Thermally Induced Reversible Martensitic Phase Transition and Self-Healing in Nickel Glycinamide Crystals

Martensitic phase transitions and self-healing in molecular compounds are extremely rare, and while they carry potential for rapid energy transduction, they are currently found only by serendipity. Here, we report the observation of both phenomena in a coordination compound of glycinamide (Glyam), [Ni(H₂O)₂(Glyam)₂]I₂ (1). Upon cooling to 205-213 K, the high-temperature polymorph of this material (1_RT) transforms into twinned crystals of the low-temperature polymorph (1_LT(twinned)), while upon heating, it is converted back to 1_RT in the temperature range 217-223 K. When this transformation occurs in single crystals, evolution of cracks is observed upon cooling that disappear upon heating. The forward transition 1_RT→1_LT(twinned) is completed in a few seconds, while the reverse one, 1_LT(twinned)→1_RT, proceeds up to several hours, contrary to the fast transition in polycrystalline samples. We determined the concomitant presence of 1_RT and 1_{LT(twinned )} in the same single crystal. The crystal structure analysis revealed complex inter- and intramolecular displacements of atoms during the transition. To the best of our knowledge, this is only the second report where a thermally induced reversible martensitic phase transition and self-healing are observed in crystals of a coordination compound, and indicates the potential generality of these extraordinary phenomena.