35Cl spin-lattice relaxation in incommensurate bis(4-chlorophenyl)sulfone

Journal ArticleThe 35C1 nuclear-quadrupole-resonance (NQR) spin-lattice relaxation rate in incommensurate bis(4- chlorophenyl)sulfone (BCPS) is determined by Raman processes below 60 K and by direct one-phonon processes above 60 K. The variation of the 35C1 spin-lattice relaxation rate over the incommensurate NQR frequency distribution in BCPS has been also measured at various temperatures. The results are explained in terms of phason and amplitudon contributions to the effective spin-lattice relaxation rate. In contrast to Rb2ZnCl4-type crystals the changes in the CI electric-field-gradient tensor produced by modulation wave fluctuations contain both first- and second-derivative terms

Spektri NMR jednog zrna ikosaedarskog Al-Pd-Mn kvazikristala ovisni o smjeru

Orientation-dependent 27Al quadrupole-perturbed NMR spectra of a single-grain Al72.4Pd20.5Mn7.1 icosahedral quasicrystal are reported. The dependence on orientation is a consequence of a spatially anisotropic distribution of orientations of the electric field gradient tensor at the inequivalent lattice sites. The results demonstrate that the NMR spectra of quasicrystalline single-grain samples of a macroscopic symmetry as high as icosahedral are orientation-dependent and differ from the orientation-independent spectra of powder samples, in contrast to previous reports.Ustanovili smo ovisnost kvadrupolno-smetanih spektara NMR 27Al u jednom zrnu ikosaedarskog kvazikristala Al72.4Pd20.5Mn7.1 o smjeru. Ta je ovisnost posljedica prostorno neizotropne raspodjele tenzora gradijenta električnog polja na različitim položajima u rešetci. Ishodi mjerenja pokazuju da spektri NMR kvazikristalnih jednozrnih uzoraka tako visoke simetrije kao što je ikosaedarska ovise o smjeru i razlikuju se od smjerno-neovisnih spektara praškastih uzoraka, što je suprotno ranijim izvješćima

Spektri NMR jednog zrna ikosaedarskog Al-Pd-Mn kvazikristala ovisni o smjeru

Orientation-dependent 27Al quadrupole-perturbed NMR spectra of a single-grain Al72.4Pd20.5Mn7.1 icosahedral quasicrystal are reported. The dependence on orientation is a consequence of a spatially anisotropic distribution of orientations of the electric field gradient tensor at the inequivalent lattice sites. The results demonstrate that the NMR spectra of quasicrystalline single-grain samples of a macroscopic symmetry as high as icosahedral are orientation-dependent and differ from the orientation-independent spectra of powder samples, in contrast to previous reports.Ustanovili smo ovisnost kvadrupolno-smetanih spektara NMR 27Al u jednom zrnu ikosaedarskog kvazikristala Al72.4Pd20.5Mn7.1 o smjeru. Ta je ovisnost posljedica prostorno neizotropne raspodjele tenzora gradijenta električnog polja na različitim položajima u rešetci. Ishodi mjerenja pokazuju da spektri NMR kvazikristalnih jednozrnih uzoraka tako visoke simetrije kao što je ikosaedarska ovise o smjeru i razlikuju se od smjerno-neovisnih spektara praškastih uzoraka, što je suprotno ranijim izvješćima

Evolution of magnetic and crystal structures in the multiferroic FeTe2O5Br

Neutron diffraction and nuclear quadrupole resonance (NQR) measurements were employed to investigate magnetic order in the non-ferroelectric phase preceding the low-temperature multiferroic state in FeTe2O5Br. Refnement of the neutron diffraction data and simulations of 79,81Br NQR spectra reveal that the incommensurate magnetic ordering in the non-ferroelectric state comprises amplitude-modulated magnetic moments, similarly as in the multiferroic state. The two ordered states differ in the orientation of the magnetic moments and phase shifts between modulation waves. Surprisingly, all symmetry restrictions for the electric polarization are absent in both states. The different ferroelectric responses of the two states are thus argued to arise from the differences in the phase shifts between certain modulation waves, which cancel out in the non-ferrolectric state.Comment: 9 pages, 8 figures including appendix, published in PR

1H NMR relaxometry in the TGBA* and TGBC* phases

Recent results obtained by 1H NMR relaxometry of liquid crystals having twist grain boundary (TGB) phases are here reviewed. In particular, three chiral rod-like lactate derivative mesogens were investigated. In the isotropic phase, three-exponential magnetization decay was observed in all cases and the three distinct spin-lattice relaxation times (T1) were assigned to three specific molecular groups of these molecules. In the TGBA* and TGBC* phases the magnetization decay is mono-exponential and the main features of the 1H NMR dispersion curves analyzed through a global target fitting procedure in terms of specific molecular and collective dynamics are discussed

Molecular motions of banana-shaped liquid crystals by means of NMR spectroscopy

Recent studies based on NMR spectroscopy put in evidence the occurrence of a slow motion regime in bent-shaped liquid crystals having the aromatic core formed by five phenyl rings linked by ester groups both in the isotropic and in the nematic phases. In this paper a brief overview of these NMR studies is presented and additional new relaxation data recorded on a banana-shaped mesogen are reported and discussed. In particular, proton spin-lattice relaxation times, T(1), were acquired at different Larmor frequencies from 8 MHz to 5 kHz by means of NMR relaxometry in a wide range of temperatures from the isotropic to the crystalline phases. These data confirm that NMR relaxation in bent-core liquid crystals is affected by much slower dynamics than that observed in common rod-like liquid crystals and that overall molecular reorientations are responsible of this slow motion regime. This finding is discussed in the frame of the results obtained by means of several NMR methods, such as (2)H NMR T(2) analysis and (1)H self-diffusion NMR measurements, on the same bent-core molecule

Atomic jumps in quasiperiodic Al72.6Ni10.5Co16.9 and relatad crystalline material

Al-27 NMR measurements of the transverse magnetization decay in a decagonal quasicrystal Al72.6Ni10.5Co16.9 detected at temperatures between 300 and 4 K slow, low-activation-energy, diffusive atomic jumps that exhibit properties compatible with the elementary excitations of the quasiperiodic lattices in the form of phason jumps. However, identical atomic motion was observed also in "vacancy-ordered" bcc crystalline Al50Cu35Ni15, indicating that this motion is not quasicrystals specific, but a feature of close-packed atomic structures-either periodic or quasiperiodic-that contain structural vacancies

Mn magnetism in icosahedral quasicrystalline Al72.4Pd20.5Mn7.1

The unusual magnetism of manganese atoms in an icosahedral Al72.4Pd20.5Mn7.1 single-grain quasicrystal was studied by Al-27 nuclear magnetic resonance (NMR), magnetic susceptibility, and electrical resistivity measurements. Between room temperature and 50 K both the Al-27 NMR linewidth and the frequency shift exhibit a Curie-Weiss-type, 1/(T-theta) temperature dependence. At lower temperatures a significant narrowing of the linewidth, concomitant with a reduction of the frequency shift is observed. These features can be explained by a gradual reduction of the local exchange magnetic field at the position of the Al-27 nuclei that is transferred from the manganese d moments via the conduction electrons. Two possible origins of this phenomenon are discussed: (i) the Kondo-like screening of manganese moments by the conduction electrons and (ii) the ''resistivity'' -damping of the RKKY interaction due to increasing electron localization at low temperatures

1H NMR Relaxometric Study of Molecular Dynamics in a “de Vries” Liquid Crystal

Liquid crystals that exhibit de Vries smectic A phases are promising materials for new generations of ferroelectric liquid crystal displays and other electro-optical devices. We investigated the molecular dynamic properties of a rod-like de Vries liquid crystal compound, namely the 9HL, a (S)-hexyl lactate derivative, in the whole mesophasic range. This is the first molecular dynamics’ investigation on a de Vries phase, and the interest of this system is related to the understanding of the structural and supramolecular organization of de Vries SmA phases, which has been a subject of a broad scientific debate in the last years. Proton NMR spin–lattice relaxation times, T1, were measured in the range 100 MHz to 5 kHz, thus covering a very wide range of motional regimes. 1H NMR dispersion curves and temperature-dependent relaxation rates were analyzed with a global minimum target fitting approach and the main molecular motions, namely reorientational diffusion, translational self-diffusion, layer undulation, and tilting director fluctuations were fully characterized. The molecular dynamics’ behavior observed across the SmA–SmC* phase transitions of 9HL was consistent with the proposed cluster diffuse cone model for the de Vries SmA phase