[NiL3]X2 (where L=N-phenylethane-1,2-diamine and X=I− and ClO4−), [NiL2X2] (X is Cl−, Br−, NCS−, 0.5SO42− or 0.5SeO42−) and [NiL2(H2O)2](NO3)2 have been synthesized from solution and their thermal study has been carried out in the solid phase. [NiL2Cl2] upon heating undergoes irreversible endothermic phase transition (142–152C, ΔH=0.35 kJ mol−1) without showing any visual colour change. This phase transition is assumed to be due to conformation changes of the diamine
chelate rings. NiLCl2 and NiL2.5I2 have been prepared pyrolytically from [NiL2Cl2] and [NiL3]I2 respectively in the solid state. [NiL2(H2O)2](NO3)2 upon heating undergoes deaquation-anation reaction without showing any visual colour change. [NiL2X2] (X is Cl−, Br−, NCS−), [NiL2(H2O)2](NO3)2 and [NiL2(NO3)2] possess trans-octahedral configuration, whereas, [NiL2X2] (X is 0.5SO42− or 0.5SeO42−) are having cis-octahedral configuration. Amongst the complexes, only NiLCl2 shows unusually high (5.1 BM at 27C) magnetic susceptibility value.
Phase transitions which depend on grain size induce very interesting properties in materials such as zirconia or barium titanate.
A new and rigorous thermodynamic treatment of this type of phase transition is proposed with consideration of the surface
phenomena. An interpretation is given of the observed differences when the material—particularly BaTiO3—under consideration is a fine grain powder or is a fine grain ceramic.
Authors:K. Gavrichev, A. Bolshakov, D. Kondakov, A. Khoroshilov, and S. Denisov
Differential scanning calorimetry, differential thermogravimetry, X-ray analysis and electronic microscopic studies of thermal
transformations of PbO2 were carried out. Formation of fine dispersed (less than 100 nm) particles of α-PbO was observed at PbO2 thermal decomposition at heating to 580°C. Reverse reaction of Pb3O4 formation from PbO was found at cooling and annealing at 400°C in air. At heating of α-PbO to 650°C the particle growth to
1 μm with formation of β-PbO took place. Thermal decomposition with formation of β-PbO particles with size from 0.3 to 1 μm
at PbO2 heating to 650°C was observed. Transition from PbO to Pb3O4 at cooling of sample heated to 650°C was not detected. Interpretation of observed phenomena from the point of view of particle
size influences on the shift of α-PbO↔β-PbO phase transition temperature and on the chemical activity of phases are presented.
Authors:S. Abboudy, A. E. Hamed, M. E. Kassem, and L. I. Abou-El-Nasr
The influence of stoichiometric ratio on the phase transition of sodium potassium sulphate (NaxK1−x)2SO4 crystals is investigated in the temperature range 300–500 K. The stoichiometric ratiox is chosen to bex=0.2, 0.3, 0.4, 0.8 and 0.9. The study has been carried out by using differential scanning calorimetry, DSC technique. The value of the specific heat,Cp, at the transition temperature,Tc, increases asx is increased up tox=0.4 and then decreases for higher values ofx. The change in the specific heat, ΔCp, is following a relation of the form ΔCp ΔCp α(T−Tc/Tc)−α withα = 0.12 which is in a fair agreement with the 3d Ising model. Insertion of Cu2+ ions into the lattice of (NaxK1−x)2SO4 crystals leads to a multiple peak in the temperature dependence ofCp. The results are discussed from thermodynamical point of view.
A DSC investigation of severaln-alkylammonium salts is reported. The results of this investigation can be used to help decide which of several crystalline forms should be studied by X-ray diffraction for the best correlation between the liquid and solid states.
Authors:M. E. Kassem, A. E. Hamed, M. El-Osairy, and A. M. Okaz
A specific heat study of (Li1−xKx)2SO4 — LKS — mixed crystals has been made in a wide concentration rangex covering the temperature range 300–800 K. The phase diagram was determined. It is shown that a variation of the Li+:K+ concentration changes the critical behaviour of the specific heat. The results are discussed in terms of scattering mechanisms of energy carriers.
For the purpose of finding temperature standards for DTA at the temperature below 150°, some solid transitions of low molecular organic compounds were measured by differential scanning calorimetry. Among fourteen materials, we have concluded to select transitions for hexachloroethane (71°) and hexamethylbenzene (110°) as candidates for such standards.
Dried samples of ammonium nitrate (AN) containing 1, 3, 5 mol% KNO3, RbNO3 and CsNO3 were investigated with temperature resolved X-ray diffraction. The samples were cycled with 2 temperature programs from −70° to 100° and −70° to 150°C, resp. DSC measurements were made for comparison.