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A phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds

n-Alkane/urea inclusion compounds are crystalline materials in which n-alkane ‘guest’ molecules are located within parallel one-dimensional ‘host’ tunnels formed by a helical hydrogen-bonded arrangement of urea molecules. The periodic repeat distance of the guest molecules along the host tunnels is...

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Autores principales: Couzi, Michel, Guillaume, François, Harris, Kenneth D. M.
Formato: Online Artículo Texto
Lenguaje:English
Publicado: The Royal Society Publishing 2018
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6030336/
https://www.ncbi.nlm.nih.gov/pubmed/30110443
http://dx.doi.org/10.1098/rsos.180058
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author Couzi, Michel
Guillaume, François
Harris, Kenneth D. M.
author_facet Couzi, Michel
Guillaume, François
Harris, Kenneth D. M.
author_sort Couzi, Michel
collection PubMed
description n-Alkane/urea inclusion compounds are crystalline materials in which n-alkane ‘guest’ molecules are located within parallel one-dimensional ‘host’ tunnels formed by a helical hydrogen-bonded arrangement of urea molecules. The periodic repeat distance of the guest molecules along the host tunnels is incommensurate with the periodic repeat distance of the host substructure. The structural properties of the high-temperature phase of these materials (phase I), which exist at ambient temperature, are described by a (3 + 1)-dimensional superspace. Recent publications have suggested that, in the prototypical incommensurate composite systems, n-nonadecane/urea and n-hexadecane/urea, two low-temperature phases II and ‘III’ exist and that one or both of these phases are described by a (3 + 2)-dimensional superspace. We present a phenomenological model based on symmetry considerations and developed in the frame of a pseudo-spin–phonon coupling mechanism, which accounts for the mechanisms responsible for the I ↔ II ↔ ‘III’ phase sequence. With reference to published experimental data, we demonstrate that, in all phases of these incommensurate materials, the structural properties are described by (3 + 1)-dimensional superspace groups. Around the temperature of the II ↔ ‘III’ transition, the macroscopic properties of the material are not actually associated with a phase transition, but instead represent a ‘crossover’ between two regimes involving different couplings between relevant order parameters.
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spelling pubmed-60303362018-07-17 A phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds Couzi, Michel Guillaume, François Harris, Kenneth D. M. R Soc Open Sci Chemistry n-Alkane/urea inclusion compounds are crystalline materials in which n-alkane ‘guest’ molecules are located within parallel one-dimensional ‘host’ tunnels formed by a helical hydrogen-bonded arrangement of urea molecules. The periodic repeat distance of the guest molecules along the host tunnels is incommensurate with the periodic repeat distance of the host substructure. The structural properties of the high-temperature phase of these materials (phase I), which exist at ambient temperature, are described by a (3 + 1)-dimensional superspace. Recent publications have suggested that, in the prototypical incommensurate composite systems, n-nonadecane/urea and n-hexadecane/urea, two low-temperature phases II and ‘III’ exist and that one or both of these phases are described by a (3 + 2)-dimensional superspace. We present a phenomenological model based on symmetry considerations and developed in the frame of a pseudo-spin–phonon coupling mechanism, which accounts for the mechanisms responsible for the I ↔ II ↔ ‘III’ phase sequence. With reference to published experimental data, we demonstrate that, in all phases of these incommensurate materials, the structural properties are described by (3 + 1)-dimensional superspace groups. Around the temperature of the II ↔ ‘III’ transition, the macroscopic properties of the material are not actually associated with a phase transition, but instead represent a ‘crossover’ between two regimes involving different couplings between relevant order parameters. The Royal Society Publishing 2018-06-13 /pmc/articles/PMC6030336/ /pubmed/30110443 http://dx.doi.org/10.1098/rsos.180058 Text en © 2018 The Authors. http://creativecommons.org/licenses/by/4.0/ Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.
spellingShingle Chemistry
Couzi, Michel
Guillaume, François
Harris, Kenneth D. M.
A phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds
title A phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds
title_full A phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds
title_fullStr A phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds
title_full_unstemmed A phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds
title_short A phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds
title_sort phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds
topic Chemistry
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6030336/
https://www.ncbi.nlm.nih.gov/pubmed/30110443
http://dx.doi.org/10.1098/rsos.180058
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