“…The range of behaviours includes ferroelectricity, where a spontaneous and switchable polarization is exhibited in (typically) polar systems under an electric field (Horiuchi et al, 2012); antiferroelectricity, where symmetry-opposed polar sub-units present in a material may be aligned on application of an electric field and can be coupled with a crystallographic phase transition (Tolé dano & Guennou, 2016); piezoelectricity, where the system shows a mechanical response to the field with change in, for example, the lattice/structural parameters (Werling et al, 2013); proton-transfer behaviour (Rode et al, 2016;; and the enhancement of nonlinear optical properties (Bai et al, 2013). A range of materials exist that exhibit electric field responses in the solid state, including metal oxides, metal-organic frameworks (Zhang & Xiong, 2012), hydrogen-bonded organic molecular crystals (Stroppa et al, 2011;Owczarek et al, 2016; and ionic solids (Li et al, 2015;Schmalzried & Smolin, 1998;Zhang et al, 2018;Rodzevich et al, 2017). Related to their dielectric properties, these materials can have applications as pressure sensors (Haertling, 1999), actuators (Wersing et al, 2008), memory devices (Amanuma et al, 2000, Dawber et al, 2005 and capacitors (Bouregba et al, 2003).…”