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

Abstract

Temperature dependent crystal structures are reported for the co-crystal of 5,5′-dimethyl-2,2′-bipyridine (55DMBP) and bromanilic acid (H₂ba) across its phase transitions. 55DMBP–H₂ba 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 P(σ₁σ₂σ₃)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 P 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–H⋯N 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 H₂ba, thus leading to an exchange between O1–H1⋯N1 and O2–H1⋯N1 hydrogen bonds. This mechanism is essentially different from the incommensurability in phenazine-chloranilic acid.