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Structure and thermodynamics of the superionic transition in compounds with the fluorite structure

Paul Fossati 1 Alain Chartier 1 A. Boulle
1 LM2T - Laboratoire de Modélisation, Thermodynamique et Thermochimie
SCCME - Service de la Corrosion et du Comportement des Matériaux dans leur Environnement : DEN/DPC/SCCME
Abstract : Some AX2 binary compounds with the fluorite structure (space group Fm-3m) are well-known examples of materials exhibiting transitions to ionic superconducting phases at high temperatures but below their melting points. This transition is characterised by lambda-shaped peaks in constant-volume heat capacity and linear thermal expansion coefficients , as well as more gradual changes in enthalpy and entropy. The high-temperature superionic states have been described before as either highly defective crystals or part-crystal, part-liquid states where the A ions retain their crystalline order whilst the B ions undergo partial melting. However, no detailed description of the structure of these phases exists. We present here the results of our investigation of the structural changes that occur during superionic transitions and the structural characteristics of the resulting superionic materials. This work is based on atomic-scale molecular dynamics modelling methods as well as computational diffraction techniques. We employed a set of empirical potentials representing several compounds with the fluorite structure to investigate any potential-dependent effect. We show the limitations of our current understanding of the superionic transition in fluorites, as well as the importance of small-scale structure changes and how they are crucial to explain the thermodynamical properties of these materials at high temperatures.
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https://hal-cea.archives-ouvertes.fr/cea-03394603
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Submitted on : Friday, October 22, 2021 - 10:30:04 AM
Last modification on : Tuesday, November 16, 2021 - 4:13:20 AM

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  • HAL Id : cea-03394603, version 1

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Paul Fossati, Alain Chartier, A. Boulle. Structure and thermodynamics of the superionic transition in compounds with the fluorite structure. Journées annuelles du GDR TherMatHT, Sep 2021, Lyon, France. ⟨cea-03394603⟩

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