J.M. Janik

Anisotropy of Self-diffusion in a Liquid Crystal Studied by Neutron Quasielastic Scattering

Abstract The coefficient D for self-diffusion in nematic p-azoxyanisole has been determined from measurements of the energy width of quasi-elastically scattered neutrons. The directional dependence of D was studied by polarizing the substance along different directions relative to the scattering vector. For an unpolarized substance we measured D= 1.4 × 10−5 cm2 sec−1, whereas for a polarized substacnce we found D∥ = 1.8 × 10−5 cm2 and D⊥ = 1.0 × 10−5 cm2 sec−1 for diffusion parallel with and perpendicular to the optical axis, respectively. These results are in qualitative agreement with the anisotropy of the viscosity coefficient, as measured by Miesowicz (Nature 136, 261 (1935); 158, 27 (1946)).

Neutron quasielastic scattering results for Me(NH3)6(XY4)2, Me(NH3)6(XY3)2 and Me(NH3)6X2 compounds, compared with the calorimetric and Raman line width data — a new analysis

Abstract Quasielastic neutron scattering data for Ni(NH3)6(CIO4)2, Mg(NH3)6(BF4)2, Ni(NH3)6(NO3)2, Mg(NH3)6(NO3)2, Ni(NH3)6Cl2 and Ni(NH3)6Br2 were subjected to a new analysis in which not only the correlation times for NH3 uniaxial reorientations but also a new parameter “excess elasticity” were determined. The new parameter gave us information concerning anharmonic librations of the NH3 groups. All neutron data were compared with the calorimeter and Raman line width data. An attempt to ascribe the phase transformations to transitions from hindered to free reorientations of the NH3 groups or/and the anions was made.

Reorientations in [Mg(H2O)6](ClO4)2 studied with proton magnetic resonance and quasielastic neutron scattering

Abstract Proton magnetic relaxation measurements on [Mg(H 2 O) 6 ](ClO 4 ) 2 revealed two processes responsible for the temperature dependence of T 1 . One is connected with H 2 O 180° flips about the symmetry axes and the other is connected with a tumbling of the complex cation. Quasielastic neutron scattering measurements gave another evidence of H 2 O 180° flips. The reorientational correlation times derived from NMR, which in the 273–325 K region are of the order of picosecond, coincide perfectly well with those derived from QNS.

Stochastic NH3-rotation in [Ni(NH3)6](ClO4)2 crystals studied by the neutron incoherent scattering method

Quasielastic neutron scattering were spectra measured for polycrystalline [Ni(NH3)6](ClO4)2. The spectra have been interpreted using models describing the stochastic NH3-rotational motions. The two models: 120°-jumps and the uniaxial rotational diffusion were used in the fitting procedure. The results of the fitting show that a preference should be given to the 120†-jump model. A comparison with an IR profile study was also made. Both the existence of 120°-jumps and the discontinuity in the correlation times observed at the phase transition can qualitatively be explained by the Bates and Stevens picture.

Neutron scattering by liquid crystals. A new analysis

Abstract This paper presents a new analysis of all (old and new) quasi-elastic neutron scattering results obtained for seven members of the PAA series (n = 1,…, 7). The analysis is based upon a discussion of the parameter p, which is the deviation of the intensity of the elastic component from the intensity demanded by a reorientational model. If the value of this parameter is negative, it may be interpreted as an indication of an additional motion which has not been taken into account in the model. The first model analysed in this way for all seven substances in the nematic phase was that of the rotational diffusion of the whole molecule in its stretched (trans) conformation, around the long axis. A very large negative values of p (p ≊ -60 per cent) was obtained, clearly showing that some motions have been neglected. The second model was that of the rotational diffusion of moieties consisting of benzene rings plus alkoxy side-chains. The diffusion occurs around the benzene para-axes, and the alkoxy side-...

Comparison of calorimetry and neutron scattering results concerning phase transitions in /Ni(NH3)6/(NO3)2 with the raman band profile study

Abstract Previously obtained calorimetry results revealed an existance of three solid phases in /Ni(NH 3 ) 6 /(NO 3 ) 2 . Neutron quasielastic scattering results, also obtained previously, gave evidence of quick reorientations of NH 3 groups around NiN axes in phases I and II. In the Raman profile study now reported we show that the NO − 3 groups reorient quickly in phase I, whereas they are fixed in phase II. Connection of NH 3 and NO − 3 reorientations with entropy of I↔II transition is discussed.

Incoherent quasielastic neutron scattering and Raman band shape study of the NH3 and BF-4 reorientations in [Ni(NH3)6](BF4)2

Abstract QNS measurements carried out on polycrystalline [Ni(NH3)6](BF4)2 revealed that the NH3 reorientations about Ni-N axes are occurring in all three phases with an activation energy Ea ⋍ 2kJ/mol. A discontinuity in the correlation times vs. inverse temperature at the phase I/phase II transition (140K) was observed. Two (extreme) models describing the stochastic NH3 reorientational motions were used to interpret the QNS spectra: the 120°-jumps and the uniaxial rotational diffusion. The quality of the fits for all spectra is quite good for both models, but according to the χ2 test, a small preference should be given to the 120°-jump model. Raman band shape measurements carried out on the same sample reveal BF-4 reorientations in phase I. These reorientations are abruptly stopped at the phase I/phase II transition.

Neutron scattering study of molecular motions in the naphthalene–tetracyanobenzene complex

Neutron diffraction measurements performed with the naphthalene–tetracyanobenzene complex showed the occurrence of the phase transition at ∼70 K in the thermal expansion of lattice parameters. Incoherent inelastic neutron scattering measurements showed many similarities between the phonon density of states of the complex on one hand and of the pure naphthalene on the other. There are also differences, which indicate a coupling between the external and the internal modes in the complex. Quasielastic neutron scattering showed an occurrence of fast stochastic jumps of naphthalene molecules in the complex. The two positions between which these rotational jumps occur differ by ∼36° and a residence time is of the order of 8 ps at room temperature.

Calometry, proton magnetic resonance and quasielastic neutron scattering studies of [Mg(NH3)6] (NO3)2

Abstract Adiabatic calorimetry measurements show that [Mg(NH3)6](NO3)2 goes from a room temperature phase I to an intermediate phase II below 221 K and then to a phase III below about 110 K, when cooled. On heating, the III to II transition occurs around 145 K. The structural phase transitions and the hysteresis are also reflected in the correlations times of the NH3 reorientation motion as derived from NMR relaxation and quasielastic neutron scattering.

An adiabatic calorimetry study of [Ni(NH3)6](NO3)2

Abstract Specific heat vs. temperature was measured for polycrystalline [Ni(NH3)6](NO3)2 in the temperature range 83–256 K. Two anomalies, one at ≃197 K and one at ≃247 K were found. The corresponding entropy values are 1.21 and 6.05 e.u. In this respect the substance to be similar to the previously measured [Ni(NH3)6](BF4)2, [Ni(NH3)6](ClO4)2 and [Mg(NH3)6](ClO4)2. However, further foundings are different: the intermediate phase, normally existing between 197 and 247 K, may be undercooled until ≃100 K, being metastable below 197 K.