The Abnormal Compressive Behavior Of Metal-Organic Framework Materials Under High Pressure

The abnormal compressive behavior including negative linear compressibility（NLC）,negative area compressibility（NAC）,zero linear compressibility（ZLC）and zero area compressibility（ZAC）is a class of rare and highly special compression of crystal unit cells.NLC/NAC means that there is/are one/two specific direction（s）in the unit cell expanding coupled to the reducing of unit cell volume when compressed uniformly;while ZLC/ZAC describes a rarer property that one/two specific direction（s）in the unit cell neither expand（s）nor shrink（s）under hydrostatic pressure.The abnormal mechanical response gives the abnormal compressive materials extensive applicational prospect,such as optical devices used in deep-sea environments,anti-seismic materials,ultra-sensitive pressure sensors,artificial muscles and next-generation body armor.Additionally,NLC materials can be compounded with other materials to prepare composite materials with ZLC or ZAC.However,due to the specificity of abnormal compressive behavior,the number of abnormal-compression materials is extremely rare,especially,the ZAC materials have never been found.In addition,the known NLC value of NLC material is very small（or the large NLC value can only exist in a very narrow pressure range）,which greatly weakens the applicational value of NLC materials.Therefore,seeking more abnormal compression materials or materials with large negative or even zero compressibility values become the most important in this research field.We discovered one of the most extreme NLC materials,InH（BDC）₂,that possessing the largest value of NLC.We perpormed high pressure ADXRD and Raman experiments on InH（BDC）₂ by using DAC.The results showed an extreme NLC along the c-axis in the pressure range of 0–0.53 GPa.The NLC value,-62.4 TPa^-1,is one of the largest among the known NLC materials.Compared to other traditional large NLC materials（Ag₃[Co（CN）₆]-I and Zn[Au（CN）₂]₂-I）without the highly toxic CN^-and noble metal Au⁺/Ag⁺composites,InH（BDC）₂ is much safer and more economical and therefore is more practically valuable.Further analysis of the experimental results of ADXRD and Raman showed that it was the closing effect ofβ-quatz-like framework hinging that resulted in the NLC of the c-axis,which is further confirmed by the DFT calcultions.It is worth pointing out that the closing effect of the framework was also the reason for the negative thermal expansion（NTE）of the material in the c-axis direction.This discovery reveals that there is a very close internal relationship between the NLC and the NTE in framework materials.NTE is more readily measured than NLC,presenting a useful method for screening likely NLC candidates,which will greatly reduce the energy and time for seeking NLC materials.More importantly,we believe that there will be materials with large NLC in the systems ofβ-quartz or similar framework structures such as"wine-rack"structure and"scissors"structure.To enlarge the NLC value,a more flexible framework and more rigid organic chains are expected,which has important guiding significance for the design of materials with large NLC in the future.We put forward a new mechanism for NLC.We carried out the high pressure ADXRD experiments by using DAC on Co（SCN）2（pyrazine）2.The results showed that the NLC along the X₃=0.84a-0.55c direction occurred in the pressure range from 0 to1.00 GPa.Further analysis of the crystal structures at high pressures indicated that the mechanism of NLC of X₃ is the molecular layer sliding parallelled to the ab-plane.The high pressure theoretical simulation of Co（SCN）₂（pyrazine）₂ crystals by first-principles calculation showed that the compression process of Co（SCN）2（pyrazine）2 was as follows:when the external pressure increased,the interlayer spacing d（001）decreased sharply due to the absence of interlayer strong chemical bonds,and then the repulsion force between the negative charged NCS···SCN pairs distributed in the interlayer increased.The repulsion force was then transferred to the plane framework of Co（pyrazine）₂²⁺through N2-Co coordination bonds and drived the plane Co（pyrazine）₂²⁺framework to slide along the a-axis direction.Based on the large PLC of the X₁ axis,the NLC occurred eventually in the X₃ axis.However,besides the positive promotion of molecular layer slippage,the NLC of X₃ axis was simultaneously inhibited by the positive compression of the a-axis.Therefore,the NLC effect was relatively small,only 1.99 TPa^-1 in 0–1.00 GPa.When the pressure is higher than 1.00 GPa,the compression of X₃ axis caused by positive compression of a-axis exceeded the extension of X₃ axis caused by molecular layer slippage,so the X₃ axis was no longer elongated.The NLC was hence turned to PLC.Molecular layer sliding is a new mechanism of NLC.The discovery of this mechanism has important inspiring and guiding significance for seeking,designing and synthesising of new NLC materials in layered monoclinic crystal materials or other related systems.We discovered the first material that exhibit near zero area compressibility.we studied the structural evolution of[C（NH₂）₃][Cd（HCOO）₃]crystals under high pressure.The ADXRD experiments results indicated the near ZAC of the ab-plane in the pressure range of 0–0.53 GPa.Further analysis on the high pressure structures indicated that the near ZAC of the ab-plane and the PLC along the c-axis direction can be ascribed to the triangular pyramid structure in the top of the unique rhombohedral frameworks.The first-principle calculations have the similar results with the ADXRD experiments:upto 0.5GPa,the computational a-axis and c-axis were 99.99%and 97.38%of the original scale comprared to 99.97%and 97.38%for the ADXRD results,respectively.Further analysis of the theoretical simulation structures showed that the main factors for the reduction of the length of the rhombohedral framework strut r were the contaction of the Cd-O bond and the reduction of Cd-O-C angle,while the increase of the intraframeworkθwas mainly caused by the increase of O-Cd-O angle.[C（NH₂）₃][Cd（HCOO）₃]is the first reported near ZAC single material.The ZAC single materials are readily prepared and free of limitations of the physical and chemical properties of each component compared to the ZAC composites.This work opens a new research field:the study of ZAC materials under high pressure,which is of great significance to seek more ZLC and ZAC materials and their applications in the future.