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Spettroscopia Raman e Grandi Sorgenti (Synchrotron Facilities) Marco Milanesio Dipartimento Scienze e Tecnologie Avanzate Università del Piemonte Orientale, Alessandria, Italy E-Mail: [email protected] http://www.mfn.unipmn.it/~marcomi
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Storage ring producing high brilliance X-ray (1000-1000000 more intense than in normal diffractometers)
HRPD + XAFS
The good news: Synchrotron allows X-ray Powder Diffraction (XRPD) in situ studies (down to ms time res.)
Environmental chambers are used to condition the sample environment: Temperature, pressure, atmosphere, irradiation, laser in situ during XRPD
When a phase undergoes a phase transition its diffraction pattern changes
1) The availability of 3rd generation synchrotrons allowed the use of XRPD experiments at in situ conditions.
2) The addition of an extra dimension, such as time or temperature allowed the investigation of structural properties during solid-state transformations
Template Burning inside TS-1 and Fe-MFI Molecular Sieves: An in Situ XRPD Study. Milanesio, M.; Artioli, G.; Gualtieri, A. F.; Palin, L.; Lamberti, C.J. Am. Chem. Soc.; 2003; 125(47); 14549-14558
Sometime overcame by Raman!!
The bad news: the limits of in situ XRPD
Raman spectroscopy can give complementary information to XRPD, being extremely sensitive to small structural distortions, to changes in the hydration/protonation states, to surface modifications and to changes in the charge/defect distribution and short-range ordering.
Because of the perfect space, time and reaction-coordinate correlation between Raman and XRPD and no bias due to different sample holder/conditioning modes
Difficult or impossible to synchronize separate XRPD and Raman experiments
XRPD: phase transition from the triclinic to rhombohedral phase at 323 K
Raman spectroscopy: small variation of the peak at 775 cm-1 related to rotation of the side chain O-Si-C-C torsion angles
This case study allowed i) to test the first Raman/XRPD implementation with a NIR laser (1064 nm) and ii) to design the better setup for the permanent Raman installation at the SNBL
References on the simultaneous experiments
Raman/Single crystal XRD
Gas blower + Cryostream nitrogen blower:
from 80 to 1300 K
Helium cryostat down to 5-10 K
Diamond Anvil cell: from 0 to 50 GP
Lamp- and laser- induced excitation:
Gas pumping system: from vacuum to 30 bars
Case study 1 - The stearate-hydrotalcite (Mg/Al layered hydroxide) thermal treatment and decomposition (almost no mineralogy!)
Example of application to a low order synthetic nanocomposite organic-inorganic material –conformational/chemical sensitivity of Raman
At 362 K phase transition due to the swelling of the layers
A new peak appeared at 2θ=0.81° (d=42.4 Å). Its intensity increased up to 425 K and its 2θ moved to 0.72° (d=48.2 Å).
The C-H stretching modes (2800-3000 cm-1) and the symmetric and asymmetric C-C stretching modes (at 1044 and 1110 cm-1) are sensitive to the changes in the trans-gauche conformation of the CH2 units and to the packing of the embedded phase.
At high temperature: the chains assume folded gauche conformations and inter-digitation is reduced (Raman).
Raman/XRPD the conformational features of the organic moiety and the structural rearrangement of the inorganic moiety
At room temperature: all-trans stearate chains form an inter-digitated bi-layer while (Raman).