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Issue 46, 2016
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Structural distortions in the high-pressure polar phases of ammonium metal formates

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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.

Graphical abstract: Structural distortions in the high-pressure polar phases of ammonium metal formates

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Publication details

The article was received on 30 Aug 2016, accepted on 19 Oct 2016 and first published on 19 Oct 2016


Article type: Paper
DOI: 10.1039/C6CE01891B
Citation: CrystEngComm, 2016,18, 8849-8857
  • Open access: Creative Commons BY license
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    Structural distortions in the high-pressure polar phases of ammonium metal formates

    I. E. Collings, M. Bykov, E. Bykova, M. G. Tucker, S. Petitgirard, M. Hanfland, K. Glazyrin, S. van Smaalen, A. L. Goodwin, L. Dubrovinsky and N. Dubrovinskaia, CrystEngComm, 2016, 18, 8849
    DOI: 10.1039/C6CE01891B

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