Thermal Expansion of the Acetylene-Ammonia Co-crystal under Titan's Conditions

Acetylene and ammonia are known to form a stable orthorhombic co-crystal under the surface condi-tions of Saturn's moon Titan (1.5 bar, 94 K). Such material represents a potential new class of organic minerals that can play an important role in Titan's geology. In this work, the thermal expansion of this co-crystalline system has been derived from in situ powder X-ray diffraction data obtained between 85 and 120 K. The results indicate significant anisotropy, with the majority of the expansion occurring along the c axis (~ 2% over the temperature range of interest). Rietveld refinements reveal little chang-es to the structure as compared to that previously reported by Boese et al. (2009). The expansion is consistent with the alignment of C-H…N interactions along the chains in the a and b axes, and the weak intermolecular bonding in the structural layers along the c axis

已知乙炔与氨在土星卫星土卫六（Titan）的表面条件（1.5巴，94 K）下可形成稳定的正交晶系（orthorhombic）共晶。该物质代表一类潜在的新型有机矿物，可在土卫六的地质作用中发挥重要作用。本研究通过85至120 K区间采集的原位粉末X射线衍射（in situ powder X-ray diffraction）数据，推导得到该共晶体系的热膨胀特性。结果显示该体系存在显著的各向异性，绝大部分热膨胀沿c轴方向发生（在目标温度区间内膨胀量约为2%）。里特维尔德精修（Rietveld refinements）结果表明，与Boese等人2009年报道的晶体结构相比，本次测得的结构几乎无变化。该热膨胀特性与a、b轴方向上沿链状结构排列的C-H…N相互作用，以及沿c轴方向结构层内较弱的分子间键合作用相符。