Center for Molecular Modeling - P. Matthys

EPR and ENDOR analysis of Fe3+ impurity centers in fluoroelpasolite lattices

wim — Mon, 03 Oct 2011 07:13:34 +0000

F. Loncke, H. De Cooman, N.M. Khaidukov, H. Vrielinck, E. Goovaerts, P. Matthys, F. Callens

Physical Chemistry Chemical Physics (PCCP)

9 (39), 5320-5329

2007

Abstract

Fe3+ ions in hexagonal and cubic fluoroelpasolite crystals (AI2BIMIIIF6) have been investigated in a combined Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) study. A detailed analysis of the ENDOR spectra for the nearest 19F and 23Na shells in X (9.5 GHz) and Q band (34 GHz) allowed the complex EPR spectra to be disentangled and to determine the spin Hamiltonian parameters for the various S = 5/2 Fe3+ centres. W-band (95 GHz) EPR measurements as a function of temperature were performed to provide unambiguous evidence about the absolute signs of the Zero Field Splitting (ZFS) and SuperHyperFine (SHF) parameters for Fe3+ in Cs2NaAlF6 as already determined from the ENDOR work. It could be concluded that all principal 19F hyperfine values were positive, in agreement with earlier assignments in the literature for related systems. A comparative analysis of the 19F SHF data for Fe3+ at a perfectly octahedral site in the cubic crystal, and at two slightly trigonally distorted environments in the hexagonal crystals, indicates that the metal-to-ligand distance changes upon doping. The obtained set of parameters concerning one defect in various analogous environments can furthermore be used to test different methods of theoretical calculations for ZFS and SHF values.

DOI

http://dx.doi.org/10.1039/B704406B

Private attachment

07 phys. chem. chem. phys. 9 5320 loncke.pdf

Q-Band EPR and ENDOR of Low Temperature X-Irradiated β-d-Fructose Single Crystals

wim — Fri, 30 Sep 2011 13:52:47 +0000

G. Vanhaelewyn, E. Pauwels, F. Callens, M. Waroquier, E. Sagstuen, P. Matthys

Journal of Physical Chemistry A

110 (6), 2147–2156

2006

Abstract

β-d-Fructose single crystals were in situ X-irradiated at 80 K and measured using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques at Q-band (34 GHz) microwave frequencies. The measurements revealed the presence of at least four carbon-centered radicals stable at 80 K. By means of ENDOR angular variations in the three principal crystallographic planes, six proton hyperfine coupling tensors could be determined and were assigned to four different radicals by the aid of EIE. Two of the radicals exhibit only β-proton hyperfine couplings and reveal almost identical EIE spectra. For the other two radicals, the major hyperfine splitting originates from a single α-proton hyperfine coupling and their EIE spectra were also quite similar. The similarity of the EIE spectra and hyperfine tensors led to the assumption that there are only two essentially different radical structures. The radical exhibiting only β-proton hyperfine couplings was assigned to a C3 centered radical arising from H3 abstraction and the other radical suggested to be an open-ring species with a disrupted C2−C3 bond and a double C2−O2 bond. A possible formation mechanism for the latter open-ring radical is presented. By means of cluster density functional theory (DFT) calculations, the structures of the two radicals were determined and a fairly good agreement between the calculated and experimental hyperfine tensors was found.

DOI

http://dx.doi.org/

Private attachment

06 j. chem. phys. A 110(6)2147 vanhaelewyn.pdf

Field-frequency ENDOR spectroscopy of paramagnetic defects in insulating materials

wim — Mon, 17 Oct 2011 10:31:10 +0000

H. Vrielinck, F. Loncke, H. De Cooman, M.A. Tarpan, D. Zverev, P. Matthys, F. Callens

Talk

Conference / event / venue

16th International Conference on Defects in Insulating Materials (ICDIM) 2008

Aracaju, Brazil

Sunday, 24 August, 2008 to Friday, 29 August, 2008