Searching for mixtures of planetary ices V. Naden Robinson, 1,3 - - PowerPoint PPT Presentation

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Searching for mixtures of planetary ices V. Naden Robinson, 1,3 - - PowerPoint PPT Presentation

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Searching for mixtures of planetary ices V. Naden Robinson, 1,3 Miriam Marquez 1 , J. Christiansen 1 , Y. Wang, 2 Y. Ma, 2 and A. Hermann 1 1 School of Physics and Astronomy, The University of Edinburgh 2 State Key Laboratory for Superhard

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Searching for mixtures of planetary ices

  • V. Naden Robinson,1,3 Miriam Marquez1, J. Christiansen1, Y. Wang,2
  • Y. Ma,2 and A. Hermann1

1 School of Physics and Astronomy, The University of Edinburgh 2 State Key Laboratory for Superhard Materials, Jilin University 3 International Centre for Theoretical Physics, Trieste

Workshop on Crystal Structure Prediction, ICTP – January 2019

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  • E. Fermi:

Notes on thermodynamics and statistics, 1953

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Earth

~ 1022-25 GPa ~ 20,000 GPa

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High Pressure

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Experiments

Diamond Anvil Cell (DAC)

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Shock Compression Double Stage NMR Conductivity

X-ray Neutron Raman-IR Camera

[Static] [Dynamic] Generally non equilibrium

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Experiments

Diamond Anvil Cell (DAC)

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Shock Compression Double Stage NMR Conductivity

X-ray Neutron Raman-IR Camera

[Static] [Dynamic] Generally non equilibrium

DOI: 0.1063/1.4930134

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High Pressure

  • Strange Phenomenon

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High Tc

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Exotic states of matter

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Exotic states of matter

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https://www.americanscientist.org/article/on-neptune-its-raining-diamonds

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Partial Melts

K

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Machine Learned Force-field

[1] Woolman, Gavin, et al. "Structural and electronic properties of the alkali metal incommensurate phases." Physical Review Materials 2.5 (2018): 053604. [2] Emma E McBride, Keith A Munro, Graham W Stinton, Rachel J Husband, Richard Briggs, H-P Liermann, and Malcolm I McMahon. Submitted

40 GPa 200 K

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Partial Melts

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Submitted

1000 K

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Crystal Structure Prediction

  • Simplest form: Relax many random unit cells → lowest energy

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many random unit cells Energetic ranking Potentials Ab initio (DFT) QMC Machine Learning

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Crystal structure prediction

  • Particle-swarm optimization
  • Up to four formula units, 500,000 + structures
  • Pressures 5, 10, 20, 30, 50, 80, 100 … 1000 GPa

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Planetary ice mixtures

Our picture of icy planets Planetary ices

  • Mixtures of H2O, NH3, CH4
  • Mixing vs de-mixing?
  • Stoichiometric compounds?
  • Ground state vs high

temperature?

Exoplanets: super-Earths & mini-Neptunes

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Planetary ice mixtures

H2O NH3 CH4

1:1 1:2 2:1 7:4:1

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Ammonia hydrates

Three known phases:

  • AMH: ammonia monohydrate, (H2O)(NH3)
  • ADH: ammonia dihydrate, (H2O)2(NH3)
  • AHH: ammonia hemihydrate, (H2O)(NH3)2

C.W. Wilson et al., J. Chem. Phys. 2015. J.S. Loveday, R.J. Nelmes, Phys. Rev. Lett. 1999.

Experimental phase diagrams:

AMH ADH AHH

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Ammonia water

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High pressure?

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Binary Searches

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[NH3:H2O] phase diagram

* PBE ground state results

  • AMH, ADH: decompose below 1 Mbar
  • AHH, AQH: stable up to 5 Mbar
  • Stabilization by water ionization
  • High pressure favors mixtures with

NH3:H2O ≥ 2

4:1 2:1 24

[1:1] [2:1] [4:1] [1:2]

  • V. Naden Robinson et al., J. Chem. Phys. 149, 234501 (2018).
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Ionization of Water

AMH [1:1] P4/nmm AMH [1:1] P43 AQH [4:1] I4/m AHH [2:1] I4/m

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Proton Transfer

  • V. Naden Robinson et al., J. Chem. Phys. 149, 234501 (2018). 26

[4:1] [2:1]

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Ionization Energy Cost

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AHH [2:1] Higher-pressure phases

Stability to 550 GPa

  • V. Naden Robinson et al., Proc. Natl. Acad. Sci., 2017. 28
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DMA approximants

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In all hydrates

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  • V. Naden Robinson et al., J. Chem. Phys. 149, 234501 (2018).
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More ammonia-rich phases?

Ammonia Quarterhydrate

ADH AMH AHH

AQH (H2O)(NH3)4 O2-(N2H7+)2

Berthold et al, Angew. Chem. Int. Ed., 1988.

I∙(N2H7)

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Full NH3-H2O phase diagram

* PBE ground state results

  • AMH, ADH: decompose below 1 Mbar
  • AHH, AQH: stable up to 5 Mbar
  • Stabilization by water ionization
  • High pressure favors mixtures with

NH3:H2O ≥ 2

  • V. Naden Robinson et al., submitted.

4:1 2:1

T

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Finite Temperature

Harmonic approximation

ADH [1:2] AHH [2:1] AMH [1:1] AQH [4:1]

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Computing simple mixtures

  • M. Bethkenhagen et al., J. Phys. Chem. A, 2015
  • Crystal structure searches
  • Ab initio MD based on ground state structures
  • Observables: EOS, conductivities, viscosities

[1:1] (H2O)(NH3)

[1:2] (NH3)(H2O)2

  • X. Jiang et al., Phys. Rev. B, 2017.

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32 molecules 384 molecules

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Phase Diagrams

[1:1] [4:1] [1:2] [2:1] 35

NVT AIMD

CASTEP

ttotal = 1.53 ns Up to: 1280 atoms 384 molecules

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Plasticity

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ADH [1:2]

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  • X. Jiang et al., Phys. Rev. B, 2017.

384 molecules

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Plasticity

https://www.youtube.com/watch?v=dqizfwcwzvE ADH[1:2] 10 GPa 700 K 38

& double well

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Plasticity

AHH[2:1] 500 GPa 1000 K 39

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AMH [1:1]

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  • M. Bethkenhagen et al., J. Phys. Chem. A, 2015

144 molecules 32 molecules

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Full NH3-H2O phase diagram

  • AMH, ADH: decompose below 1 Mbar
  • AHH, AQH: stable up to 5 Mbar
  • Stabilization by water ionization
  • High pressure favors mixtures with

NH3:H2O ≥ 2

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Full NH3-H2O phase diagram

  • AMH, ADH: decompose below 1 Mbar
  • AHH, AQH: stable up to 5 Mbar
  • Stabilization by water ionization
  • High pressure favors mixtures with

NH3:H2O ≥ 2

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Full NH3-H2O phase diagram

4:1 2:1 43

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Summary & Acknowledgements

Ammonia-water mixtures

  • AHH: sequence of fully de-protonated, ionic

structures

DMA approximants Change in superionicity?

  • AMH: updating high-pressure evolution

H-bond network topologies

  • Full binary H2O-NH3 phase diagram

New quarter-hydrate, AQH Ammonia-rich phases favored

  • Superionic, plastic, and melt lines

categorized for all mixtures

Collaborators

Yanchao Wang Yanming Ma Miriam Marquez

Funding & Computing

Supervisor

Andreas Hermann

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Jacob F. Christiansen

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Research Structure

Crystal Structure Searching → Stable Mixture Stabilities → Finite Temperature MD

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Crystal structure prediction

  • Particle-swarm optimization
  • CASTEP total energy calculations
  • Up to four formula units, 200,000 + structures
  • Pressures 5, 10, 20, 30, 50, 80, 100 … 1000 GPa

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3

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Liquid Electrides

Li [90 GPa, 1500 K]

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1.0 0.0

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AHH phase evolution

Amma P-3m1 AHH-I AHH-II ps-DMA Amma P-3m1 Pnna Pnma H2O+NH3 P [GPa] 5.5 23 65 110 180 505 540

H2O OH- O2-

Above 65 GPa:

  • Fully deprotonated water molecules
  • Ionic structures O2-(NH4+)2

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Disordered region

  • V. Naden Robinson et al., Proc. Natl. Acad. Sci., 2017. 50
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What next?

  • Independent of XC functional
  • IR and Raman Spectroscopy
  • Heat these up
  • Low pressure regime
  • (Superionic) Alloys in the Icy Planets

H2O NH3 CH4

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What's inside?

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ADH [1:2] rcut= 1.15 Å

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Molecular Nature

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Unit types

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ADH [1:2]

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BAC

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Bond life times

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ADH [1:2]

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Number of O/N-H bonds

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AQH [4:1]

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Decomposition chemistry H2, N2, NO

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AHH [2:1]

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Diffusion differences

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AMH [1:1]

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Equation of State

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Preliminary data on superionicity

Ab-initio MD on AHH phases

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Superionicity (WIP)

AQH AHH ADH AMH

Thanks to Jacob F. Christiansen

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Revisiting AMH

No stable phases at high pressures?

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AMH structure searching

Phase Stable to AMH-I 2.5GPa AMH-II 3.5GPa P4/nmm 35GPa P43 140GPa P21m 470GPa

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AMH high-pressure phases

P4/nmm (2012) Ima2 (2015) P43 (2017)

H-bond chain topologies (70GPa) H-bond symmetrisation (140 GPa)

C.f. alkali hydroxides P212121 LiOH, CsOH

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Ionic phase transitions

H2O + NH3 → OH- + NH4+

A.D. Fortes et al., J. Chem. Phys. 2001. G.I.G. Griffiths et al., J. Chem. Phys. 2012.

AMH-I

~5GPa

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Ammonia hemihydrate

(H2O)(NH3)2 — molecular phases

  • CASTEP code
  • PBE functional
  • 1000 eV cutoff
  • Ultrasoft PP
  • rO,N=1.2Å, rH=0.6Å

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Ammonia hemihydrate

Searching for disorder

P-1 Cm C2/m Low symmetry, half-ionic phases (OH-)(NH4+)(NH3)

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Full ionisation of water

AHH-P-3m1 Charge density: Electron Localization Function (ELF): …and hydrogen bonding Ionic structures…

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Finite temperature stability

Harmonic approximation

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AQH h-bond Symmetrization

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ISIS – AMH DMA formation

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