Random fields and quantum effects in proton glasses

The effects of quantum fluctuations on the proton glass phase in mixed hydrogen-bonded ferro-antiferroelectric systems are considered. The system is described in terms of the infinite-ranged Ising pseudospin glass model in a transverse tunneling field in the presence of random parallel fields. The stability limit of the high-temperature proton glass phase is determined within the thermofield dynamic approach, and the behavior or linear and nonlinear susceptibility is evaluated.     

Magnetic resonance of the weak organic magnet TDAE-C60

The charge transfer compound TDAE-C₆₀ (TDAE — tetrakis-dimethylamino-ethylene) is so far the only known C₆₀ based molecular ferromagnet. The ESR spectra of TDAE-C₆₀ single crystals demonstrate that freshly grown regularly shaped single crystals show at first an AFM (antiferromagnetic)-like behavior which changes upon annealing at room temperature in vacuum to a FM (ferromagnetic) one. It thus seems that annealing induces a change in the relative orientation of the distorted C ₆₀ ^? ions resulting in a change in the sign of the effective magnetic interactions between neighboring C ₆₀ ^? ions. A structural transition in the TDAE methyl protons at 250 K seems to be connected with this process. Here we review the structure, the ESR spectra, the proton NMR spectra and the¹³C NMR spectra of TDAE-C₆₀.     

Proton-14N double resonance study of the structural phase transitions in the perovskite type layer compound (CH3NH3)2CdCl4

Using a proton-nitrogen double resonance technique we have determined the quadrupole coupling of¹⁴N in the room temperature orthorhombic (Cmca), the low temperature tetragonal (P4₂/ncm), and the monoclinic low temperature (P2₁/c) phases of (CH₃NH₃)₂CdCl₄. In all these phases all nitrogens are chemically equivalent demonstrating that the disorder in the orientations and H-bonding arrangements of the CH₃-NH₃ groups in theC m c a andP4₂/ncm phases is indeed dynamic and not static. In the monoclinic phase the¹⁴N quadrupole coupling constant equalse ² qQ/h=880 kHz and the asymmetry parameter is=0.20, wherease ² qQ/h=790 kHz,=0.1 in the tetragonal low temperature phase ande ² qQ/h=726 kHz,=0.21 in the room temperature orthorhombic phase. The observed increase in the¹⁴N quadrupole coupling constant on going from the orthorhombic phase to the tetragonal low temperature phase which is coupled with a simultaneous decrease in the asymmetry parameter can be understood in terms of a partial freezing in of the dynamic disorder in the C-N bond directions whereas the¹⁴N quadrupole coupling tensor in the monoclinic phase is characteristic of a frozen in C-N bond in a deformed lattice, where the N-H — Cl bonds are of different length.     

17O NQR study of the ferroelectric phase transition in PbHPO4

The temperature dependence of the ¹⁷O NQR spectra of the O-H---O bonded oxygens in PbHPO₄ have been studied using a proton-¹⁷O double resonance technique. The results show that the protons move between two off-centre sites above T_c and freeze into one of the possible equilibrium sites below T_c.     

NMR study of diffusion of butane in linear polyethylene

The diffusion coefficient of butane in linear polyethylene at room temperature as a function of the vapor pressure of butane was measured by the spin-echo method with a pulsed magnetic field gradient. For the Special morphology of randomly oriented stacks of parallel lamellas the detour factor is 1/3. As long as the blocking factor and migration through the lamellas can be neglected, the local diffusion coefficient D_a of the small molecules through the amorphous layers in the direction parallel to the lamellas is three times the apparent diffusion coefficient D derived from the decay of the amplitude of the spin echo under the assumption of an infinitely extended homogeneous medium. The diffusion coefficient and the spin–spin relaxation time both increase exponentially with increasing pressure, i.e., butane concentration in the polymer, while the spin-lattice relaxation time is pressure independent and seems to be determined by interaction with the amorphous polyethylene matrix.