T. Stanek

An adiabatic calorometry study of [Mg(H2O)6](ClO4)2

Abstract Specific heat vs. temperature was measured for polycrystalline [Mg(H2O)6](ClO4)2 in the temperature range 90-350 K. Four anomalies in specific heat curve are found at about 108 K, 168 K, 273 K and 325 K. The entropy changes indicate a configurational disorder of ClO-4 ions in the high temperature phases. The results are compared to those previously obtained for [Ni(H2O)6](ClO4)2. On the basis of analogy one may expect an existence of a phase transition below 90 K for [Ni(H2O)6](ClO4)2 not yet detected.

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.

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.

Adiabatic calorimetry and neutron diffraction studies of phases and phase transitions in [Ni(ND3)6](NO3)2

Abstract Calorimetry, X-ray and neutron diffraction measurements have been performed on [Ni(ND 3 ) 6 ](NO 3 ) 2 on heating and cooling to study the phase transitions which occur, and to determine the structure of the different phases which are formed. The high temperature phase, phase I, has a face centered cubic structure of space group Fm3m and transforms on cooling to a phase II of simple cubic structure, space group Pa3. On further cooling an orthorhombic deformation leads to phase III. Below 80 K a further splitting of the neutron peaks gives evidence of a phase IV of monoclinic structure.

Odd-even alternation of freedom of alkyl chain molecular motions in nematic and isotropic liquids as observed by the neutron incoherent scattering

Abstract Quasielastic and elastic neutron incoherent scattering patterns were obtained for nematic and isotropic liquid phases of substances belonging to the PAA-homologous series (n=1–5). The spectra were explained as arising from intramolecular reorientations of and in the alkoxy tails. The correlation time region of these motions is several ps. The intensity of the elastic component of the spectra exhibits the odd-even effect. It is explained as correlated to the orientational order parameter S which shows also an odd-even alternation, and causes an alternation of steric hindrance to molecular motions. The results for nematic and isotropic phases are similar. We believe that this is due to a survival of the short range orientational order to the isotropic phase. The substance with n=2 seems to be exceptionally mobile, which still needs an explanation.

Intense quasielastic neutron scattering of 4,4′-diethyoxyazoxybenzene and its connection with the inelastic neutron scattering spectrum

Abstract This is a continuation of a previous study of the odd-even effect for compounds of the series of 4,4′-di-n-alkyloxyazoxybenzenes. The reduction of the elastic component of the neutron scattering spectra (the elasticity depression), studied earlier at constant ΔT from the clearing point, has now been studied at constant temperature. The ethyoxy member of the series shows an exceptionally large elasticity depression; this is interpreted as evidence of a small steric hindrance for molecular motions due to the high nematic order parameter. This statement has been corroborated by the inelastic neutron scattering measurements for solid compounds of the series.

Existence of stochastic rotational motions in the glass form of nitric acid monohydrate

Abstract Incoherent quasielastic neutron scattering spectra were measured of the glass form of the nitric acid monohydrate. The spectra indicate that stochastic proton jumps take place in the glass form. Two models of such jumps were tested: one connected with H 2 O-groups reorientations by 180°, and one with H 3 O + -ions reorientations by 120°. Reorientational correlation times are of the order of several picoseconds.

Quasielastic neutron scattering (QNS) study of cation rotation in (CH3NH3)5Bi2Cl11, (CD3NH3)5Bi2Cl11 and (CH3NH3)5Bi2Br11

Abstract Rotation of (CH3NH3)+ cations in crystalline (CH3NH3)5Bi2Cl11, (CD3NH3)5Bi2Cl11 and (CH3NH3)5Bi2Br11 was studied by QNS. The model of instantaneous 120° jumps of whole cations around their C–N axes was fitted to the experimental data. The parameters describing reorientation obtained for two types of cations agree well with the results of NMR experiments.