ALBERT

Notizen: In the present paper we studied the thermodynamical behaviour under high pressure oftwo MTe2-type compounds (M = Pd, Pt) by applying the thermodynamical method, which weelaborated in previous studies [1,2]. The two discussed compounds are representatives of the CdI2structure 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 thebonds. Its direct consequence is cleavage obtaining, lubrication properties, anisotropic physicproperties. One of the most interesting points stands on the possibility for realising interactionsbetween the layers of different types of ions. That could be done under high pressure by any of thefollowing transformation processes: (i) a phase transition to the typical pyrite structure; (ii) a phaserearrangements changing the parameters of the crystal cell but keeping the 2D-type structure. Thecomputation of the volumetric thermodynamical functions showed that both PdTe2 and PtTe2 do notundergo 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 inthe volumetric entropy generation was due to different phase rearrangements, we calculated thelongitudinal thermodynamical functions. In such a way we detected that both PdTe2 and PtTe2undergo a phase rearrangement. The change along one of the space axis in both compounds wascompensated by the reverse change along the other space axis. Like this no changes at thevolumetric level were observed. The longitudinal calculations gave an explanation for thedifferences in entropy generation at volumetric level: beyond the rearrangement point PdTe2decreases its entropy generation, i.e. its new arrangement is somehow stable under increasingpressure. While, beyond its rearrangement point PtTe2 increases its entropy generation, i.e. even inthe new arrangement it loses stability under increasing pressure. We conclude that both PdTe2 andPtTe2 do not undergo a classical phase transition at volumetric level. At longitudinal level bothcompounds undergo phase rearrangement. A difference between PdTe2 and PtTe2 is observed intheir entropy generation beyond the rearrangement point

Notizen: In the present study we elaborated a thermodynamical model for analysis of isothermalphase transformations under high pressure. Our study was provoked by the necessity to characterisethe behaviour of MTe2 chemical compounds (M = Pd, Pt) while subjected isothermally to highpressure. As known [1] MTe2 powders are representatives of the CdI2 structure type. This structuretype is a bi-dimensional one and as such is atypical for the big family of lamellar MQ2-typedichalcogenides (M = Pd, Pt; Q = S, Se, Te). Specific of lamellar structure is the strong ionicity ofthe bonds. One of the most interesting points stands on the possibility for realising interactionsbetween the layers of different types of ions. That could be done under high pressure by any of thefollowing transformation processes: (i) phase transition to the typical pyrite structure; (ii) phaserearrangement changing the parameters of the crystal cell but keeping the 2D-type structure. In thisframework our aim was to elaborate a thermodynamical model for analysis of such isothermalphase transformations under high pressure. Our analysis model is designed to answer the followingquestions: (i) if the treated compound undergoes a classical phase transition or a phaserearrangement; (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 bothvolumetric and longitudinal Gibbs free energies and their partial derivatives. We recognise thetransformation to be: (i) a phase transition when it affects the volumetric Gibbs free energy and itspartial derivatives; (ii) a phase rearrangement if it affects the longitudinal Gibbs free energy and itspartial 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 theGibbs free energy (both volumetric and longitudinal). For characterising the degree of stability ofthe treated compound under high pressure we calculate its entropy generation (volumetric andlongitudinal, respectively) during the treatment process. The established model was further appliedto PdTe2 and to PtTe2 while subjected isothermally to high pressure