Papers by Author: Dimitar Roussev

Abstract: In the present paper we studied the thermodynamical behaviour under high pressure of two MTe2-type compounds (M = Pd, Pt) by applying the thermodynamical method, which we elaborated in previous studies [1,2]. The two discussed compounds are representatives of the CdI2 structure type, which is bi-dimensional and as such is atypical for the big family of lamellar MQ2- type dichalcogenides (Q=S, Se, Te). Specific of lamellar structure is the strong ionicity of the bonds. Its direct consequence is cleavage obtaining, lubrication properties, anisotropic physic properties. 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) a phase transition to the typical pyrite structure; (ii) a phase rearrangements changing the parameters of the crystal cell but keeping the 2D-type structure. The computation of the volumetric thermodynamical functions showed that both PdTe2 and PtTe2 do not undergo any classical phase transition [1]. But we observed a curious difference in their stability: PtTe2 loosed its stability quite fast and PdTe2 was quite stable. Aiming to clarify if the difference in the volumetric entropy generation was due to different phase rearrangements, we calculated the longitudinal thermodynamical functions. In such a way we detected that both PdTe2 and PtTe2 undergo a phase rearrangement. The change along one of the space axis in both compounds was compensated by the reverse change along the other space axis. Like this no changes at the volumetric level were observed. The longitudinal calculations gave an explanation for the differences in entropy generation at volumetric level: beyond the rearrangement point PdTe2 decreases its entropy generation, i.e. its new arrangement is somehow stable under increasing pressure. While, beyond its rearrangement point PtTe2 increases its entropy generation, i.e. even in the new arrangement it loses stability under increasing pressure. We conclude that both PdTe2 and PtTe2 do not undergo a classical phase transition at volumetric level. At longitudinal level both compounds undergo phase rearrangement. A difference between PdTe2 and PtTe2 is observed in their entropy generation beyond the rearrangement point.

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.