A polymorph lost and found: The high-temperature crystal structure of pentacene

A well-defined structural phase transformation is observed in bulk single crystals of pentacene, whereas pentacene powders heated above the phase-transformation temperature do not always fully convert, and upon 1 cooling, coexistence of the two polymorphs is observed down to room temperature. The first-order phase transformation is isostructural: the close-packed herringbone-type layers shift against each other, keeping the same symmetry.