Phase sequence in diisopropylammonium iodide: avoided ferroelectricity by the appearance of a reconstructed phase

Crystals of diisopropylammonium iodide are synthesized, grown and characterized. Two phases: P21/m (Z = 1) and P212121 (Z = 2) are observed. In contrast with analogous compounds no polar phase occurs, despite a critical-like electric behaviour. A phenomenological theory is proposed to describe the thermodynamics of the whole family of diisopropylammonium halides

Organic-inorganic hybrid crystals, (2,4,6-CH 3 PyH) 3 Sb 2 Cl 9 and (2,4,6-CH 3 PyH) 3 Bi 2 Cl 9 . Crystal structure characterization and tunneling of CH 3 groups studied by 1 H NMR and neutron spectroscopy

The crystal structures of (2,4,6-CH3PyH)3Sb2Cl9 (TMPCA) and (2,4,6-CH3PyH)3Bi2Cl9 (TMPCB) (Py – pyridine) have been determined at 100 K by the single crystal X-ray diffraction method. TMPCA and TMPCB crystallize in the monoclinic C2/c and triclinic P1 polar space group, respectively. In both cases the asymmetric part is comprised of three nonequivalent 2,4,6-trimethylpyridinium cations and a discrete M2Cl93− anion. The Bi2Cl93− moiety forms a face-sharing bi-octahedron, whereas in a case of Sb2Cl93− we deal with two pyramids connected by a corner. The inelastic neutron scattering spectra (INS) were recorded for TMPCA at low temperatures (4–50 K). Two peaks on each side of the central elastic line have been observed at ca. 4.8 and 2.9 μeV, the high energy peak exhibits an excitation energy value equal to ca. 6 meV. For TMPCA and TMPCB the 1H NMR spin–lattice relaxation times, T1, have been measured in the temperature region 15–410 K. The flattening of the T1 (spin–lattice) vs. reciprocal temperature, 1/T, dependence between 30 K and 15 K indicates the incoherent tunneling effect of the methyl group being treated as the quantum rotor. The conclusions drawn from the 1H NMR results as regards to the tunneling of the CH3 groups in the pyridinium cations are consistent with the tunneling peaks observed in the INS spectra

Structure and tunneling splitting spectra of methyl groups of tetramethylpyrazine in complexes with chloranilic and bromanilic acids

The crystal and molecular structure of the 2,3,5,6-tetramethylpyrazine (TMP) complex with 2,5-dibromo-3,6-dihydroxy-p-quinone (bromanilic acid, BRA) has been studied and the results are compared with TMP CLA (2,5-dichloro-3,6- dihydroxy-p-quinone (chloranilic acid, CLA) complex. The X-ray structure of TMP BRA complex indicates the formation of dimeric units, in which two BRA - anions are connected by two O-H···O (2.646(2) Å) hydrogen bonds, whereas the cations and anions are joined together by strong N+-H···O- (2.657(2) Å) hydrogen bonds. The results are analyzed in terms of both the methyl group surroundings and the C-H···O and N+- H···O- (or N···H-O) bridge formations. Both effects, the strength of the N+- H···O- hydrogen bonds and steric hindrance for the rotations, are responsible for the CH3 group dynamics. For the TMP CLA and TMP BRA complexes, the inelastic neutron backscattering spectra were also investigated. In the case of TMP CLA, four tunneling signals have been observed in the energy range ±30 ÎeV, which indicates four inequivalent methyl groups in the crystal structure at the lowest temperature. No tunneling splitting is observed in the case of the TMP BRA complex, most probably due to the overlapping with the elastic peak. The tunneling results are consistent with the 1H NMR spin-lattice relaxation time investigations in a wide temperature range, which also point to the CH 3 group tunneling effect in the case of TMP CLA. © 2014 American Chemical Society

Dynamics of Ferroelectric Bis(imidazolium) Pentachloroantimonate(III) by Means of Nuclear Magnetic Resonance ¹H Relaxometry and Dielectric Spectroscopy

Some of haloantimonates­(III) and halobismuthates­(III) are ferroelectric. Bis­(imidazolium) pentachloroantimonate­(III), (C₃N₂H₅)₂SbCl₅ (abbreviation: <b>ICA</b>) is the first example of such compounds with a one-dimensional anionic chain which exhibits ferroelectric properties. The relation between the ionic dynamics and network structure and the ferroelectric features is not clear. Here Nuclear Magnetic Resonance (NMR) ¹H spin–lattice relaxation experiments at 25 MHz are reported for <b>ICA</b> in the temperature range of 80 K-360 K, covering ferroelectric-paraelectric and structural phase transitions of the compound occurring at 180 and 342 K, respectively. The relaxation process is biexponential in the whole temperature range indicating two dynamically nonequivalent types of imidazolium cations. Temperature dependences of both relaxation contributions allow for identifying three motional processes. Two of them are cation-specific – <i>i.e</i>. they are attributed to the two types of imidazolium cations, respectively. The third process involves both types of cations, and it is characterized by much lower activation energy. Moreover, the relaxation data (combined with ¹H second moment measurements) show that the ferroelectric-paraelectric phase transition mechanism is governed, to a large extent, by the anionic network arrangement. The NMR studies are complemented by dielectric spectroscopy experiments performed in the vicinity of the Curie temperature, <i>T</i>_<i>C</i> = 180 K, to get insight into the mechanism of the ferroelectric-paraelectric phase transition. The dielectric dispersion data show critical slowing down of the macroscopic relaxation time, <i>τ</i>, in <b>ICA</b> when approaching <i>T</i>_<i>C</i> from the paraelectric side, indicating an order–disorder type of ferroelectrics

Structure and Tunneling Splitting Spectra of Methyl Groups of Tetramethylpyrazine in Complexes with Chloranilic and Bromanilic Acids

The crystal and molecular structure of the 2,3,5,6-tetramethylpyrazine (TMP) complex with 2,5-dibromo-3,6-dihydroxy-p-quinone (bromanilic acid, BRA) has been studied and the results are compared with TMP CLA (2,5-dichloro-3,6-dihydroxy-p-quinone (chloranilic acid, CLA) complex. The X-ray structure of TMP BRA complex indicates the formation of dimeric units, in which two BRA^<i>–</i> anions are connected by two O–H···O (2.646(2) Å) hydrogen bonds, whereas the cations and anions are joined together by strong N⁺–H···O^<i>–</i> (2.657(2) Å) hydrogen bonds. The results are analyzed in terms of both the methyl group surroundings and the C–H···O and N⁺–H···O^<i>–</i> (or N···H–O) bridge formations. Both effects, the strength of the N⁺–H···O^<i>–</i> hydrogen bonds and steric hindrance for the rotations, are responsible for the CH₃ group dynamics. For the TMP CLA and TMP BRA complexes, the inelastic neutron backscattering spectra were also investigated. In the case of TMP CLA, four tunneling signals have been observed in the energy range <i>±</i>30 μeV, which indicates four inequivalent methyl groups in the crystal structure at the lowest temperature. No tunneling splitting is observed in the case of the TMP BRA complex, most probably due to the overlapping with the elastic peak. The tunneling results are consistent with the ¹H NMR spin–lattice relaxation time investigations in a wide temperature range, which also point to the CH₃ group tunneling effect in the case of TMP CLA