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Thermal and nuclear quantum effects in the hydrogen bond dynamical symmetrization phase transition of delta -AlOOH

Yael Bronstein, Philippe Depondt and Fabio Finocchi
Thermal and nuclear quantum effects in the hydrogen bond dynamical symmetrization phase transition of delta -AlOOH
European Journal of Mineralogy (June 2017) 29 (3): 385-395

Abstract

We conducted ab initio molecular dynamics simulations of the delta phase of the hydrous mineral aluminium oxide hydroxide (AlOOH) at ambient temperature and high pressure. Nuclear quantum effects were included through a Langevin dynamics in a bath of quantum harmonic oscillators. We confirm that under increasing pressure delta -AlOOH undergoes a phase transition from a P2 (sub 1) nm structure with asymmetric and disordered O-H bonds to a stiffer Pnnm phase with symmetric hydrogen bonds, which should be stable within the pressure and temperature ranges typical for the Earth's mantle.The transition is initially triggered by proton tunneling, which makes the mean proton position to coincide with the midpoint of the O-O distance, at pressures as low as 10 GPa. However, only at much larger pressures, around 30 GPa as previously found by other calculations, the Pnnm phase with symmetric hydrogen bonds is stable from the classical point of view.The transition is also characterized through the analysis of the H-O stretching modes, which soften considerably and fade out around 10 GPa in the P2 (sub 1) nm structure, when thermal and nuclear quantum effects are taken into account in the simulations. At variance, the harmonic picture is not adequate to describe the highly anharmonic effective potential that is seen by the protons at the transition. Finally, we propose that the picture of a dynamical transition to the high-symmetry and proton-centered Pnnm phase, which is brought about by the onset of proton tunneling, could be confirmed by quasi-elastic neutron scattering and vibrational spectroscopy under pressure.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 29
Serial Issue: 3
Title: Thermal and nuclear quantum effects in the hydrogen bond dynamical symmetrization phase transition of delta -AlOOH
Affiliation: Universite Pierre et Marie Curie, Institut des Nanosciences de Paris, Paris, France
Pages: 385-395
Published: 201706
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 54
Accession Number: 2017-077516
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201741
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