A solid-state phase transformation in octafluoronaphthalene has been discovered at 266.5K on cooling, and at 15K higher on heating. The symmetry of both phases is found to be the same, namely monoclinic with space group P21/c. The unit cell parameters change by up to 10%, but the integrity of a single crystal, which shatters on cooling, is good enough for a single-crystal structure determination. This has been done in both phases to a sufficient accuracy that a mechanism for the transformation can be proposed. Molecules which lie parallel to one another shear to a new parallel position, the shear movement being equal to one carbon-carbon bond of the naphthalene skeleton. In this process the molecules reorient, but are still related by the same symmetry operations. This transformation, although not unique, is probably the first of its kind to be discovered in molecular systems.
Pawley, G. S., & Dietrich, O. W. (1975). A Phase Transformation with no Change in Space Group Symmetry: Octafluoronaphtalene. JOURNAL OF PHYSICS C-SOLID STATE PHYSICS, 8(16), 2549-2558. https://doi.org/10.1088/0022-3719/8/16/008