Hydrogen%20bonding%20in%20minerals%20under%20pressure

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• Hydrogen bonding in minerals under pressure
• Bjoern Winkler
• Goethe University Frankfurt a. M.
• b.winkler_at_kristall.uni-frankfurt.de

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Contents

• Motivation
• Methods
• diffraction
• spectroscopy
• modelling
• Examples
• diaspore
• the hydrogarnet substitution
• nominally anhydrous minerals wadsleyite

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Motivation

• Hydrogen bonding plays an important role in a
  very large number of processes
• Numerous aspects are studied
• structure and dynamics of hydrogen bonds in
  hydrous phases
• incorporation of hydrogen (water) into
  nominally anhydrous phases
• where are the hydrogen atoms ?
• how much water can be incorporated ?
• effects of anharmonicity
• Test for theories

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The Hydrogen Bond
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Applicability of diffraction techniques

• single crystal x-ray in DAC
• large P range but hydrogens difficult to
  localize
• single crystal neutron in DAC
• moderate P - neutron Laue diffraction
  (feasibility study worked on VIVALDI_at_ILL)

x-ray powder diffraction at P ? neutron powder
diffraction deuteration necessary, limited P-
range
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IR spectroscopic characterisation

• n(OH) 2700 - 3600 cm-1
• powder and single crystal IR spectroscopy as
  well-established tools, but quantitative
  measurements require calibration
• correlations between n(OH) and d(O...O), d(O-H),
  d(H...O) at ambient pressure well established
  (Libowitsky (1999) and references therein)

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Modelling of hydrogen bonded systems
Force field (empirical potential) models dont
work too well for hydrous silicates and related
materials

• Parameter-free approach
• modeling of crystals periodic boundary
  conditions
• density functional theory (DFT)
• athermal limit, Born-Oppenheimer Approximation
• lattice dynamics from DFPT or finite displacement
  approach
• plane waves pseudopotentials or LCAO basis set

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• Example I diaspore, a-AlOOH

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Diaspore, a-AlOOH
Pbnm, Z 4 a 4.401(1) Å b 9.421 (4) Å c
2.845(1) ŠV 117.68 (8) ų intermediate
H-bond relatively high symmetry relatively
small unit cell simple chemistry
Busing and Levy 1958
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High pressure single crystal diffraction
Single-crystal structure analysis of AlO(OH) at
50 GPa hydrogen atoms cannot be located
(Friedrich et al., 2007, Am. Min) a-AlOOH to d-
AlOOH at 18 GPa Diaspore metastable 30 GPa (and
reflections still narrow)
Diaspore crystal (30 x 20 x 10 µm³) at 52 GPa
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Compressibility (experiment and model)
Exp. and DFT B0 151(2) GPa Literature 85 -
230 GPa
Exp. Open symbols with error bars DFT Filled
symbols
B. Winkler et al. (2001) Eur. J. Mineral 13,
343. A. Friedrich et al. (2007) PCM 34, 145. A.
Friedrich et al. (2007) Am. Mineral. 92, 1640
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Diaspore

• structural behaviour very well predicted
• calc. Winkler et al (2001)
• exp. Friedrich et al. (Phys. Chem. Min, 2007
  Am. Min., 2007)
• predict smooth decrease of a to 9.5o at 50 GPa

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Hydrogen bond stretching motions of diaspore,
AlOOH
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Pressure-induced shift of n(OH)

• predicted shift does not follow correlation
  established at ambient pressure
• but O-H...O is slightly kinked in diaspore
• no experimental data yet

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Hydrogen bonding in diaspore
prediction of dispersion relation for
OH-stretching frequencies shows unexpectedly
large wave vector dependence implies non-local
dynamics
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European Synchrotron Radiation Facility
circumference 844 m energy 6 GeV 40 beamlines
http//www.saxier.org/aboutus/saxs.shtml
http//www.esrf.eu/AboutUs/GuidedTour/
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How to do the experiment?
kin
Q(1.11.11.1)
kout
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IXS set-up
Spot size 30 x 60 ?m2 (H x V)
sample
detector
E
i
Monochromator Si(n,n,n) reflection, n 7 -13
Q 89.98 ?1 tunable by temperature
E
f

Q 4 ?/ ? sin(?)
Analyzer Si(n,n,n) reflection, n 7 -13 Q
89.98 ?2 constant
? 2 d(T) sin?
?d/d ?E/E -?(T)?T ? 2.58.10-6 at RT
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Diaspore
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IXS of hydrogen bonding in diaspore, AlOOH

• there is dispersion of the OH-stretching
  vibration this is not a fully localised mode
• dispersion can be rationalized by simple
  electrostatic model of the H-H interaction

Winkler B. et al., PRL, 2008
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• Example II katoite

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The hydrogarnet substitution

• Garnets X3Y2Z3O12
• X 8-fold coordinated
• Y 6-fold coordinated
• Z 4-fold coordinated
• O 4-fold coordinated
• GrossularCa3Al2Si3O12
• Pyrope Mg3Al2Si3O12

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Compressibility of garnets
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Compressibility of garnets
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The hydrogarnet substitution

• Katoite end-member hydrogrossular
• model for hydrogarnet substitution
• SiO4 replaced by (OH)4
• is there an unusual pressure-induced behaviour
  of the OH bond ?

Nobes et al. (2000) Am. Min., 85, 1706-1715
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Katoite

• Compressibility well described
• compression mechanism for Al,Ca,O in agreement
  with experiment
• O-HO shows conventional behaviour
• hydro-pyrope always unstable w.r.t. components
  due to small size of Mg
• Nobes et al. (2000) Am. Min., 85, 1706-1715
• prompted new experiments by Lager et al., 2005

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Katoite
Fluorinert Lager van Dreele, 1996 Theory
Nobes et al., 2000a, 2000b DAC Lager et al.,
2002 SME Lager et al., 2005
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Katoite
Theory Nobes et al., 2000a, 2000b SME Lager et
al., 2005
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Katoite IR (Lager et al., 2005)
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• Example III zoisite

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Zoisite

• two structurally closely related silicates
  orthozoisite and clinozoisite
• orthozoisite - nearly linear OH...O
• clinozoisite - kinked OH...O (167)
• hydrogen bond has intermediate strength (n(OH)
  3100 cm-1 )

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Zoisite

• pressure-induced shifts are very different
• Winkler et al. (1989)
• orthozoisite dn/dP -34 cm-1/GPa
• Bradbury and Williams (2003)
• clinozoisite dn/dP -5 cm-1/GPa
• generally dn/dP -2 - -5 cm-1/GPa
• some exceptions with (small) blue shifts

Winkler et al. (1989)
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Model calculations

• DFT based
• plane wave basis set / norm conserving
  pseudopotentials and DFPT using the CASTEP code
• atom centered basis set (TZ2P) and finite
  displacement approach using SIESTA code
• athermal limit
• no correction for anharmonicity
• anharmonicity will generally red-shift the
  stretching frequency (by 100 cm-1)
• GGA will generally blue-shift the OH-stretching
  frequency
• recent study (Balan et al., 2008) has shown that
  this results in fortuitous error cancellation

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Results

• very good agreement between independent models
  and between models and experiment for structures,
  elastic behaviour and lattice dynamics
• pressure-dependence of n(OH) of zoisite is indeed
  anomalous
• theo -35 cm-1/GPa,
• exp -34 cm-1/GPa
• linear bond 2.5 elongation of O-H at 10 GPa in
  orthozoisite
• kinked bond in clinozoisite remains kinked, only
  0.5 elongation

doesnt follow structure-frequency correlation

Winkler et al., Phys. Chem. Min (2008)
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• Example IV wadsleyite

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Wadsleyite - structure

• ?-Mg2SiO4 - stable in the transition zone
  (410-525 km depth)
• structure orthorhombic or slightly monoclinic
  (Smyth et al. 1997)
• structure with fully ordered H-defects suggested
  by Smyth (1994)

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Elastic constants
relaxed structure with total energy E0 and volume
V0 after straining the crystal, the energy is
V is the volume of the strained crystal
and
is the pressure taken at V0
is the elastic energy proportional to the strains
The elastic constants are then
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Elasticity of hydrous wadsleyite

• earlier study (Kiefer et al., 2001) used DFT-LDA
  thermal correction for anhydrous wadsleyite
• here DFT-GGA, both hydrous and anhydrous
  wadsleyite
• most drastic change in c55
• B decreases by 15
• exp Holl et al., 2008 get a decrease in B by 12
  for partial hydration

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Wadsleyite - phonons

• exp. (Kohn et al., 2002, Deon and Kochmüller,
  2008)
• complex IR-spectra between
• 3200 3700 cm-1
• DFT model
• IR active modes at 3240 cm-1
• and 3265 cm-1
• OH-flip induces significant changes (modes at
  3370 and 3590 cm-1)
• pressure dependence
• nearly linear red-shift of 2 cm-1/GPa

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• Example V molar absorption coefficients

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Take home messages

• Diffraction studies for the localisation of
  hydrogen positions in minerals are demanding
• Dont use spectroscopy-structure correlation
  established at ambient pressure to infer hydrogen
  positions at high pressures
• DFT models work very well for hydrogen bonds
• (if all assumptions are fulfilled reasonably well
  and the structural model is appropriate)
• Recent developments allow to predict intensity
  changes of Raman and IR spectra as a function of
  pressure, compute molar absorption coefficients

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Acknowledgements

• Frankfurt group, especially D. Wilson, A.
  Friedrich
• ESRF Michael Krisch and Alexei Bosak
• Keith Refson, Victor Milman, Julian Gale, R.
  Nobes, E.V. Akhmatskaya, J. White
• HydroMin collaboration E. Balan, K. Wright, M.
  Blanchard, S. Delattre, M. Lazzeri, F. Mauri, J.
  Ingrin
• Funding DFG, BMBF, DAAD, ESF, CECAM, Psi-k