Hexafluoropropene: a model system for muons in organic fluorides

In fluorine containing compounds, it is well known that the muon is usually preferentially drawn towards the electronegative fluorine atoms. By implanting close to fluorine the muon spin may interact strongly with one or two fluorine spins. In the case of ionic fluorides (such as CaF2, NaF, etc),
the muon and two fluorine ions form a strong, linear ‘hydrogen bond’ (the so-called F–μ–F state), with a usual F–μ bond length of around 1.2 Angstroms. Our recent work on the structrually complicated compound PERFECTA revealed a very unique F--mu--F state, which was only visible at around 4 K, and we have so far been unable to analyse this. Here, we propose an experiment on the model organic fluoride hexafluoropropene, which is an organic fluoride which crystallises below about 115 K into a triclinic unit cell with minimal strucural disorder, but has many possible muon sites which we will be able to distinguish between. This will not only aid our analysis of PERFECTA, but will also make the analysis of organic fluorides with µSR far more quantitative and effective.

在含氟化合物中，学界已公认μ子（muon）通常会优先被电负性较强的氟原子所吸引。当μ子被植入至靠近氟原子的位置时，其自旋可与一个或两个氟原子的自旋发生强相互作用。对于离子型氟化物（如氟化钙CaF₂、氟化钠NaF等），μ子与两个氟离子会形成强线性的‘氢键’（即所谓的F–μ–F态），其F–μ键长通常约为1.2埃（Angstrom）。我们此前针对结构复杂的化合物PERFECTA开展的研究，发现了一种极为独特的F–μ–F态，该态仅在约4开尔文（K）的温度下可被观测到，且截至目前我们尚未能完成对此态的分析。在此背景下，我们提议针对模型有机氟化物六氟丙烯（hexafluoropropene）开展一项实验：该有机氟化物在约115开尔文以下会结晶为三斜晶胞，且结构无序度极低，但存在多个可能的μ子占位位点，我们可对这些位点加以区分。此项实验不仅将助力我们对PERFECTA的分析工作，还能让基于µSR的有机氟化物分析工作更具定量性与实效性。