fe2WO6 have been investigated by synchrotron and neutron diffraction vs temperature, combined with magnetization and dielectric properties measurements. The monoclinic P21/a crystal structure of ß-Fe2WO6 has been determined and consists of an original network of zigzag chains of FeO6 and WO6 octahedra sharing trans and skew edges, connected through corners into a 3D structure. Magnetization measurements indicate an antiferromagnetic transition at TN = 264 K, which corresponds to a ↑↑↓↓ nearly collinear ordering of iron moments inside sequences of four edge-sharing FeO6 octahedra, as determined by neutron diffraction. A canting of the moments out of the ac plane is observed below 150 K, leading to a noncollinear antiferromagnetic structure, the P21/a' magnetic space group remaining unchanged. These results are discussed in comparison with the crystal and magnetic structures of γ-Fe2WO6 and with the magnetic couplings in other iron tungstates and trirutile Fe2TeO6.
RESUMEN
Conjugated polymers have been widely studied as flexible, versatile semiconductors in organic electronics. However, the material stability is one of the problems limiting their applications. Thus, understanding the degradation process of conjugated polymers is crucial. In this work, we monitored the chain scission of the model polymer MEH-PPV in chloroform solutions under different conditions by assessing its molecular weight using gel permeation chromatography and optical spectral measurements. We showed that changes in the UV-VIS spectrum can be seen only when the degradation has already progressed substantially. The fluorescence spectrum was found to be almost totally insensitive to the degradation stage of the polymers. We demonstrate that chain scission in solutions happens even in the dark leading to a 15% decrease of the molecular weight after just one day of storage. If exposed to room light, the chain length decreases by about 10 times over one day of exposure. Using stronger light intensity or enriching the solution with oxygen accelerates the degradation process dramatically. The rate of the reaction follows approximately a square root dependence with light intensity and oxygen concentration. We conclude that some extent of polymer degradation is difficult to avoid in common laboratory practices since to prevent it, one needs to work in an oxygen-free atmosphere in the dark. Preparation of polymer films from partially degraded solutions might lead not only to losing the connection between the molecular weight and the opto-electronic properties but also to unintentional doping of the semiconductor by products of chain scission reactions.
