High-pressure Studies On Hydrogen-bonded Organic Crystals:Thiourea Dioxide And D,L- Mandelic Acid

In recent years, hydrogen-bonded organic molecule crystals have attract more and more attention of the scientists. The inner of organic molecule crystals involve the combination of strong intramolecular covalent bonds with weak intermolecular forces such as hydrogen bonding, van der waals forces and electrostatic interactions. Pressure is considered to be an important method of investigating hydrogen-bonded organic molecular crystals.The main work of this paper is using the diamond anvil cell(DAC) combined with in situ Raman spectroscopy, in situ angle-dispersive X-ray difraction(ADXRD) and first-principle calculations to investigate hydrogen-bonded organic crystals. Moreover, we mainly discussed the coordination between hydrogen bonding and van der waals force. We draw the conclusions as follow.The obvious changes in both the external vibration peaks and internal vibration peaks at 3.7 GPa indicated that there was a phase transition in thiourea dioxide crystal(initial phase to high pressure phase). When pressure released to 2.9 GPa, the new high pressure phase came back to the initial phase. We can say that the phase transition was reversible and there was a small hysteresis of the transition in thiourea dioxide crystal. In order to further provide the existence of phase transitions, we conducted in situ angle-dispersive X-ray difraction of thiourea dioxide. XRD diffraction pattern changed obviously at 3.8 GPa, which agreed well with the dramatic changes in Raman spectra at 3.7 GPa. Similarly, XRD diffraction pattern came back to the initial ones. The above results once again showed that there was a reversible phase transition in thiourea dioxide crysatl. Due to the poor quality of the diffraction pattern, we can not get the structure information of new high pressure phase. However, we can perform Pawley refinement. The new high pressure phase still belongs to orthorhombic with a space group Pbam. In addition, we performed the first-principle calculations and established the corresponding theoretical model. Then, we used the calculated model to study the effect of pressure on thiourea dioxide crystals so as to better understand the process of phase transition. Based on the model, the much more weaker interlayer hydrogen-bonded networks occurred to have great distortions with increasing pressure. We believe that the phase transition is mainly due to the great distortions of much more weak interlayer hydrogen-bonded networks.Moreover, we performed in situ Raman spectroscopy study of D,L- mandelic acid organic hydrogen-bonded crystal. With pressure increasing to 0.6 GPa, there were some dramatic changes in the Raman spectra. The Raman peaks at 96 cm-1 and 122 cm-1 disappeared. There were three new Raman peaks at 63 cm-1, 114 cm-1 and 126 cm-1. For the internal vibration region, there were ten new Raman peaks at 244 cm-1, 302 cm-1, 324 cm-1, 375 cm-1, 390 cm-1, 487 cm-1, 705 cm-1, 724 cm-1, 748 cm-1 and 760 cm-1 separately. Adding that, the C=O stretching vibration Raman peak(1718 cm-1) split and there was a new Raman peak at 2968 cm-1. The above changes of Raman spectra indicated that there was a pressure-induced phase transition in D,Lmandelic at 0.6 GPa from Pbca to P21/c. Upon total release of pressure, the Raman spectra returned to its initial state, implying this transition was reversible. By comparing the crystal structure at ambient and high pressure, we think that the phase transition is mainly due to the changes of molecule arrangements and the great distortions of O-H···O hydrogen-bonded networks.In summary, hydrogen-bonded organic crystals thiourea dioxide and D,Lmandelic both have reversible phase transtion under high pressure. The phase transition in thiourea dioxide crystal is mainly due to the great distortions of much more weak interlayer hydrogen-bonded networks. However, the phase transition is mainly due to the changes of molecule arrangements and the great distortions of O-H···O hydrogen-bonded networks. There is no doubt that hydrogen bonding play an important role in the determining the material structure and the property. Thus, high pressure studies on hydrogen-bonded organic crystals can help us understand the essence of hydrogen bonding and the coroperation between hydrogen bonding and van der waals interactions under high pressure. In addition, it has profound significance for physics, chemistry and material.