Ordered Phases in n-Heptacosane (C₂₇H₅₆). Structure Determination of the Mdci Phase

n -Heptacosane (C 27 H 56 ) is known to have a rich polymorphism. With increasing temperature, the following phase transitions are observed: Oi → Odci → Mdci → RIII ···> RIV → L. The Mdci form is known to be isomorphous with the modification B in n -tritriacontane (C 33 H 68 ). Two possible space groups were given in the literature for the B-form of C 33 H 68: monoclinic ( Aa ) and orthorhombic ( A 2 1 am ). We have determined the structure of the n -heptacosane at 325 K combining lattice energy minimization calculations and Rietveld refinement using accurate X-ray diffraction data. We have shown that the Mdci form of heptacosane is monoclinic ( Aa, Z = 4). Moreover, at each phase transition Oi → Odci → Mdci, a substantial decrease in intensity of the 0 0 l reflections and an increase in the interlayer spacing due to end-gauche defects are observed. In addition, each phase transition is characterized by subtle appearance and disappearance of reflections observed at large angles of diffraction. These characteristics are powerful tools for phase identification in binary or ternary phase diagrams of n -alkanes.