Credit: © 2010 ACS

The thermosalient phenomenon is also known as the jumping crystal effect, an apt name to describe crystals that undergo phase transitions that cause them to literally jump. The movement is caused by anisotropic changes in the crystals' shape and in some cases can be several centimetres. Although most examples known so far have been discovered serendipitously — usually on hot-stage microscopes — crystals with this interesting property could find use as actuators. The stress put on the crystal to make it jump, however, also causes extreme difficulty in making a full structural analysis of the subsequent phase, and thus the thermosalient effect remains poorly understood.

Now, Panče Naumov, Joel Berstein and colleagues from Osaka University and Ben-Gurion University in Israel have fully studied1 the phenomenon in crystals of oxitropium bromide (OXTB), a compound originally developed to treat bronchial diseases. On heating to 328 K, crystals of OXTB were seen to jump. Heating them rapidly through this temperature resulted in disaggregation, but slower heating retained the crystallinity. This allowed the structures of both phases to be fully determined using single-crystal X-ray diffraction.

Although the team found that the changes in unit cell and atomic positions did not seem to be strikingly large, they did discover a conformational difference between the oxitropium molecules in the two phases. Although the rigid tricylcic part of the molecule changes little, the flexible ester linkage allows different orientations of two other terminal groups. The thermal strain built up on heating transfers through this ester bridge and triggers the phase transition.