Room-Temperature Polar Order in [NH₄][Cd(HCOO)₃] - A Hybrid Inorganic–Organic Compound with a Unique Perovskite Architecture

Abstract

We report on the hybrid inorganic-organic ammonium compound [NH4][Cd(HCOO)3], which displays a most unusual framework structure: instead of the expected 4(9)·6(6) topology, it shows an ABX3 perovskite architecture with the peculiarity and uniqueness (among all the up-to-date reported hybrid metal formates) that the Cd ions are connected only by syn-anti formate bridges, instead of anti-anti ones. This change of the coordination mode of the formate ligand is thus another variable that can provide new possibilities for tuning the properties of these versatile functional metal-organic framework materials. The room-temperature crystal structure of [NH4][Cd(HCOO)3] is noncentrosymmetric (S.G.: Pna21) and displays a polar axis. DFT calculations and symmetry mode analysis show that the rather large polarization arising from the off-center shift of the ammonium cations in the cavities (4.33 μC/cm(2)) is partially canceled by the antiparallel polarization coming from the [Cd(HCOO)3](-) framework, thus resulting in a net polarization of 1.35 μC/cm(2). As shown by second harmonic generation studies, this net polarization can be greatly increased by applying pressure (Pmax = 14 GPa), an external stimulus that, in turn, induces the appearance of new structural phases, as confirmed by Raman spectroscopy.

Quantifying Thermal Disorder in Metal–Organic Frameworks: Lattice Dynamics and Molecular Dynamics Simulations of Hybrid Formate Perovskites

“…The spontaneous polarization is calculated to be 2.6 μC cm –2 , which is somewhat less than the 3.48 μC cm –2 estimated in ref (8) based on the static position of the NH 3 + group relative to the cage alone. This suggests that the polarization arising from the molecule is partially compensated by the cage as previously found for [NH 4 ][Cd(HCO 2 ) 3 ] 13 and [CH 3 CH 2 NH 3 ][Mn(HCO 2 ) 3 ]. 40 …”

An extended Tolerance Factor approach for organic–inorganic perovskites

“…This finding also includes hybrid formates with an anti–anti formate connectivity; however, for syn–anti bridged formates, the size of the ReO 3 -like cavity is reduced and even TFs below 0.8 lead to a perovskite-like architecture. 21 …”

A Series of Bimetallic Ammonium AlNa Formates

“…For 2 hyz to 6 dma , the TF values are 0.83–0.97, thus perovskite structures are expected. Perovskite phases have also been observed for amCd , the dense or pressed amMn , and one phase of K[Co(HCOO) 3 ] if considering the similar size of K + and NH 4 + , however, shorter syn–anti formate bridges are involved in these structures. Compound 1 am belongs the trigonal space group R .…”