Anna Migdał-Mikuli

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.

Melting and thermal decomposition of [Ni(H2O)6](NO3)2

The two-stage melting process and the thermal decomposition of [Ni(H2O)6](NO3)2 was studied by DSC, DTA and TG. The first melting point at 328 K is connected with the small and the second melting point at 362 K with the large enthalpy and entropy changes. The thermal dehydration process starts just above ca. 315 K and continues up to ca. 500 K. It consists of three well-separated stages, but the sample mass loss at each stage depends on the experimental regime. However, irrespective of the chosen regime, the total of registered mass losses in stage one and two amounts to three H2O molecules per one [Ni(H2O)6](NO3)2 molecule. The remaining three H2O molecules are gradually freed in the temperature range of 440‐500 K in the third stage of the dehydration. Above 580 K, anhydrous Ni(NO3)2 decomposes into NO and NiO. The gaseous products were identified by quadrupole mass spectrometer (QMS), and the solid product was identified by X-ray diffraction (XRD) analysis. # 2001 Elsevier Science B.V. All rights reserved.

Phase Transitions in Crystalline [M(H2O)6](ClO4)2 (M=Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd and Hg)

The results of DSC measurements in the temperature range 140–370 K on nine crystalline compounds of the type [M(H2O)6](ClO4)2, where M=Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd and Hg, are discussed. Anomalies detected in the DSC curves are related to the existence of solid-solid phase transitions and/or to the melting points of these compounds. In consequence of two different hypothetical structural modifications of [Fe(H2O)6](ClO4)2, two DSC curves are obtained. For the compounds with M=Fe, Cd and Hg, new phase transitions have been discovered. The transition temperatures of the other phase transitions are in good agreement with literature data obtained by adiabatic calorimetry. For the compounds with M=Mg, Ni and Cd, DTA measurements were also carried out and the melting points of theses compounds were established.

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.

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.

Phase transition and molecular motions in [Co(NH3)6](ClO4)3 studied by differential scanning calorimetry and infrared spectroscopy

Abstract The phase transition in [Co(NH 3 ) 6 ](ClO 4 ) 3 at T c h =103.25 K (on heating) and at T c c =102.68 K (on cooling) was determined by differential scanning calorimetry. Fourier transform far infrared and middle infrared spectra were measured in the temperature range of 25–295 K. The spectra did not show significant changes at the phase transition temperature. However, there were some characteristic changes in the temperature dependence of the full width at half maximum of the ρ r (NH 3 )F 1u mode. It suggests that the discovered phase transition is connected with a sudden change of the orientational dynamics of the NH 3 groups.

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.

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.

Incoherent quasielastic neutron scattering study of NH3 reorientations in various phases of [Ni(NH36] (NO3)2

Abstract QNS measurements carried out on polycrystalline [Ni(NH 3 ) 6 ] (NO 3 ) 2 revealed that the NH 3 reorientations about Ni-O axes are occuring in all phases. In the high-temperature phase these reorientations, which have a correlation time of the order of picoseconds, occur via an activation process with a barrier of ≈ 1.5 kcal/mol. Below 247 K there exist two phases, a metastable one and a stable one. The NH 3 reorientations in the metastable phase occur via activation over a barrier of ≈0.5 kcal/mol. In the stable phase these orientations occur probably via a reorientation-phonon coupling because the activation barrier is apparently zero. The perfect agreement between the results obtained in the QNS and the calorimetric measurements is pointed out.

Low-temperature phase transition in [Mn(OS(CH3)2)6](ClO4)2 studied by single crystal X-ray diffraction, infrared absorption and Raman scattering spectroscopies.

Single crystal X-ray diffraction studies of [Mn(OS(CH3)2)6](ClO4)2 have shown that the low temperature phase transition, detected by differential scanning calorimetry (DSC) at about 223 K, is associated with the crystal symmetrys reduction from an orthorhombic crystallographic system (Fdd2, No. 43) to a monoclinic one (Cc, No. 9). The analysis of the full width at half maximum of the bands connected with: δd(OClO)F2 and ρ(CH3) vibrational modes in the FT-IR and FT-RS spectra, respectively, registered in the function of temperature, proved that the reorientational motions of ClO4- anions and CH3 groups from (CH3)2SO ligands, began to slow down at temperatures below the phase transition at about 223K. Mean values of activation energy for ClO4- reorientation in the high temperature phase I and low temperature phase II are: Ea(I)≈14 kJ mol(-1) and Ea(II)≈10 kJ mol(-1), respectively. Analogous values for CH3 reorientation are: Ea(I)≈23 kJ mol(-1) and Ea(II)≈1 kJ mol(-1), respectively.