Characteristic feature of the ferroelectric phase transition in Rochelle salt

Rochelle salt – a structural reinvestigation with improved tools. I. The high-temperature paraelectric phase at 308 K

“…We observe that all of the water O atoms have the largest extension of their displacement ellipsoids directed approximately towards the split pair K1/K1 ii as if to enhance the K-O interactions. Strong anisotropy in these water molecules has been noted and discussed in several previous studies of RS (Suzuki & Shiozaki, 1996;Solans et al, 1997;Shiozaki et al, 1998Shiozaki et al, , 1999. Apparently the directional relationship to the K1 site pointed out above has not been appreciated.…”

A new avenue to relaxor-like ferroelectric behaviour found by probing the structure and dynamics of [NH3NH2]Mg(HCO2)3

“…The 2 H spectra are particularly sensitive to the dynamics of the cation, while the 13 C spectra show significant changes to the host framework, as observed via the formate ion. As most clearly seen in the 13 C spectra, the intermediate region contains features of the low and high temperature regions, consistent with a first-order transition, but there is a clear evolution of the low temperature structure towards the high temperature limit, i.e. the intermediate region is not a simple first-order transition involving separate domains of two materials.…”

“…the intermediate region is not a simple first-order transition involving separate domains of two materials. Another important feature of the 13 C NMR results is that the low temperature spectra can only be explained in terms of significant static disorder; rather than consisting of a limited number of resonances corresponding to crystallographically distinct carbon sites, there is a continuous distribution of chemical shifts between 170 and 175 ppm, implying a distribution of local environments. This is consistent with the disorder of H positions of the formate ligand observed in the diffraction experiments being static, which creates a continuous distribution of local formate environments.…”

“…Table 1 The mean FWHM of the quasielastic peak and calculated residency times for the modelled hydrogen positions using equations (10)(11)(12)(13) which rotation about N-N is always fast compared to the spectral lineshape (100s kHz). The elastic incoherent structure factor (EISF) comprises contributions of the total scattering that are solely elastic and how these change as a function of Q.…”

“…8,9 One of the earliest discovered and most widely utilised classes of ferroelectrics are perovskites, such as BaTiO 3 , 10 whose properties stem from the B site titanium cation, which has a degree of mobility within the BaO 3 cage. 11 Rochelle's salt was one of the first reported ferroelectric materials, but it was not until decades later, following a neutron diffraction study, that the origin of the ferroelectricity was recognised as being hydrogen bond driven; 12,13 such ferroelectricity derives from the displacement of the hydrogen atom within the intermolecular bond, highlighting that ferroelectric properties are not restricted to inorganic perovskites. [14][15][16] Relaxor ferroelectrics are a subclass of ferroelectric materials used in capacitors for energy storage and piezoelectric sensing, particularly where high polarisation over a broader range of temperatures is needed.…”