Kinetic Monte Carlo simulations of proton conductivity.

The kinetic Monte Carlo method is used to model the dynamic properties of proton diffusion in anhydrous proton conductors. The results have been discussed with reference to a two-step process called the Grotthuss mechanism. There is a widespread belief that this mechanism is responsible for fast proton mobility. We showed in detail that the relative frequency of reorientation and diffusion processes is crucial for the conductivity. Moreover, the current dependence on proton concentration has been analyzed. In order to test our microscopic model the proton transport in polymer electrolyte membranes based on benzimidazole C(7)H(6)N(2) molecules is studied.

The hydrogen dynamics of CsH5(PO4)2 studied by means of nuclear magnetic resonance.

We have investigated the hydrogen dynamics of cesium pentahydrogen diphosphate, CsH(5)(PO(4))(2), by means of nuclear magnetic resonance (NMR) spectroscopy, in order to address the question of why there is no superprotonic phase transition in this compound, in contrast to other structurally similar hydrogen-bonded ionic salts, where a superprotonic transition is frequently found to be present. The analysis of the NMR spectrum and the spin-lattice relaxation rate revealed that the temperature-dependent hydrogen dynamics of CsH(5)(PO(4))(2) involves motional processes (the intra-H-bond jumps and the inter-H-bond jumps at elevated temperatures, as a mechanism of the ionic conductivity) identical to those for the other H-bonded superprotonic salts. The considerably stronger H-bond network in CsH(5)(PO(4))(2) prompts the search for a higher superprotonic transition temperature. However, due to the relatively weak bonding between the {[H(2)PO(4)]}∞ planes in the [100] direction of the CsH(5)(PO(4))(2) structure by means of the ionic bonding via the cesium atoms and the small number of H bonds in that direction (where out of five H bonds in the unit cell, four are directed within the {[H(2)PO(4)]}∞ planes and only one is between the planes), the bonds between the planes become thermally broken and the crystal melts before the H-bond network rearranges via water release into an open structure typical of the superprotonic phase. Were the coupling between the {[H(2)PO(4)]}∞ planes in the CsH(5)(PO(4))(2) somewhat stronger, the superprotonic transition would occur in the same manner as it does in other structurally related hydrogen-bonded ionic salts.

The influence of hydrostatic pressure on the ferroelectric phase transition in (C(3)N(2)H(5))(5)Bi(2)Cl(11).

Single crystals of pentakis (imidazolium) undecachlorodibismuthate (III); (C(3)N(2)H(5))(5)Bi(2)Cl(11), have been studied by the dielectric method at ambient and hydrostatic pressures. At ambient pressure the crystals undergo a ferroelectric phase transition of order-disorder type at 166 K. The character of the dielectric permittivity anomalies is typical of the ferroelectric phase transition close to second-order type. The magnitude of the Curie constants of paraelectric (C(+)) and ferroelectric (C(-)) phases is of the order of 10(3) and the (C(+))/(C(-)) ratio is near 2. At elevated pressures the maximum dielectric permittivity corresponding to the (T(c)) value gradually decreases with increasing pressure. In addition, above about 15 MPa the dielectric permittivity shows a step-like behaviour as well as a temperature hysteresis, which is typical of first-order phase transitions. The Curie-Weiss constant (C(+)) rapidly increases with increasing pressure, reaching a value of the order of 10(5) at about 181 MPa. The pressure-induced change in the phase transition type is continuous and reversible. The phase boundary has been found to decrease nonlinearly with increasing pressure with the initial slope of dT(c)/dp = -3.9 × 10(-2) K MPa(-1). The results have been discussed within the phenomenological model of phase transitions.

Effect of hydrostatic pressure on N(CH3)4+ cation motion in ferroelectric N(CH3)4H(Cl3CCOO)2.

The proton spin-lattice relaxation time in ferroelectric N(CH3)4H(Cl3CCOO)2 has been studied under isobaric conditions at pressures 0.1, 200 and 400 MPa over a wide range of temperature. The data indicate that the dominant relaxation mechanism for T1 can be attributed to the classical CH3 group reorientation of N(CH3)4+ cation. The influence of pressure on methyl group reorientation of N(CH3)4+ cation was analysed.

1H NMR study of N(CH3)4H(ClF2CCOO)2.

The proton spin-lattice relaxation times and 1H NMR second moments were measured over a wide range of temperature. The results were compared with those of the 19F NMR relaxation that we obtained earlier. For both nuclear species, the evolution of the longitudinal magnetizations with time is observed to be strongly bi-exponential and were in good quantitative agreement with the cross-relaxation theory.

35Cl NOR and 19F NMR relaxation studies of CClF2 group dynamics in N(CH3)4H(ClF2CCOO)2.

The reorientation of CClF2 groups in N(CH3)4H(ClF2CCOO)2 has been studied using pulsed NQR and NMR techniques. The temperature dependence of both chlorine (35Cl) NQR and fluorine (19F) NMR spin-lattice relaxation has been measured T1Q of chlorine is attributed to the sum of two contributions: the reorientation of CClF2 groups and the modulation of the electric field gradient (EFG) produced by the motion of the N(CH3)4+ cations. The activation energies were determined for both kinds of motion. The fluorine relaxation is dominated by an intramolecular 19F-1H dipolar interaction. In analysis of cross-relaxation effects the spectral density functions have been evaluated using the motional parameters obtained from NQR data.