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Institute
Selected allotropes of phosphorus are investigated
by different levels of density functional theory
(DFT) calculations to evaluate the relative stability orders with a special focus on the role of van der Waals interactions.Phosphorus is an excellent reference system with a large number of allotropes. Starting from low-dimensional molecular (0D, white P) and polymer structures (1D, P nanorods) to layered (2D, black P) and tubular structures (2D and 3D, crystalline forms of red P), covalent structure motifs are interconnected by van der Waals interactions. They are a key factor for the correct energetic description of all P allotropes. A comparative study is carried out within the local density approximation (LDA) and the generalized gradient approximation (GGA), with and without implementation of a dispersion correction by Grimme (GGA-D2). Our intention is to achieve a reasonable agreement of our calculations with experimental data, the plausibility of energy values, and the treatment of long-range interactions. The effect of van der Waals interactions is exemplified for the interlayer distances of black phosphorous and its electronic structure.
Phosphorus displays fascinating structural diversity and the discovery of new modifications continues to attract attention. In this work, a complete stability range of known and novel crystalline allotropes of phosphorus is described for the first time. This includes recently discovered tubular modifications and the prediction of not-yet-known crystal structures of [P12] nanorods and not-yet-isolated [P14] nanorods. Despite significant structural differences, all P allotropes consist of covalent substructures, which are held together by van der Waals interactions. Their correct reproduction by ab initio calculations is a core issue of current research. While some predictions with the established DFT functionals GGA and LDA differ significantly from experimental data in the description of the P allotropes, consistently excellent agreement with the GGA-D2 approach is used to predict the solid structures of the P nanorods.
Changes of the electrical conductance of a number of inorganic copper(I) conductive compounds including Cu₁₂Sb₄S₁₃,CU₂O, CuI, CuBr and CuCl were tested on exposure to different gases and vapors, including ammonia, nitrogen monoxide, octanethiol, hydrogen chloride, acetic acid, formic acid, ethanol, humidity and piperidine. The sensitivity of the resistivity to ammonia was estimated as 2.7%/ppm for CuBr,1.4%/ppm for Cu₂O, 1.2%/ppm for CuCl, 0.013%/ppm for CuI, but only 0.0028%/ppm for Cu₁₂Sb₄S₁₃. The highest selectivity for ammonia was observed for Cu₁₂Sb₄S₁₃.
The sensitivity patterns of these materials to the tested gases and vapors are very different which makes them applicable for sensor arrays for identification and quantification of these species.