Negative Linear Compressibility in [NH3NH2]Co(HCOO)3 and Its Structural Origin Revealed from First Principles

First-principles density functional theory computations are used to predict negative linear compressibility in hybrid organic-inorganic perovskite [NH2NH3][ Co(HCOO)(3)]. Negative linear compressibility is a rare exotic response of a material to pressure associated with expansion along one or two lateral directions. Detailed structural analysis revealed that [NH2NH3][Co(HCOO)(3)] responds to pressure through tilting of its relatively rigid units, CoO6 polyhedra, and (HCOO)(-1) ligand chain. The (HCOO)(-1) units form a wine-rack geometry which is well described with the strut-hinge model. Within the model, the struts are formed by the rigid units, while hinges are their relatively flexible interconnects. Under pressure, the hinge angle increases which leads to the expansion along the direction subtended by the angle. Interestingly, at zero pressure the linear compressibilities in [NH2NH3][Co(HCOO)(3)] are all positive. As pressure increases, the lowest linear compressibility value turns negative and increases in magnitude. Comparison with the literature suggests that such a trend is likely to be common to this family of materials. Mechanical properties of [NH2NH3][Co(HCOO)(3)] are highly anisotropic.

Automated summary

The study predicts negative linear compressibility in [NH2NH3][Co(HCOO)(3)] using first-principles density functional theory calculations, revealing the exotic response of the material to pressure and showing a common trend in this family of materials. The structural analysis and the strut-hinge model describe how the material responds to pressure through the tilting of rigid units and flexible interconnects, leading to the expansion along specific directions. Additionally, the anisotropic mechanical properties of [NH2NH3][Co(HCOO)(3)] are highlighted.