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Trimetaphosphimates decompose not only upon heating but also in water. Hydrolysis of trimetaphosphimic acid leads to the formation of diimidotriphosphoric acid by ring opening. Single-crystal structure determination using synchrotron radiation revealed the oxonium salt of the latter acid with double stacks of rod-shaped anions along [001]. Anharmonic displacement parameters were used to model slight disorder of part of the anions. These are interconnected by hydrogen bonds within and between the stacks. Powder X-ray diffraction confirmed phase purity. At temperatures above 120 °C, diimidotriphosphoric acid decomposes into unknown crystalline compounds. It thus seems a promising precursor for the synthesis of oxonitridophosphates at moderate temperatures. Graphical abstract

The narrow band gap semiconductors Bi2Te3 and Sb2Te3 are well known for their room temperature thermoelectric performance. Recently, they were shown to serve as model systems of three-dimensional topological insulators with a bulk band gap and very robust, spin-resolved surface states due to a special band inversion in their periodic bulk. Evidently, it is of interest to investigate the special surface states with a local probe like nuclear magnetic resonance (NMR). However, especially the bulk NMR of these materials shows peculiar and rather unexpected phenomena, e. g., a magnetic field induced deformation of the local charge symmetry and excessive line broadening due to a special internuclear coupling. Here we report a comprehensive account of the room temperature NMR properties of the spin-1/2 nucleus 125Te of single crystalline Bi2Te3 and Sb2Te3. This includes a very unusual spin-lattice relaxation that seems not to be reflected in the NMR shift, as well as an exchange enhanced NMR line broadening.

Cu

Cu2Se is a high ZT material consisting of cheap and earth abundant elements. Its high thermoelectric performance is mainly based on very low thermal conductivity associated with mobile atoms. This comes with the drawback of a critical voltage, over which the material decomposes to elemental copper and a copper-deficient Cu2−xSe phase. Composites of copper selenide with graphite do not enhance the critical voltage significantly as copper selenide is not completely encapsulated by graphite. Thermoelectric properties of pellets of such composites obtained by mixing and pressing Cu2Se powder and graphite flakes are comparable to pristine Cu2Se, but thermal stability increases remarkably. Samples with different grain sizes show that the reproducibility of measurements is worse for larger grains, which in addition lead to inferior thermoelectric performance. Higher graphite contents increase both thermal and electrical conductivity, and an optimum of the thermoelectric figure of merit ZT is observed for fine-grained samples with 4 wt. % graphite.