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