The Extraordinary Mechanical Response Of Hydrogen-bonding Crystals Under High Pressure

Stimuli-responsive materials refer to the material that exert phasical or mechanical responses under external stimuli like temperature, light, force, magnetic field and so on. And materials deformed under external stimuli to produce mechanical changes or change their morphologies are called mechanical-responsive materials. The mechanical responses of materials are mainly includes crystal bending, jumping, stretching, splitting, burst etc. These mechanical responses are considered to have potential applications as biological actuators and artificial muscles. DACs as an efficient tool to generate the high or extra-high pressure have widely used in high-pressure science. In this paper, the high pressure which is generated by DACs is introduced into the study of mechanical responses of crystals. As a result, we investigate the mechanical response of crystals through the combination of DAC technology and the mechanochemistry. So far, the studies about mechanical response of crystals are mainly concentrated in thermosalient(TS) and photosalient(PS) materials which have the ability to convert optical energy or thermal energy into mechanical energy at a macroscopic level. In this paper, we discussed the several mechanical effects of three materials by the meaning of the combination of mechanochemistry and DAC technology. On the one hand, the ammonium oxalate monohydrate crystal showed a negative linear compressibility(NLC) properties and the ammonium bicarbonate crystal had an anisotropic compressibility under pressure. On the other hand, N-methylurea(NMU) powder crystals underwent a solid-state recrystallization process, while the NMU single crystal crushed under pressure.Firstly, we provide an observation of a counterintuitive mechanical response in two crystals under high pressure generated by DAC technique. High-pressure synchrotron powder XRD measurement indicated that ammonium oxalate monohydrate shows a mechanical NLC property along b-axis from 5.1 GPa to 11.5 GPa,while the b-axis is slightly compressed from 0 GPa to 5.1 GPa. The conjunct analysis of high-pressure Raman spectra and DFT calculations further confirmed the NLC property in ammonium oxalate monohydrate and indicated that the ―wine-rack‖ hydrogen bonding motifs along b-axis result in the NLC along b-axis. In ammonium bicarbonate crystal, we observed an anisotropic compressibility up to about 2 GPa using high-pressure synchrotron powder XRD measurement accompanied with DFT calculations. It is demonstrated that anisotropic compressibility in ammonium bicarbonate crystal is induced by the ―wine-rack‖ hydrogen bonds along b-axis and c-axis. Based on the analysis of anisotropic compressibility in ammonium bicarbonate crystal, we proposed that the ―dual-wine-rack‖ hydrogen bonding motifs have the potential of resulting in negative area compressibility(NAC) in crystals. At last, we proposed that organic NLC materials owning hydrogen bonding ―wine-rack‖ frameworks in their structure might have a critical pressure point. Before the critical pressure, organic NLC materials undergo a precompaction process. And it subsequently exhibit NLC properties after the critical pressure. In the ammonium bicarbonate crystal, 2 GPa is too low to exhibit a mechanical response, which only leads to the anisotropic compressibility. In a word, it is demonstrate that the ―wine-rack‖ hydrogen bonding motifs in the structure result in the NLC and anisotropic compressibility in two crystals respectively. The NLC properties studied by DAC technique in this paper is supposed to be a very important content of mechanical studies in mechanochemistry.Secondly, we presented a novel mechanical effect, a solid-state recrystallization process, in NMU powder crystals using DAC technique. In-situ photographs and high-pressure powder X-ray diffraction experiments indicate that close contact of grains and the reconstructive phase transitionat at about 0.3 GPa are two prerequisites for this mechanical recrystallization process. High-pressure Raman experiments companied by density functional theory(DFT) calculations elaborate the possible mechanism of this reconstructive phase transition in NMU. Unlike the traditional mechanical responses in thermosalient(TS) and photosalient(PS) materials, we believe that the solid-state recrystallization process in NMU powder crystals is a new type of mechanical response of crystals. At last, the high pressure transmission experiments indicated that the optical transmissivity of NMU crystals in DAC sample cell exhibits an abrupt change from ~ 15 % to ~ 90 % during the recrystallization process. During the recrystallization process, the powder sample grains are fused into a large crystal which undoubtedly causes the abrupt changes of the intrinsic physical properties like refractive index, light or vapor permeability and so on. The abrupt changes of optical transmissivity during the recrystallization make NMU crystals as an excellent molecular pressure switches with optical properties or pressure-driven actuators. Additionally, the NMU crystals also have a size-dependent effect. The powder crystals of phase I is transform to phase II at about 0.3 GPa, while the single crystal of phase I is transform to phase III at about 3 GPa accompanied with the crash of the single crystal. We believe that the crush of NMU single crystal is another mechanical response under pressure which is induced by the pressure. The studies about the mechanical responses of NMU crystal broaden the content of the research in mechanical response of the crystals and have a very important significance.