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.
Original language | English |
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Pages (from-to) | 8849-8857 |
Number of pages | 9 |
Journal | CrystEngComm |
Volume | 18 |
Issue number | 46 |
DOIs | |
State | Published - 2016 |
Funding
We thank the ESRF, ISIS neutron source, and DESY for beamtimes. I. E. C. thanks Somnath Dey for useful discussions, and Dr. Andreas Schnleber and Dr. Christian Hübschle for their assistance with the low-temperature ANiF data collection. I. E. C. would like to acknowledge the Alexander von Humboldt Foundation for funding. A. L. G. acknowledges the ERC (Grant 279705) and EPSRC (Grant EP/G004528/2) for financial support. N. D. thanks the DFG for financial support through the Heisenberg Program and Project No. DU 954-8/1. N. D. and L. D. gratefully acknowledge the Federal Ministry of Education and Research (BMBF, Germany) for funding.