A Thermodynamical Model for Analysis of Isothermal Phase Transformations under High Pressure

2007 ◽  
Vol 553 ◽  
pp. 57-62
Author(s):  
Veneta Grigorova ◽  
Dimitar Roussev
Keyword(s):  
Phase Transition ◽  
High Pressure ◽  
Free Energy ◽  
Phase Transformations ◽  
Gibbs Free Energy ◽  
Structure Type ◽  
Transformation Process ◽  
Partial Derivatives ◽  
Degree Of Stability ◽  
Thermodynamical Model

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.

High pressures and the structure of solids of geochemical and geophysical interest

1992 ◽  
Vol 56 (384) ◽  
pp. 373-383 ◽  
Author(s):  
C. H. L. Goodman
Keyword(s):  
High Pressure ◽  
Free Energy ◽  
Phase Transformations ◽  
Phase Diagrams ◽  
Gibbs Free Energy ◽  
Amorphous Materials ◽  
High Pressures ◽  
Opposite Sign ◽  
Amorphous Solids ◽  
Radius Ratio

AbstractPressures of 10 GPa and above can bring about phase transformations in many oxides, an effect of great interest to geochemists and geophysicists. We can interpret such behaviour as due to the differential compressibility of 'anion' and 'cation' leading to a progressive rise in radius ratio with pressure, and hence, on the classic crystallochemical picture, eventually to an increase in co-ordination number (though with complications which make prediction difficult). More generally, pressure affects Gibbs free energy G directly; for oxides a pressure of 5 GPa gives, very roughly, the same contribution to G as 100°C in temperature (though with opposite sign). Thus high pressure significantly affects the shape and structure of phase diagrams, showing increasingly important effects above, say, 10 GPa—but again prediction can be difficult. However these two complementary approaches to the effects of pressure, helpful though they can be conceptually, are 'crystal-based' and totally neglect another rather littleknown but potentially important effect--the formation of amorphous solids; 'polymers' and glasses. Since amorphous materials are 'non-equilibrium' they are not readily dealt with theoretically; also, since they are difficult to detect by standard crystallographic techniques, they can be overlooked experimentally. The pressure-induced formation of amorphous solids could have significant implications for both geochemistry and geophysics.

Thermodynamical Behaviour of PdSe2 while Subjected Isothermally to High Pressure – Volumetric Level Analysis

2008 ◽  
Vol 273-276 ◽  
pp. 271-276
Author(s):  
Veneta Grigorova
Keyword(s):  
Phase Transition ◽  
High Pressure ◽  
Entropy Generation ◽  
Structure Type ◽  
Temperature Level ◽  
First Order ◽  
Order Of Phase Transition ◽  
Temperature Levels ◽  
Level Analysis ◽  
Compression Temperature

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.

A tetragonal polymorph of SrMn2P2made under high pressure – theory and experiment in harmony

2017 ◽  
Vol 46 (21) ◽  
pp. 6835-6838 ◽  
Author(s):  
Weiwei Xie ◽  
Michał J. Winiarski ◽  
Tomasz Klimczuk ◽  
R. J. Cava
Keyword(s):  
Phase Transition ◽  
High Pressure ◽  
Tetragonal Phase ◽  
Ambient Pressure ◽  
Structure Type ◽  
Space Group ◽  
Type Space ◽  
Tetragonal Phase Transition ◽  
Theory And Experiment

A trigonal–tetragonal phase transition in SrMn2P2is proposed and confirmed experimentally under high pressure. At ambient pressure, SrMn2P2crystallizes in the primitive trigonal La2O3structure type (space groupP3̄m1) in blue. Under high pressure, the tetragonal ThCr2Si2structure type (space groupI4/mmm) in red is more stable.

Pressure-Induced Nano-Crystallization of Y2O3

2011 ◽  
Vol 1297 ◽  
Author(s):  
Stuart Deutsch ◽  
Jafar F. Al-Sharab ◽  
Bernard H. Kear ◽  
Stephen D. Tse
Keyword(s):  
Phase Transition ◽  
Grain Size ◽  
High Pressure ◽  
Phase Transformation ◽  
Monoclinic Phase ◽  
Transformation Process ◽  
Coarse Grained ◽  
Nanocrystalline State ◽  
Phase Transformation Process ◽  
Reversible Phase

ABSTRACTA reversible-phase transformation process to convert coarse-grained polycrystalline cubic-Y2O3 directly into the nanocrystalline state is being developed. The process involves a forward cubic-to-monoclinic phase transition under high pressure and a backward transformation from monoclinic-to-cubic under a lower pressure. The process has been used to reduce the grain size of fully dense cubic-Y2O3 from 300 μm to 0.1 μm. A surface modification effect, comprising a columnar-grained structure, has also been observed. Preliminary work indicates that the surface structure is modified, apparently formed by interaction between the graphite heater and sample.

Comparison between the Thermodynamical Behaviour of PdTe2 and PtTe2, while Subjected Isothermally to High Pressure

2007 ◽  
Vol 553 ◽  
pp. 63-68
Author(s):  
Veneta Grigorova ◽  
Dimitar Roussev ◽  
Stephane Jobic
Keyword(s):  
Phase Transition ◽  
High Pressure ◽  
Entropy Generation ◽  
Direct Consequence ◽  
Structure Type ◽  
Different Types ◽  
The Difference ◽  
Thermodynamical Functions ◽  
Transformation Processes ◽  
Lubrication Properties

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.

scholarly journals Magnetic Phase Transition, Elastic and Thermodynamic Properties of L12-(Ni,Cu)3(Al,Fe,Cr) in 3d High-Entropy Alloys

Crystals ◽  
2020 ◽  
Vol 10 (12) ◽  
pp. 1102
Author(s):  
Li Ma ◽  
Zhi-Peng Wang ◽  
Guo-Hua Huang ◽  
Jin-Li Huang ◽  
Ping-Ying Tang ◽  
...  
Keyword(s):  
Phase Transition ◽  
Free Energy ◽  
Gibbs Free Energy ◽  
Magnetic Phase Transition ◽  
Magnetic Phase ◽  
High Entropy Alloys ◽  
High Entropy ◽  
Structure Phase ◽  
Temperature Dependences ◽  
Significant Temperature

The phase stability and elastic properties of paramagnetic (PM), ferromagnetic (FM) and antiferromagnetic (AFM) phases in L12-(Ni,Cu)3(Al,Fe,Cr) alloy are first investigated using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA). The result shows the AFM structure phase of the three is the most stable in the ground state. Calculated elastic constants show that all the phases are mechanically stable, and have uncovered that L12-(Ni,Cu)3(Al,Fe,Cr) can achieve good strength and ductility simultaneously. Then, crucial thermal properties are described satisfactorily using the Debye–Grüneisen model, showing heat capacity, Gibbs free energy G, the competitive contribution of entropy −TS and enthalpy H exhibiting significant temperature dependences. Moreover, the magnetic phase transition thermodynamics was studied, which suggests that −TS has a primary contribution to Gibbs free energy and may play a key role in the phase transition. The present results can benefit the understanding of the mechanical, thermodynamic and magnetic properties of the L12 structure phase in 3d high-entropy alloys.

Controlling CoSi2 nucleation : the effect of entropy of mixing

2000 ◽  
Vol 611 ◽  
Author(s):  
C. Detavernier ◽  
R.L. Van Meirhaeghe ◽  
K. Maex ◽  
F. Cardon
Keyword(s):  
Phase Transition ◽  
Free Energy ◽  
Gibbs Free Energy ◽  
Small Difference ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Si Substrate ◽  
Entropy Of Mixing ◽  
Nucleation Barrier

ABSTRACTIt is generally known that nucleation effects strongly influence the CoSi to CoSi2 phase transition. According to classical nucleation theory, the small difference in Gibbs free energy between the CoSi and CoSi2 phase is responsible for the nucleation barrier. Adding elements that are soluble in CoSi and insoluble in CoSi2 will influence the entropy of mixing, and thus change ΔG. In this way, the height of the nucleation barrier may be controlled.By depositing Fe or Ge (respectively replacing Co and Si in the CoSi lattice) in between the Co and the Si substrate, we were able to increase the nucleation barrier. In the presence of Ni, the nucleation barrier is lowered, and low-resistive disilicide is formed at lower temperatures.

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