# ZIF-75 under Pressure: Negative Linear Compressibility and Pressure-Induced Instability
## 5 Conclusions  
In this study, the behavior of the zeolitic imidazolate framework ZIF-75 under the effect of pressure was studied by means of periodic density functional methods using basis sets composed of plane waves and norm-conserving pseudopotentials. Initially, the structural results showed that, while at room temperature, ZIF-75 is tetragonal, space group \(141/a\); at zero temperature, ZIF-75 is monoclinic, space group C\(2/c\). This result should be verified experimentally by the refinement of X-ray diffraction data at a low temperature. The study of the computed crystal structures under increasing pressures demonstrates that ZIF-75 undergoes a pressure-induced phase transition at relatively low pressures of \(P\approx 0.1\) GPa. Furthermore, in the range of pressure going from \(P=-0.06\) to \(0.1\) GPa, ZIF-75 exhibits the negative linear compressibility effect since the dimension of the crystal along the [0 1 0] crystallographic direction increases very rapidly as the pressure increases. This material may therefore be appropriate for many of the potential applications associated with the NLC effect. In this sense, one advantage of this material over other NLC materials [89] is its composition, since it does not contain metallic elements and its synthesis is relatively simple [2]. However, the presence of the pressure-induced phase transition limits the range of applicability of this material, not only in the applications where pressure plays a significant role but also in the most common applications in which the material operates under stressed conditions. One example is gas adsorption and storage, since the gas molecules' collisions with the walls of the channels may exert significant pressure on them [9; 30; 32; 45; 83]. The behavior of MOF materials under pressure is an extremely important topic from the point of view of their various applications. In summary, the results found in the present work are significant because they have shown, in the first place, that ZIF-75 undergoes a pressure-induced phase transition at small applied pressures near \(1.0\) GPa. The implications of this finding are twofold: (a) the new material obtained after the application of pressure, also microporous, may be a functional material, and the possible applications of this material should be experimentally investigated; (b) the presence of a pressure-induced phase transition limits the applicability of ZIF-75 in the common applications of this material, such as gas and compound absorption, separation, and storage, and catalysis, to pressures smaller than \(1\) GPa. Secondly, ZIF-75 exhibits a strong negative linear compressibility effect under applied pressures from \(P=-0.06\) to \(0.1\) GPa, with a large series of potential applications. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app122010413/s1](https://www.mdpi.com/article/10.3390/app122010413/s1)]([https://www.mdpi.com/article/10.3390/app122010413/s1](https://www.mdpi.com/article/10.3390/app122010413/s1)), Figure S1: X-ray diffraction patterns of ZIF-75 derived from the tetragonal and monoclinic structures; Table S1: Most intense reflections in the X-ray diffraction pattern of tetragonal ZIF-75; Table S2: Unit cell volume and lattice parameters of ZIF-75 under the effect of different external pressures. Conceptualization, F.C. ; investigation, F.C. and V.T. ; writing--original draft preparation, F.C. ; writing--review and editing, F.C. and V.T. All authors have read and agreed to the published version of the manuscript. The Ministry of Science, Innovation and Universities financially supported the present work through the projects PGC2018-094814-B-C21 (FC) and FIS2016-77726-C3-1-P (VT). The CTI-CSIC computer center is gratefully acknowledged for the time we were able to spend with the supercomputer which was necessary for carrying out this study. We also want to acknowledge the help of [PERSON] for reading the original manuscript and their many comments, which have allowed us to improve it significantly.