title: 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.
authors: Department of Fundamental Chemistry, University of A Coruña , 15071, A Coruña, Spain.
authors: L C
authors: LC
authors: Gómez-Aguirre
authors: B
authors: B
authors: Pato-Doldán
authors: A
authors: A
authors: Stroppa
authors: S
authors: S
authors: Yáñez-Vilar
authors: L
authors: L
authors: Bayarjargal
authors: B
authors: B
authors: Winkler
authors: S
authors: S
authors: Castro-García
authors: J
authors: J
authors: Mira
authors: M
authors: M
authors: Sánchez-Andújar
authors: M A
authors: MA
authors: Señarís-Rodríguez