From ambient- to high-pressure dielectric response of perovskite formamidinium manganese formate

Abstract

In this work, we compare the dielectric properties of single-crystal and pelletized perovskite-like formamidinium-templated inorganic–organic hybrid [NH₂CHNH₂][Mn(HCOO)₃] (FAMn) at ambient pressure. Furthermore, we report pressure-dependent dielectric measurements of pelletized FAMn that allow us to determine the equation of state, as well as changes in the dielectric properties and the rotational motion of formamidinium (FA) cations in the crystal lattice under mechanical stress. Our experiments show that the mechanism of FA cation motion is complex, encompassing six different reorientation paths around three two-fold symmetry axes. Multiple reorientation possibilities of FA cations inside the crystal lattice lead to their irregular molecular dynamics. High-pressure experiments allowed observing that formate anions together with manganese cations form a rigid skeleton, which is stable over a wide temperature and pressure range. Consequently, the hydrostatic pressure exerts a surprisingly small influence on the FA dynamics compared to the related compounds containing the same type of molecular cations. Finally, we reveal a different role of interactions between the A-site organic and the B-site metal cations with the X-site linkers in controlling the dynamics of the A-site moieties under various pressure–temperature conditions. Our results are supported by the molecular dynamics simulations.