Reversible facile single-crystal-to-single-crystal polymorphic transition accompanied by unit cell volume expansion and twinning

Reversible single-crystal-to-single-crystal phase transformation between two enantiotropic polymorphs of a pharmaceutical salt, diphenhydramine citrate, in both directions is accompanied by a transient expansion in the unit cell, allowing rotation of phenyl rings. Twinning of the lower symmetry polymorph facilitates the exceptional reproducibility and the lack of observable hysteresis in the phase change.