Anisotropic 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 conditions 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 majori-ty of the expansion occurring along the c axis (~ 2% over the temperature range of interest). Rietveld refinements reveal little changes 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.