Phase behaviour and driving forces behind ferroelectricity in MDABCO-based perovskites Hamish Yeung 1 , S.D. Gale1, H.J. Lloyd 1,2 , H. Blakiston 1 , L. Male 1 , M.R. Warren 2 , L.K. Saunders 2 , P.A. Anderson 1 1 School of Chemistry, University of Birmingham, B15 2TT, U.K. 2 Diamond Light Source, Harwell Science and Innovation Campus, Didcot, OX11 0DE, U.K. Perovskites, such as the canonical ferroelectric BaTiO 3 , have long been considered state-of-the-art in design of new FE materials owing to desirable properties and tunability of their composition. The recent discovery of a new subset of hybrid organic–inorganic perovskites based on the A-site MDABCO dication (MDABCO = N-methyl- N′-diazabicyclo[2.2.2]octonium) has led to renewed interest in hybrid inorganic–organic ferroelectric materials, owing to their properties that are comparable to BaTiO 3 . [MDABCO]NH 4 I 3 , in particular, has a large spontaneous polarization, high phase transition temperature and facile switchingthrough eight possible polarisation directions. 1 Until recently, only a handful of compositions of MDABCO-based perovskites had been reported and the driving forces that underpinned the emergence of spontaneous polarisation were poorly understood. In 2021 we showed using coarse-grained and atomistic models that the key factors driving spontaneous polarisation include orientation of the MDABCO dipole moment along the pseudo-cubic 〈 111 〉 direction, strain coupling and dipolar interactions. 2 Hydrogen bonding was also thought to be important in determining the magnitude of polarisation and coercive field; however, its influence on phase transitions to the high-symmetrycentrosymmetric phase remained unclear. This presentation discusses some of our work on the discovery of new [MDABCO]BX 3 compositions (Fig. 1), where B is an alkali metal cation and X is a halide anion, and its implications for the design of new MDABCO- based perovskites. 3 We show that new phases can help to define compositional limits for the pseudo-cubic perovskite structure vs. competing structures, such as hexagonal forms. In addition, access to a wider range of compositions enables us to determine correlations between a variety of structural features and phase transition temperatures, which point towards key design principles for new ferroelectric materials. References 1. H.-Y. Ye , Y.-Y. Tang , P.-F. Li , W.-Q. Liao , J.-X. Gao , X.-N. Hua , H. Cai , P.-P. Shi , Y.-M. You and R.-G. Xiong, Science, 2018, 361, 151–155. 2. D. J. W. Allen , N. C. Bristowe , A. L. Goodwin and H. H.-M. Yeung, J. Mater. Chem. C, 2021, 9, 2706. 3. S. D. Gale, H. J. Lloyd, L. Male, M. R. Warren, L. K. Saunders, P. A. Anderson and H. H.-M. Yeung,CrystEngComm, 2022, 24, 7272–7276.
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