orthoborate crystallizes in the vaterite-type structure and has two polymorphous
forms, viz. a low- und a high temperature one. DTA measurements of YBO3
confirmed a reversible phase transition with a large thermal hysteresis. The
phase transition has been accurately characterized by the application of different
heating and cooling rates (β). Consequently, the extrapolation of the
experimental data to zero β yields the transition points at 986.9C
for the heating up and at 596.5C for the cooling down cycle. These values
correspond to samples just after treatment at 1350C. For samples with
a different ‘thermal history’ other phase transition temperatures
are observed, (e.g. after having performed several heating and cooling cycles).
The linear relationship between the associated DTA signal ΔT=Tonset–Toffset and the square root
of the heating rate β was confirmed, but the relation between Tonset and square root of β
is not found here.
From the empirical data a good linear fitting
between Tonset and
ln(β+1) can be derived.
From the kinetic analysis (Kissinger
method) of the phase transformation of YBO3 an apparent
activation energy of about 1386 kJ mol–1
for heating and of about 568 kJ mol–1 for
cooling can be determined
Authors:D. Vollmer, J. Vollmer, R. Strey, H. Schmidt, and G. Wolf
Differential scanning microcalorimetric measurements on phase transitions in water-oil-surfactant mixtures are presented,
demonstrating that this method is highly sensitive towards small heat changes connected with structural transitions in the
samples. The values for the latent heat of phase transitions are determined and the results are compared with predictions
from mean field theory, emphasizing the role calorimetric experiments can play to identify the most important contributions
to the free energy describing the mixtures. Doing this, the present status of the understanding of temperature dependent phase
transitions in microemulsions is reviewed.
An adiabatic calorimetry was used for some investigations of equilibrium and non-equilibrium phase transitions. For one of
the substances studied (4,4′-di-n-heptyloxyazoxybenzene) it was possible to determine temperature dependence of an order parameter and number of clusters of
high temperature phase in a region of a phase transition. For another substance (liquid 3,4 dimethylpiridine) an anomaly on
the specific heat curves was interpreted as being responsible for a decay of molecules’ clusters.
Non-equilibrium phase transitions were investigated for some liquid crystal substances. The process of transformation between
metastable and stable phases was described quantitatively. The conclusions obtained concern the stability of metastable phases.
Authors:M. Rodová, J. Brožek, K. Knížek, and K. Nitsch
Phase transitions in ternary caesium lead bromide (CsPbBr3) were studied by means of DSC, TMA and high temperature X-ray diffraction. The samples were prepared from the solution by
water evaporation and from the melt. on the DSC curves as well as on the temperature dependence of the lattice constants of
CsPbBr3 only two effects were found belonging to the earlier published phase transitions at 88 and 130C and no further effects.
Linear thermal expansion coefficient α of individual CsPbBr3 modifications were calculated from both TMA and high temperature X-ray diffraction. The structural parameters of the room
temperature orthorhombic phase were refined and the results are presented. CsPbBr3 prepared from the solution contained about 10% of CsPb2Br5 and so the DSC curve of pure CsPb2Br5 was also measured and an effect at a temperature of 68.5C was found.
Authors:V. A. Drebushchak, S. N. Dementiev, and Yu. V. Seryotkin
stellerite, with two close peaks one by one. The first one (∼200 °C) is narrow and high and the second one (∼250 °C) is broader and smaller. The first peak was reported to be the first-order phasetransition with the step change of 3.24% in the UC volume and
There are several approaches for the description of phase transitions (PT). leading to different classification schemes:ApproachClassification scheme bythermodynamicorder (after Ehrenfest)kineticreversibility, quenchability, hysteresisstructuralrelationship of crystallographic axes before and after PTmechanisticactual migration path of atoms during PT The most important techniques for investigating PT include DTA and DSC thermo-optical analysis, melastic neutron scattering, IR and Raman spectroscopy and elastic X-ray and neutron diffraction. The latter, as dependent on the temperature, can give indications on the mechanism of PT. The appropriate application of these techniques is demonstrated on the example of the polymorphism of KPO3.
Authors:J. Igartua, G. Aguirre-Zamalloa, I. Ruiz-Larrea, M. Couzi, A. López-Echarri, and T. Breczewski
The specific heat of N(CH3)4CdBr3 from 50 to 300 K has been measured by adiabatic calorimetry, using both static and dynamic methods. The obtained results
have permitted a careful study of the ferro-paraelectric phase transition the crystal shows at 160 K. The available spectroscopic
data have been used to generate a reliable baseline which accounts for the normal lattice contribution to the specific heat.
These results allow for an accurate estimation of the phase transition thermodynamic functions: ΔH=2620 J·mol−1 and ΔS=18.04 J·(mol°C)−1. These high values are in agreement with the predictions of the 6 well potential Frenkel model.
Authors:M. Leitão, C. Nobre, J. Jesus, and J. Redinha
The study of trans- and cis-1,2-cyclohexanediol by infrared spectroscopy was performed. The variation of the maximum frequency and of the bandwidth of the OH stretching vibration give evidence of the role played by hydrogen bonding in the solid and liquid phases of both isomers and allows to follow the phase transitions. A solid rotator phase is shown for the cis compound.
The influence of the temperature program parameters of an ODSC experiment on the calculated “reversing” and “kinetic” signals has been studied. Mixed orthophosphate salts of KMPO4 (where M=Ni2+, Co2+ and Fe2+) which present at least one structural phase transition have been used for this purpose. On these crystalline compounds we have shown that the non reversing heat flow is partly associated with the formation and disappearance of ferroelastic and ferroelectric domain walls. However a proper choice of the temperature program parameters is important so that the calculated “reversing” and “kinetic” curves have the supposed physical meaning according to the assumptions made for the calculations.