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

Hybrid organic-inorganic perovskites [NHNH][X(HCOO)] (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 octahedra and -HCOO ligands remain relatively rigid while XO octahedra tilt significantly, which leads to highly anisotropic mechanical properties and expansion along certain directions. These trends are common for the three materials considered.