X-ray Absorption Spectroscopy studies at the GILDA

Transcription

X-ray Absorption Spectroscopy studies at the GILDA
X-ray Absorption Spectroscopy studies at the GILDA
beamline (ESRF): the role of ab-initio structural modeling
in data analysis
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Layout
➲
The GILDA beamline
●
●
●
➲
Ab-initio structural modeling for XAS analysis
●
●
●
➲
Layout
XAS station
Novel experimental techniques: XEOL and TR-XAS
BVM
DFT
MD (-DFT)
Conclusion
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
News from the GILDA beamline at ESRF
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Beamline layout
Optic Hutch
•beam sizing
•mono-chromatization
• focalization
Control room
• Remote
instrumentation control
• Data analysis
3 Experimental cabins
• XAS Hutch (Instrumentation for XAS experiments)
• Diffraction Hutch (Instrumentation for XRD experiments)
• “Open Hutch” (Open to user’s experimental apparata)
F. d’Acapito et al. ESRF Newsletter 30 (1998), 42
A new MONO configuration
Si311
Si311
1st crystal split in two
●Si(311): 6-30keV
●Si(755) 18-90 keV
●
Si755
●
Access a wide energy range without intervention.
●
XAS
3 experimental chambers
• Standard sample environment
• User’s sample environment
• ReflEXAFS
Fluorescence detectors
•
2* multi elements HP-Ge
Resolution 200 eV @ 6.4 keV
Max CR 80kcps/element
Digital data collection
X-ray Emission Yield from sample (Arb. Units)
Excit. energy 28170 eV
•
•
•
•
2 10
G. Ciatto et al. JSR 11 (2004), 278-283
Compton
Emission
fit
gaussian
background
Elastic
In-K
4
2.2 10
4
4

2.4 10
2.6 10
Energy (eV)
4
2.8 10
4
3 10
4
Time structures at ESRF
●
16 bunches (spacing = 176 ns)
●
4 bunches (spacing = 704 ns)
●
X-ray pulse ~100ps.
●
●
Stroboscopic pump-probe data collection possible even on a BM
beamline provided that
●
Full bunch rate
●
Reversible processes
●
Process timescale << bunch spacing
Typical experimental layout
Data acq.
XAS
detect.
Sample
ESRF
RF
signal
Driver
Pulse generator
●
sample excited in phase with the bunches
●
Fixed time delay pump-probe
●
Use of slow detectors.
XEOL
st
1 Lens
2nd Lens
Fibre
Sample
Detector
Detectors: Si-PIN diode and PMT
Range 300 – 1100 nm
2014: filters for band selection
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
The role of Structural modeling in the XAS data analysis
(and comprehension)
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
➲
XAS provides easily
– Distance with neighbors
– Number and nature of neighbors
–
–
So, what ?
–
➲
➲
➲
➲
Need of tools for interpreting the data in a more general vision.
Comparison with theories for structural prediction.
Easier job due to the increase of computing power in recent
years.
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Bond Valence Model
I. D. Brown, The Chemical Bond in Inorganic Chemistry: The Bond Valence Model, Oxford University Press, 2002.
➲
➲
➲
➲
➲
➲
➲
Empirical method, useful for bonds with high ionic character
Defines a relation between the ith cation valence Vi, bond
length rij and number of neighbors
V i =∑bonds j sij
r ' o−r ij
B
si=e
Cation-anion parameters r'0 and B tabulated basing on crystal
structures
(http://www.iucr.org/resources/data/datasets/bond-valence-parameters)
Calculator:
http://www.esrf.eu/UsersAndScience/Experiments/CRG/BM08/Users/UserUtilities
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Smalt: Co based potash glass (very basic).
●
CoII ions, blue hue
● Tetrahedral environment
● Degradation to grey color with time (and humidity).
● K moves out from the glass grains. Loosing K the
glass becomes acidic.
●
●
More acid environment: Co switches to octahedral
sites
●
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Deterioration of smalt pigment
1
BVM Co
0.8
2+
0.6
s
ij
N=4
0.4
N=6
0.2
1.94 A
2.09 A
0
1.6
1.8
2
2.2
2.4
2.6
2.8
3
3.2
R (Ang. )
ij
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Modelign a octahedron distortion
CoII-O case
● BVM permits to
modelize the
distortion keeping
constant the valence
● Easily implemented
in XAS fitting codes.
●
Using Bond Valence Model for XAS analysis
F. d'Acapito et al. JNCS accepted 2014
Mg+Er -doped Silica Optic Fiber preforms
● Looking for possible structural origin of the
modified optical response with growing Mg
content
●
Density Functional Theory
➲
➲
➲
Necessary in a general case
Useful when bond parameters are not available.
Several powerful codes
●
●
●
➲
VASP
AbInit
Quantum Espresso...
Predictions
●
●
Structure
Site energetics
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
M. Rovezzi et al.
●
Main issue in Magnetic SMC: precipitates.
●
QE code
● LSD-GGA-U
● U fixed from previous studies.
●
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Beamline SAMBA @ SOLEIL
● Structure simulated with DFT
● XANES calculated with Real Space
methods (FeFF)
● Evidence of Co
-VO complexes by
Zn
looking at the 1-2 valley and 3 peak.
● Complexes linked to the
magnetization saturation.
●
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
What makes amethyst different
from other Fe-bearing quatz.
● Hypotesei
●
Fe4+
●
Fe3+ with Li or H
●
●
Fe mainly as Fe3+
●
Fe2+ < 20%
●
●
●
DFT
● LSD-GGA-U
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Site of Fe in amethyst
Fe3+ + H+
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Fe4+
Color centers easily created in LiF upon X-ray exposure
● Useful for X-ray imaging.
● Pb added to increase sensitivity
●
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
EXAFS data
DFT
●VASP
●64 atoms supercell
●GGA
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Site energetics
Equilibrium
F2
PbF2
LiF
2µF=EF
µPb+2µF=EPbF2
µLi+µF=ELiF
̂ μ= ⃗
A∗⃗
E
⃗ =μ
Â−1∗E
⃗
E f =∑i N i∗μ i
Doping process
PbLi+VLi-f
 PbLi+VLi-n
Pbi+2VLi-f  Pbi+2VLi-n
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
MD-(DFT)
➲
Useful to simulate whole XAS/XANES spectra
●
●
Careful ! Classic MD fails at low temp.
Computationally heavy
●
➲
Cases involving complex environments
●
●
Many bond distances / ligands
High configurational disorder
●
➲
Additional Support to static DFT calculations
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
BM29 beamline, ESRF
● Hg-H O pair potentials
2
determined by QM methods
● Classical MD
● Equilibration time 5ns
● Snapshots every 12.5 ps
●
●
MXAN code for XANES
generation
●
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Corresponding
structure with 2
shells of H2O
molecules
Experimental XANES data (dots) with the MDderived XANES spectrum
Evidence of 7 coordination for Hg
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Method:
● Loeffen & Pettifer PRL 76 636
● Vila, Rehr, Rossner, Krappe, PRB 76 014301.
●
Simulate the structure via DFT
● Calculate the dynamical matrix
● Calculate the DWF for the most important paths
● Simulate the spectrum
●
●
●
DM codes non available for any DFT code.
●
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
TEM from M. Jamet et al. Nature materials 5 (2006), 653
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Energetics of the various structures
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
XAS data simulation via MD-DFT
DFT:
●LSDA-GGA
●96 atoms spc
MD
●1000 time steps, 2fs
●NVT
●T target 300K
●XAS: Last 200 frames
averaged
●
The interface disorder of the
NC permits to reproduce the
XAS data
●
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Found Zn and Fe in yellow Pb animonates
● Presumably added on purpose
● Do they enter the structure of the crystal ?
●
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Zn site: static DFT
Zn in Pb
● Zn-O: 2*1.762, 6*2.679
● Zn-Pb: 6* 3.553
● Zn-Sb: 6* 3.669
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Zn site: spectrum simulation
MD-DFT
NVT, 300K
● 1000 steps
● 2fs/step
● Last 200 spectra averaged
●
●
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr
Conclusion
➲
➲
➲
➲
GILDA: well adapted to XAS on diluted systems
New opportunities: TR-XAS and XEOL
Ab-initio methods necessary for a more complete
interpretation of XAS data
●
●
●
Site geometry
Site energetics
Dynamics
F. d'Acapito, CNR-IOM-OGG, dacapito@esrf.fr