Proton Disorder in NH···N Bonded [dabcoH]⁺I⁻ Relaxor: New Insights into H-Disordering in a One-Dimensional H₂O Ice Analogue

Abstract

Protons in the NH þ 3 3 3 N hydrogen bonds linking the cations into strictly linear aggregates in the crystal of 1,4-diazabicyclo[2.2.2]octane hydroiodide ([C 6 H 13 N 2 ] þ 3 I -, dabcoHI) remain disordered, and the average crystal symmetry (space group P6m2) remains unchanged, down to 1.5 K, as evidenced by single-crystal neutron-diffraction, but against the expectation based on the third law of thermodynamics. This proton disorder reveals the role of the thermal-activation and proton-tunneling processes for the polarizability of hydrogen bonds and for the formation of polar nanodomains, resulting in the unique property of an anisotropic giant dielectric response in the chemically homogeneous compound. The dielectric constant exceeding several thousand in the direction of the crystal [z] axis between 150 and 300 K can be explained by the fluctuations of nanodomains, whereas the dielectric constant of about 100 at 10 K can be due to the residual contribution of tunneling protons in hydrogen bonds. The proton transfers disproportionate the dabco monocations into dications and neutral molecules and generate high crystal-field fluctuations. The consideration of possible crystal-symmetry changes shows that ferroelectric proton ordering would lead to the lowered symmetry of space group P3m1 within the hexagonal system, and antiferroelectric proton ordering would require a transformation to an orthorhombic phase; however, the proton-site correlations are too weak for these transformations to take place, and the resultant structure remains disordered. The crystal symmetry and the lack of FowlerBernal rules distinguish dabcoHI from the H 2 O ice-I h , and are essential for the dielectric properties of this substance.

Chiral to Chiral Phase Transition in a Novel Hydrogen Bond Type Molecular Ferroelectric

“…Usually, these hydrogens motions trigger the displace of N, O, or halogen atoms and result the residual polarization for the ferroelectric properties. A typical case for that are KH 2 PO 4 (KDP) crystals which is well‐known to undergo a ferroelectric phase transition from I 2 d to Fdd 2 at 123 K. However, to our knowledge, most hydrogen bond type ferroelectric molecules still mainly focuses on the four families, namely, i) salts of tartaric acid, such as Rochelle salt;ii) Alkali metal phosphate or arsenate dihydrogen salt family, such as KDP;iii) Organic salt of hydrogen such as triglycine sulfate (TGS);iv) Alum family such as methylamine jarosite, only small amount new hydrogen bond type ferroelectric molecules such as croconic acid were documented recently, the new systems of hydrogen bond type ferroelectric molecules especially accompanying with chiral to chiral phase transitions are still very spare, and the exploration of novel ferroelectric molecules in homochiral system which triggered by H‐bonds remains a great challenge. As we know, a H‐bonds ferroelectric must meet some essential conditions such as having a good electric hysteresis loop and a clear dielectric anomaly as well as the necessary crystallographic requirements of one of the ten polar point groups in the ferroelectric phase ( C 1 , C 2 , C s , C 2v , C 4 , C 4v , C 3 , C 3v , C 6 , and C 6v ) …”

The staple role of hydrogen and halogen bonds in crystalline (E)-8-((2,3-diiodo-4-(quinolin-8-ylthio)but-2-en-1-yl)thio)quinolin-1-ium triiodide

“…ÁÁÁN bonded aggregates, where the H atom is disordered over two nitrogen atoms, have drawn attention as organic ferroelectrics. A wide range of compounds described in the works of Katrusiak and co-authors [6,7], such as the salts of 1,4-diazabicyclo [2,2,2]octane [8], or salts of 4,4 0 -bipyridine with mineral acids [9], are characterized by the one-dimensional structural arrangement that provides the high dielectric properties of crystals. Negative linear thermal expansion in 2-phenylimidazole crystals with quasistatic disorder of NH ?…”

Photochemical control of dielectric properties based on intermolecular proton transfer in a hydrogen-bonded diarylethene crystal

“…10 2.842 A ˚, which are within the typical range for the N-HÁ Á ÁN hydrogen bond, but longer than the distances in salts of monoprotonated diazabicyclo[2.2.2]octane (dabcoH + ), which exhibit large dielectric responses. [11][12][13][14] A noteworthy point is the positions of the hydrogen-bonding protons between the nitrogen atoms in the chains. The X-ray analysis revealed that there exist two sites of the N-H protons between adjacent nitrogen atoms and the sites are occupied in the ratio of 50 : 50.…”

“…Weak correlations between the proton sites result in almost random coexistence of short-range ordered domains of the two proton orientations rather than long-range ferroelectric or antiferroelectric orderings. [11][12][13] Dielectric properties were measured for the single-crystalline sample of 1o by using an impedance analyzer. Fig.…”

“…The observed NÁ Á ÁN distances in the hydrogen-bonded chain (2.915 A ˚and 2.842 A ˚) suggest a double-well potential energy curve of the proton coordinate between the nitrogen atoms. [11][12][13][14] Taking into consideration the symmetry of the crystal structure along the c-axis, a potential energy curve for the two equivalent orientations of the N-H protons in the hydrogen-bonded chain (Fig. 1b) is symmetric.…”