Superspace approach helps: determination of proton dynamics in the phase transition of modulated supramolecular ferroelectrics: 5,5′-dimethyl-2,2′-bipyridine and bromanilic acid

Temperature dependent crystal structures are reported for the co-crystal of 5,5 & PRIME;-dimethyl-2,2 & PRIME;-bipyridine (55DMBP) and bromanilic acid (H(2)ba) across its phase transitions. 55DMBP-H(2)ba is ferroelectric (FE) below T = 245 K and remains paraelectric (PE) at higher temperatures up to 360 K, but passes through two PE-PE phase transitions. X-ray diffraction data at 120 K reveals a ferroelectric phase (FE-I phase), which can be described as a commensurately modulated structure with superspace group P1(& sigma;(1)& sigma;(2)& sigma;(3))0 with modulation wave vector q = 0.5, 0.5, 0.5. At 250 K, the crystal transforms into the paraelectric phase PE-II, which possesses the same modulation wave vector. Above 320 K the modulation wave vector becomes incommensurate, q = (0.5000, 0.4944, 0.5221), while the superspace group remains the same in the FE-I, PE-II and PE-IC phases. Different choices of the phase of the modulation wave allow the PE-II to FE-I phase transition to be described by a phase shift in superspace. Above 338 K the satellite Bragg reflections disappear. The crystal structure at 346 K of this PE-III phase is periodic with space group P1 and a unit cell that acts as basic structure for the modulated phases. Peak profiles become very broad at 350 K and at 360 K the crystal disintegrates, and the material becomes amorphous. Anharmonic atomic displacements are found for the Br atoms in the PE-IC and PE-III phases. The FE-I phase is ferroelectric due to proton transfer within part of the O-HMIDLINE HORIZONTAL ELLIPSISN intermolecular hydrogen bonds, a mechanism similar to that of phenazine-chloranilic acid. The PE-IC phase involves modulations of the proton between two tautomeric forms of H(2)ba, thus leading to an exchange between O1-H1MIDLINE HORIZONTAL ELLIPSISN1 and O2-H1MIDLINE HORIZONTAL ELLIPSISN1 hydrogen bonds. This mechanism is essentially different from the incommensurability in phenazine-chloranilic acid.