Abstract
The high-pressure behaviour of ammonium metal formates has been investigated using high-pressure single-crystal X-ray diffraction on ammonium iron and nickel formates, and neutron powder diffraction on ammonium zinc formate in the pressure range of 0–2.3 GPa. A structural phase transition in the pressure range of 0.4–1.4 GPa, depending on the metal cation, is observed for all three ammonium metal formates. The hexagonal-to-monoclinic high-pressure transition gives rise to characteristic sixfold twinning based on the single-crystal diffraction data. Structure solution of the single-crystal data and refinement of the neutron powder diffraction characterise the pressure-induced distortions of the metal formate frameworks. The pressure dependence of the principal axes shows significantly larger anisotropic compressibilities in the high-pressure monoclinic phase (K1 = 48 TPa−1, K3 = −7 TPa−1) compared to the ambient hexagonal phase (K1 = 16 TPa−1, K3 = −2 TPa−1), and can be related to the symmetry-breaking distortions that cause deformation of the honeycomb motifs in the metal formate framework. While high-pressure Raman spectroscopy suggests that the ammonium cations remain dynamically disordered upon the phase transition, the pressure-induced distortions in the metal formate framework cause polar displacements in the ammonium cations. The magnitude of polarisation in the high-pressure phase of ammonium zinc formate was calculated based upon the offset of the ammonium cation relative to the anionic zinc formate framework, showing an enhanced polarisation of Ps ∼ 4 μC cm−2 at the transition, which then decreases with increasing pressure.
Highlights
191 K and 255 K upon cooling, which originates from a change in dynamics of the ammonium cation.[1,2,14,15,16] This order–disorder transition of NH4+ causes a structural phase transition in [NH4][M(HCOO)3] compounds, due to the changes in hydrogen bonding between N–H and oxygen of the formate linker.[1,2]In contrast, ammonium nickel formate does not exhibit polarity at low temperatures; instead, the ammonium cations become statically disordered over two sites and the structure remains in its non-polar space-group symmetry.[6]
Ammonium nickel formate does not exhibit polarity at low temperatures; instead, the ammonium cations become statically disordered over two sites and the structure remains in its non-polar space-group symmetry.[6]
The [MIJHCOO)3]− network is charge-balanced by ammonium cations that are located within the c-axis channels and interact with the host framework via weak hydrogen bonding.[1,2]
Summary
191 K and 255 K upon cooling, which originates from a change in dynamics of the ammonium cation.[1,2,14,15,16] This order–disorder transition of NH4+ causes a structural phase transition in [NH4][M(HCOO)3] compounds, due to the changes in hydrogen bonding between N–H and oxygen of the formate linker.[1,2]. A-site cation and the oxygen of the formate linker has been shown to modify the magnetic superexchange pathway in [(CH3)2NH2][Fe(HCOO)3] and may be important for exhibiting magnetoelectric coupling.[9,23]. Tuning of these physical properties in [A]ijMIJHCOO)3] compounds has been achieved by variation of the chemical compositions of the A- and M-site cations, as well as doping upon these sites, and through application of mechanical strain. In order to further investigate the highpressure behaviour of such [A][M(HCOO)3] compounds and determine the mechanism and evolution of possible pressure-induced polarisation, we measured both highpressure neutron powder diffraction on [ND4][Zn(DCOO)3] and high-pressure single-crystal X-ray diffraction on [NH4][Ni(HCOO)3] and [NH4][Fe(HCOO)3] in the pressure range 0–2.3 GPa
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