Utilising proton transfer in molecular crystals for electric field applications
2School of Chemistry, University of Birmingham, Birmingham, UK
3Department of Chemistry, University of Manchester, Manchester, UK
4ISIS Support Labs, ISIS Muon and Neutron Source, Oxford, UK
Proton transfer may occur across hydrogen bonds within molecular crystals formed of organic acids and bases. The proton transfer may be static, occurring on crystal formation and resulting in oppositely charge species, or be variable, such that multiple positions across the hydrogen bond are energetically favourable and/or are susceptible to external crystal environment. Under an applied electric field, protons may be shuttled across a hydrogen bond to generate, or switch, a spontaneous polarisation in a molecular crystal.1, 2 Organic ferro/anti-ferroelectrics exploiting a proton transfer mechanism as the switchable component are of interest where they offer a lightweight lead-free alternative for use in small devices.3
In this work, we explore proton transfer behaviour in a set of molecular crystals for future electric field applications using a combination of in situ single crystal X-ray diffraction, dielectric spectroscopy and thermal analysis techniques. Systems under study include two related substituted urea dinitrobenzoic acids co-crystal and salts where (1) is polar and a potentially switchable co-crystal whilst (2) is a non-polar salt shown to exhibit temperature dependent proton migration and has potential to be an anti-ferroelectric. We also present the electric field crystallography capabilities at Diamond Light Source on diffraction beamlines I11 and I19. We describe the system configuration including sample holders for in situ electric field single crystal (I19)4 or powder (I11) X-ray diffraction studies. Both beamlines offer the capability of time-resolved crystallography under electric fields for structural dynamics mapping on the milli (i11) and micro (I19) second time scales.
1. S. Horiuchi, K. Kobayashi, R. Kumai and S. Ishibashi, Nat. Commun., 2017, 8, 14426.
2. M. Owczarek et al., Nat. Commun., 2016, 7, 13108.
3. A. S. Tayi et al., Nature, 2012, 488, 485-489.
4. L. K. Saunders, et al., J. Appl. Crystallogr., 2021, 54, 1349-1359.
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