Stabilization of a metastable crystalline phase by twinning

POLYMORPHISM of crystal structures—alternative crystal structures resulting from variations in ionic and molecular packing1 and conformation2—is important in a number of fields in solid-state chemistry, including biomineralization3 (in the mineral phases of calcium carbonate3, for example), polymer science (polypropylene4, for example), explosives (ammonium nitrate, lead azide5), nonlinear optical materials6, ceramics and catalysis (zeolites and metal oxides7). In the pharmaceutical and fine-chemicals industries, polymorphism is central to both production process design and product activity8. Here we show that crystal twinning can stabilize a crystal polymorph that is otherwise not the most stable form. We use electron microscopy, X-ray diffraction and Raman spectroscopy to show that the apparently indefinite persistence of a metastable polymorph of terephthalic acid can be explained on this basis. If twinning can be engineered, it may therefore provide a means for stabilizing crystal phases with useful physical properties.