A crystal is normally thought of as a homogenous solid formed by a periodically repeating, three-dimensional pattern of atoms, ions, or molecules. Indeed, the regular arrangement of molecules, in a single crystal lead to many useful characteristics (in addition to diffraction!) including unique optical and electrical properties, however, molecular crystals are not typically mechanically robust. Upon the application of stress or strain, these crystals generally irreversibly deform, crack or break resulting in the loss of single crystallinity.1
We have recently discovered a class of crystalline compounds that display the intriguing property of elastic flexibility – that is they are capable of reversibly bending without deforming, cracking or losing crystallinity. A number of these crystals are flexible enough to be tied into a knot!2-3 We have developed a unique approach to determine the atomic-scale mechanism that allows the bending to occur which employs mapping changes in crystal structure using micro-focused synchrotron radiation.4 We have applied this technique to understand the deformation in both elastically and plastically flexible crystals. We have also used it to show that previous theories regarding the requirement of “interlocked” crystal packing for flexibility is incorrect.5
Most recently we have used the unique information inherent in the mechanism of flexibility that we have determined in order to determine where potential energy is stored within these crystals where they are bent and therefore located the atomic-scale origin of the restoring force which allows flexible crystals to return to their undeformed state when the force holding them is removed.6