Nature of the Volume Isotope Effect in Ice

Umemoto, Koichiro; Sugimura, Emiko; Gironcoli, Stefano de; Nakajima, Yoichi; Hirose, Kei; Ohishi, Yasuo; Wentzcovitch, Renata M.

doi:10.1103/physrevlett.115.173005

Cited by 28 publications

(49 citation statements)

References 53 publications

(56 reference statements)

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“…The stress tensor calculation can be used to optimize the unit cell considering both thermal and quantum effects. The most advanced calculations to include these effects without involving PIMD in water have been performed within the quasi-harmonic approximation (QHA) 37,38 . In this scheme the total pressure is obtained expanding the BO energy surface as a quadratic function around its minimum at each volume.…”

Section: Ab-initio Simulation On Ice XImentioning

confidence: 99%

“…Ice XI is the ordered phase of common ice formed below 72 K in the presence of a small amount of an alkali hydroxide 36 . It is commonly used to study quantum effects in water thanks to its great similarity to normal ice (Ih) 37,38 . Ice is characterized by the interplay between intra-molecular covalent OH bonds and inter-molecular hydrogen bonds.…”

Section: Introductionmentioning

confidence: 99%

“…The great difference in strength of inter-molecular and intra-molecular forces makes ice phases acquire a very broad spectra for their vibrational energies, from the very low energy rotons to the large energy vibrons. Moreover, this structure of ice experimentally exhibits at low temperature the negative thermal expansion 39 and the "anomalous isotope volume effect" [37][38][39][40] : if hydrogen is replaced by deuterium, the crystal volume expands in about a 0.1 %. This is the opposite behaviour of what is usually observed when heavier isotope is substituted in the crystal.…”

Section: Introductionmentioning

confidence: 99%

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Pressure and stress tensor of complex anharmonic crystals within the stochastic self-consistent harmonic approximation

et al. 2018

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The self-consisted harmonic approximation (SCHA) allows the computation of free energy of anharmonic crystals considering both quantum and thermal fluctuations. Recently, a stochastic implementation of the SCHA has been developed, tailored for applications that use total energy and forces computed from first principles. In this work, we extend the applicability of the stochastic SCHA to complex crystals with many degrees of freedom, with the optimisation of both the lattice vectors and the atomic positions. To this goal, we provide an expression for the evaluation of the pressure and stress tensor within the stochastic SCHA formalism. Moreover, we develop a more robust free energy minimisation algorithm, which allows us to perform the SCHA variational minimisation very efficiently in systems having a broad spectrum of phonon frequencies and many degrees of freedom. We test and illustrate the new approach with an application to the phase XI of water ice using density-functional theory. We find that the SCHA reproduces extremely well the experimental thermal expansion of ice in the whole temperature range between 0 K and 270 K, in contrast with the results obtained within the quasi-harmonic approximation, that underestimates the effect by about 25%. arXiv:1804.06793v1 [cond-mat.mtrl-sci]

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Section: Ab-initio Simulation On Ice XImentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Pressure and stress tensor of complex anharmonic crystals within the stochastic self-consistent harmonic approximation

et al. 2018

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“…Its structure is known through diffraction experiments 27,29,40,43,44,50,57 and theoretical calculations 41,52 , except for the positions of the hydrogen atoms, which are not easily accessible experimentally. Moreover, isotope effects deriving from the differences between δ-AlOOH and δ-AlOOD 46 indicate that nuclear quantum effects (NQE) play a role, as it happens in other minerals 21,56 . Several studies suggested that the ionocovalent and hydrogen bonds in δ-AlOOH become symmetric under increasing pressure, in contrast to α-AlOOH 17,59 .…”

Section: Introductionmentioning

confidence: 99%

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

Bronstein¹,

Depondt²,

Finocchi³

2017

ejm

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We conducted ab initio molecular dynamics simulations of the δ phase of the hydrous mineral 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 δ-AlOOH undergoes a phase transition from a P 21nm structure with asymmetric and disordered O-H bonds to a stiffer P nnm 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 P nnm 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 P 21nm 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 P nnm phase, which is brought about by the onset of proton tunneling, could be confirmed by quasi-elastic neutron scattering and vibrational spectroscopy under pressure.

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“…This coupling between DFT and QHA has been applied successfully by others to sample finite T and P effects and quantummechanical zero-point energies for perfect crystals [26,27] and defects in metals [28]. However, to our knowledge, employing DFT and QHA to quantitatively decipher the coupled effect of thermodynamic forces (T and P ) on a charged defect in a semiconductor has not been reported previously.…”

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confidence: 99%

Thermomechanical stabilization of electron small polarons in SrTiO3 assessed by the quasiharmonic approximation

2017

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We predict a predominance diagram for electron defects in the temperature-hydrostatic stress space for SrTiO 3 by combining density functional theory and the quasiharmonic approximation. We discovered two regimes where small polarons dominate: under tensile stress at lower temperature due to a larger relaxation volume of the defect , and under compressive stress at higher temperature due to a smaller and larger formation entropy. This provides a means to modulate the electronic conductivity via controlling the underlying charge carrier. Furthermore, the results challenge the common association between larger and charge localization by demonstrating that at high temperature the free electron can induce larger compared to the small polaron. This finding is attributed to the ability of the free electron to generate greater vibrational entropy upon finite isothermal expansion. DOI: 10.1103/PhysRevB.95.161110 An electron can move freely in a periodic crystal unless the lattice is polarizable, in which case the electron can begin to exhibit localization by deforming surrounding ions to form a so-called large polaron. Assisted by lattice vibrations, electron localization can become enhanced to be self-trapped on a single ion. The latter is termed a small polaron or a self-trapped electron. While large polarons traverse crystals in a rapid bandlike fashion, small polarons tend to hop slowly from one ion to the next [1]. Holstein predicted that in a given polarizable crystal, a transition from large to small polaron behavior occurs as the temperature increases above half the Debye temperature D [2]. Understanding and controlling the extent of electron localization in the family of metal oxides underpins their performance in various applications. For example, delocalization is desired for increasing the electronic conductivity of oxides that function for water splitting and CO 2 reduction [3]. On the other hand, the ease of creating oxygen vacancies in reducible metal oxides is generally correlated with localizing electrons on neighboring host cations; that is, reducing them [4][5][6]. Temperature and stress are readily available thermodynamic forces to tune the degree of localization of electronic defects. In this Rapid Communication, we reveal how the localization of electron polarons in the metal oxide SrTiO 3 is controlled by these forces, primarily via changes in relaxation volume of electronic defects with temperature and pressure.Strontium titanate (SrTiO 3 ) is an archetype of the versatile perovskite oxides family. The electronic defects (electrons and holes) of bulk SrTiO 3 give rise to desirable properties not possessed by the underlying perfect crystal such as superconductivity [7] [16] by suggesting that in the dilute limit (i.e., less than 1% defect per unit cell) large polarons prevail, and at higher doping concentrations small polarons prevail. Density functional theory (DFT) calculations at temperature 0 K consistently predict the predominance of free electrons over small polarons [15,16], where it is ...

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Nature of the Volume Isotope Effect in Ice

Cited by 28 publications

References 53 publications

Pressure and stress tensor of complex anharmonic crystals within the stochastic self-consistent harmonic approximation

Pressure and stress tensor of complex anharmonic crystals within the stochastic self-consistent harmonic approximation

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

Thermomechanical stabilization of electron small polarons in SrTiO3 assessed by the quasiharmonic approximation

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