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

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posted on 02.03.2015, 00:00 by L. C. Gómez-Aguirre, B. Pato-Doldán, A. Stroppa, S. Yáñez-Vilar, L. Bayarjargal, B. Winkler, S. Castro-García, J. Mira, M. Sánchez-Andújar, M. A. Señarís-Rodríguez
We report on the hybrid inorganic–organic ammonium compound [NH4]­[Cd­(HCOO)3], which displays a most unusual framework structure: instead of the expected 49·66 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 synanti formate bridges, instead of antianti 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/cm2) is partially canceled by the antiparallel polarization coming from the [Cd­(HCOO)3] framework, thus resulting in a net polarization of 1.35 μC/cm2. 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.