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

Titelangaben

Collings, Ines E. ; Bykov, Maxim ; Bykova, Elena ; Tucker, Matthew G. ; Petitgirard, Sylvain ; Hanfland, Michael ; Glazyrin, Konstantin ; van Smaalen, Sander ; Goodwin, Andrew L. ; Dubrovinsky, Leonid ; Dubrovinskaia, Natalia:
Structural distortions in the high-pressure polar phases of ammonium metal formates.
In: CrystEngComm. Bd. 18 (2016) Heft 46 . - S. 8849-8857.
ISSN 1466-8033
DOI: https://doi.org/10.1039/C6CE01891B

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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 similar 4 small mu C cm-2 at the transition{,} which then decreases with increasing pressure.