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

Abstract

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.

Keywords: coordination compound; martensitic phase transition; polymorphism; self-healing effects; twinning.

Figure 1

Optical micrographs of a single crystal recorded under polarized light before the experiment (room temperature; left panel), after the first 1_RT→1_LT(twinned) transition (188 K; center panel) and after the 1_LT(twinned)→1_RT transition (room temperature; right panel). Blue arrows show water droplets condensed from the air. The yellow arrows indicate the region where cracks occurred (center panel) and disappeared (right panel).

Figure 2

Optical micrographs of two crystals immersed in silicone oil and subjected to cooling‐heating cycles. Each photograph was taken after one cooling (−ΔT) or heating (+ΔT) run.

Figure 3

Quantitative optical monitoring of the phase transition. a) Thermal change in the hue count frequency based on stills extracted from Video S2 showing the transition 1_RT →1_LT(twinned) in the monoclinic phase 1_LT(twinned) at 188 K, and the orthorhombic phase 1_RT at 233 K. b) Fractal dimensions from the optical image data from Video S2 based on the white and black‐white D(WBW) fractal dimension (threshold, m=160), obtained by the box‐counting method.

Figure 4

Self‐healing observed by scanning electron microscopy on gold‐coated single crystal of 1. A) The crystal before cooling by liquid nitrogen; b) at 88 K; c) at 213 K; d) at 262 K. The arrows show the region where cracks occurred and disappeared.

Figure 5

Diffraction maxima in the Ewald sphere of 1_RT (at 230 K), 1_LT(twinned) (at 200 K, after the transition 1_RT→1_LT(twinned) ), and the mixture of 1_RT and two components of 1_LT(twinned) (at 230 K, during the transition 1_LT(twinned) →1_RT ). Diffraction maxima and unit cells of the two components in 1_LT(twinned) in the reciprocal space, in the images shown in the middle, are shown in blue and red, while the overlapped diffraction maxima are shown in black. In the images on the right, the diffraction maxima and unit cells of the two components in 1_LT(twinned) in the reciprocal space are shown in blue and red, 1_RT in yellow, and the overlapped diffraction maxima are in black.

Figure 6

Change in the unit cell parameters with temperature for the single crystal of 1. The parameters were obtained by cooling a crystal from 260 to 170 k. the standard deviations of the unit cell parameters are smaller than the markers on graphs (the values are available from Tables S1–S3).

Figure 7

Angles between equatorial planes of the complex cations in 1_RT and 1_LT(twinned) , and the crystal packing in two crystallographic orientations.

Figure 8

a) A schematic representation of two movements of equal probability of the complex cations in the [001] or [00$\bar{1}$ ] direction and the rotation of water molecules. b) Suggested mechanism of the formation of microfractures (orange) between grain/domain boundaries due to the formation of twinned crystals (blue and red) rotated around the b*‐axis.

Figure 9

Thermal effects accompanying the phase transition. a) DSC curve of a polycrystalline sample of 1 recorded between 200 and 230 K. The blue and red lines correspond to cooling and heating cycle, respectively. b) DSC curve of a single crystal sample that has been cycled by cooling and heating (the numbers 1–5 correspond to the ordinal numbers of the heating or cooling runs). The heating/cooling rate was 2 K min^–1. Full experimental data for the single‐crystal DSC is provided in Figure S10.