| Two-dimensional layered materials have become a hot topic in the field of materials research due to their unique structure and rich physical properties,and the connection between their structure and physical properties have attracted extensive attention in the field of high-pressure research.High pressure technology has become an effective means to regulate the crystal structure,band structure and various properties of two-dimensional layered materials.Pressure,as a thermodynamic parameter independent of temperature and chemical composition,can effectively shorten the interatomic distance of materials,increase the overlap of electron orbitals of neighboring atoms,regulate the electron spin and thus change the crystal structure,electronic structure and interatomic interactions of materials,even forming a new phase under high pressure.Thus,high-pressure research provides a new way to understand the changes in the structure and properties of two-dimensional layered materials.The interlayer interactions of 2D layered materials are coupled with intralayer interactions,and the differences in the interlayer interactions can have important effects on their crystal structure and band structure.So far,high-pressure studies have mainly focused on two-dimensional layered materials bound by van der Waals forces between layers,and the properties under high pressure of two-dimensional layered materials bound by non-van der Waals interactions have not been fully understood.Therefore,high pressure research on non-van der Waals type 2D layered materials will deepen and enrich the understanding of the structure and physical properties of 2D layered materials.In this context,several typical non-van der Waals 2D layered materials were investigated under high pressure.Firstly,we chose Cu OHX（X=F,Cl）with hydrogen-bonded interactions as a representative of hydrogen-bonded 2D materials and studied their behaviors under high pressure.At the same time,we compared them with other conventional van der Waals 2D materials,later we have studied the compressive behaviors of Bi2O2Se with electrostatic interactions under high pressure as an example of electrostatic 2D layered high pressure studies.The above systems have been investigated in this paper and the following results have been obtained.1.The behavior of Cu OHF under high pressure was investigated by high pressure XRD and Raman measurements combined with the first-principles calculation.It is found that the O-H stretching vibration frequency showed a red shift followed by a blue shift with increasing pressure before and after 7.8 GPa respectively.At the same time,it is observed from the XRD pattern that when the pressure is above 7.8 GPa,the spacing of（001）diffraction peak decreases rapidly.The structural evolution process of Cu OHF under high pressure have been successfully revealed based on the above experimental phenomena and first principles calculation.When the pressure is less than7.8 GPa,the interlayer hydrogen bonding is enhanced by the pressure-induced reduction of the interlayer spacing,and when the pressure is higher than 7.8 GPa,the interlayer hydrogen bonding weakened by the pressure-induced interlayer slip,accompanied by an isostructural phase transition.The（001）diffraction peak disappears at pressure above 22 GPa,indicating a two-dimensional to three-dimensional structural transition.Furthermore,it has been shown that the interlayer compression behavior of Cu OHF under high pressure is significantly different from that of conventional van der Waals two-dimensional materials.While the usual interlayer compression rate of van der Waals-type 2D laminates exhibits a fast followed by a slow interlayer compression rate,Cu OHF interlayer compression rate shows a slow followed by a fast one.In addition,we have successfully explained the underlying mechanism of the strange compression behavior of Cu OHF by first principles calculations.At lower pressure,the hydrogen bonds are almost perpendicular to the a-b plane,and the interlayer hydrogen bonds increase with increasing pressure,thus providing a strong enough support for the a-b plane.At higher pressure,the pressure-induced interlayer slip leads to not only a weakening of the hydrogen bonds,but a deviation of the hydrogen bonding direction from the vertical direction,and a weakening of the interlayer interactions leading to a rapid compression of the interlayer spacing.The structural evolution and phase transition mechanism of Cu OHF revealed in this study will provide a good reference for future studies on other MOHX.2.The behavior of Cu OHCl under high pressure was investigated by high pressure XRD,Raman and UV-Vis absorption spectroscopy combined with the first principle calculation.The results show that isostructural phase transitions occurred in the pressure range of 14-16 GPa due to the changes in hydrogen bonding configuration.The phase transition occurred from monoclinic（AAAA）to orthorhombic（ABAB）structure was attributed to pressure-induced changes in the stacking order between layers at 18.7-21.4 GPa,and in addition the monoclinic and orthorhombic phases can coexist over a wide range of pressure.It was also found that two electronic structural phase transitions occurred in the monoclinic phase of Cu OHCl.Most interestingly,both the XRD pattern and the Raman scattering spectrum show that the orthorhombic phase can be retained after the pressure removed.A comparison between Cu OHCl and Cu OHF also shows that both of them undergo isostructural phase transitions due to changes in hydrogen bonding configuration,but both of them show significantly different compression behavior at higher pressure,which may be due to the different strength of hydrogen bonding between them.The comparison of the compression behavior of Cu OHCl and conventional van der Waals materials under high pressure revealed that pressure can also induce a change in the stacking order between layers of Cu OHCl,the phenomenon has also been observed in van der Waals 2D materials.It was noted that the variation of the Cu OHCl layer spacing（a-axis）as a funcyion of pressure has a similar variation pattern to that of the van der Waals 2D material.However,the compression behavior for the non-layer direction（c-axis）shows a distinct difference from that of the van der Waals-type 2D material,which may be related to the fact that hydrogen bonding is directional in nature.When the direction of hydrogen bonding deviates from the perpendicular to the layer direction,the hydrogen bonding affects not only the compression behavior in the layer direction but also the non-layer direction.These results show that Cu（OH）Cl,as well as other hydrogen-bonded 2D layered materials,can provide a convenient platform for studying the effects of the crystal stacking order.3 The crystal structure and electrical transport properties of Bi2O2Se under high pressure were investigated systematically using in situ high pressure Raman spectroscopy,synchrotron X-ray diffraction,low temperature electrical transport measurements combined with First-principles calculation.Both experimental and theoretical results show that no phase transition occurred in Bi2O2Se up to 30 GPa,which shows good stability under compression.Pressure-induced superconductivity was observed in Bi2O2Se by electrical transport measurements,with a transition temperature of 3.6 K at 27.2 GPa.The combination of experiments and theoretical calculations reveals that the appearance of pressure-induced superconductivity in Bi2O2Se is related to the appearance of the flat-band structure.We suggest that the flat band in Bi2O2Se originates from the lone pair of electrons.By comparing with the van der Waals 2D layer material we found that although the electrostatic interaction is much stronger than the van der Waals interaction,Bi2O2Se exhibits similar interlayer compression behavior to the van der Waals layer materials under high pressure,which may be attributed to the fact that the two interaction forces have the same characteristics.This study not only enriches the understanding of the structure and properties of non-van der Waals two-dimensional layered materials under high pressure,but also provides a new way to achieve flat-band superconductivity. |
