Papers by Keyword: Isothermal Phase Transition

Abstract: We study thermodynamically the behaviour of PdSe2 while subjected to high pressure under isothermal conditions. The present paper continues the study started in [1]. Here we present the results of the axial calculations and analyses. Specific lengths, linear adjusted Gibbs free energy changes and linear adjusted entropy generations were studied along each spatial axis separately. We found that the first-order transition from PdS2 structure type to pyrite one at 20oC is accompanied by saltatory contraction of a and b specific lengths and respective saltatory expansion of c specific length. Under 300oC all specific lengths contract saltatory. In the transition point under 20oC PdSe2 gains saltatory stability along a and b axis and looses along c one, respectively. Besides, the loose along c axis is bigger than the gains along a and b ones. Under 300oC the transition is accompanied by slight gain of stability along all three spatial axes. Plateaux duration affects the stability of PdSe2 strongly under higher temperature.

Abstract: We study thermodynamically the behaviour of PdSe2 while subjected to high pressure under isothermal conditions. The present paper discusses the volumetric-level calculations and results. Experiments under two certain temperature levels are performed: 20oC and 300oC. Calculations and analyses are done according to the method for thermodynamical analysis developed by us in [1]. We detected the order of phase transition from PdS2 structure type to pyrite one to be first order notwithstanding the temperature level. Values of transition pressure were found to be 12.24 GPa and 9.785 GPa at 20oC and 300oC, respectively. Adjusted entropy generation during compression was calculated aiming to study stability of treated compound. Influence of compression temperature level was analysed, as well as duration of pressure plateaux.

Abstract: In the present study we elaborated a thermodynamical model for analysis of isothermal phase transformations under high pressure. Our study was provoked by the necessity to characterise the behaviour of MTe2 chemical compounds (M = Pd, Pt) while subjected isothermally to high pressure. As known [1] MTe2 powders are representatives of the CdI2 structure type. This structure type is a bi-dimensional one and as such is atypical for the big family of lamellar MQ2-type dichalcogenides (M = Pd, Pt; Q = S, Se, Te). Specific of lamellar structure is the strong ionicity of the bonds. One of the most interesting points stands on the possibility for realising interactions between the layers of different types of ions. That could be done under high pressure by any of the following transformation processes: (i) phase transition to the typical pyrite structure; (ii) phase rearrangement changing the parameters of the crystal cell but keeping the 2D-type structure. In this framework our aim was to elaborate a thermodynamical model for analysis of such isothermal phase transformations under high pressure. Our analysis model is designed to answer the following questions: (i) if the treated compound undergoes a classical phase transition or a phase rearrangement; (ii) which is the order of the phase transition or the phase rearrangement, respectively; and (iii) what is the degree-of-stability of the treated compound under high pressure. To detect if the transformation process is a phase transition or a rearrangement, we compute both volumetric and longitudinal Gibbs free energies and their partial derivatives. We recognise the transformation to be: (i) a phase transition when it affects the volumetric Gibbs free energy and its partial derivatives; (ii) a phase rearrangement if it affects the longitudinal Gibbs free energy and its partial derivatives. The order of the transformation process (phase transition or rearrangement, respectively) is determined by the order of the partial derivative of the Gibbs free energy (volumetric or longitudinal, respectively), which is discontinuous in the transformation point. Hence, we compute the two first partial derivatives (i.e., the first one and the second one) of the Gibbs free energy (both volumetric and longitudinal). For characterising the degree of stability of the treated compound under high pressure we calculate its entropy generation (volumetric and longitudinal, respectively) during the treatment process. The established model was further applied to PdTe2 and to PtTe2 while subjected isothermally to high pressure.