Negative Linear Compressibility in Organic-Inorganic Hybrid Perovskite [NH 2 NH 3 ]X(HCOO) 3 (X = Mn, Fe, Co).

Hybrid organic-inorganic perovskites [NH 2 NH 3 ][X(HCOO) 3 ] (X = Mn, Fe, Co) have a so-called "wine-rack" type of geometry that could give origin to the rare property of negative linear compressibility, which is an exotic and highly desirable material response. We use first-principles density functional theory computations to probe the response of these materials to hydrostatic pressure and predict that, indeed, all three of them exhibit negative linear compressibility above a critical pressure of 1 GPa. Calculations reveal that, under pressure, XO 6 octahedra and -HCOO ligands remain relatively rigid while XO 6 octahedra tilt significantly, which leads to highly anisotropic mechanical properties and expansion along certain directions. These trends are common for the three materials considered.