Filiep Maes

Adsorption of carbonate-derived molecules on the surface of carbonate-containing apatites

Carbonated apatites synthesized at low temperatures were dried until constant weight at 25 °C. Two 99%13C-enriched samples were examined with EPR after X-irradiation. The observed spectra are interpreted in terms of five paramagnetic radicals, i.e. on O–, two CO–3 and two CO–2 radicals. As a result of the 13C hyperfine coupling, practically no signals were found in the region around g= 2, where EPR signals originating from 12C-containing radicals (commonly called 12C signals) are to be expected. Afterwards, the apatite powders were spread out in order to ensure a good contact of the apatite surface with the surrounding atmosphere. The samples were again X-irradiated and re-examined with EPR. Strong 12C signals were detected and attributed to surface radicals. Two different paramagnetic species were tentatively identified as CO–2 and CO– radicals adsorbed on the apatite surface. The g values for these radicals are g1= 2.0030, g2= 2.0015, g3= 1.9973 for the CO–2 radical and g1= 2.0058, g2= 2.0041, g3= 2.0023 for the assumed CO– radical.

33S splittings of some sulphur centres in KCl and NaCl

A new sulphur centre has been detected in KCl by means of EPR. The defect is shown to be the same as the B centre detected already by Matthys and co-workers in NaCl. By enriching the sulphur centres with 33S it is shown to be an S2- ion in a divacancy site, in contrast to the S2- ion detected two decades ago and located in a single anion vacancy by Vannotti and co-workers. In KCl both varieties of S2- may coexist. Further experimental evidence is also presented to identify the earlier reported A centre as an interstitial S- ion.

Temperature dependence of O–3 electron paramagnetic resonance signals in KCl

An ozonide molecular ion O–3 has been detected by EPR spectroscopy in KCl single crystals doped with 2% KNO3 in the melt and X-irradiated at room temperature. The temperature dependence of the O–3 EPR signals was investigated from 10–60 K and a gradual transition from monoclinic to orthorhombic g-tensor symmetry was found. An unexpectedly strong variation of two principal g values, one corresponding to the direction perpendicular to the molecular plane, the other with the axis connecting the two outer oxygens, was observed. These results are discussed in relation to the possible allocation of the O–3 to a mono- or a tri-vacancy site. The latter model has been well established by other authors for S–3 and Se–3 defects in several alkali-imetal halides. Other defects with an electronic structure comparable with that of O–3, for instance SO–2 and SeO–2, are believed to replace only one halide ion.

ENDOR investigation of S2–, SSe– and Se2– defects in NaCl

A single-crystal ENDOR study of S2–, Se2– and SSe– defects in NaCl is presented. ENDOR signals corresponding with interactions with the nearest neighbouring Na+ ions are found. For each interaction, the superhyperfine (SHF) and nuclear quadrupole coupling tensors are determined. These ENDOR results can be explained only by assuming a monovacancy model, i.e. the S2–, Se2– or SSe– ion replaces a single Cl– ion on a lattice site. Analysis of the ENDOR data also reveals that the nearest neighbouring Na+ ions are displaced with respect to their lattice position. The size of the defect ions suggests that they will be incorporated into the NaCl lattice under great strain.

An ENDOR analysis of a diatomic sulphur defect in RbI

In this article, an ENDOR analysis of the S2- defect in RbI is presented. A complete angular variation of one set of 85Rb and 87Rb ENDOR transitions is measured. The corresponding hyperfine and nuclear quadrupole coupling tensors are determined. The orientation of the principal axes of these tensors is in good agreement with the overall D2h symmetry of the S2- ion in the lattice. The ENDOR results clearly lead to a monovacancy model, wherein the S2- ion is replacing one I- ion on a lattice site. The linewidth of the EPR signals can be explained using the ENDOR data. A comparison with previous ENDOR studies of O2- and S2- defects in rubidium halides is made.

EPR study of NaCl: CO2– and NaCl : SO2–

Two new triatomic molecular ions with C2v symmetry i.e. CO2– and SO2– have been detected in NaCl by means of EPR. By analysing the 13C hyperfine tensor of the CO2– radical the spin densities in the 2s and 2pz carbon atomic orbitals can be calculated. Since the 14N hyperfine tensor of the isoelectronic NO2 molecule in the same host lattice is also known, a comparison between the spin densities in both defects can be made. In addition, the spin densities for both defects are calculated using ab initio methods and are compared with the experimental results.As is the case for the smaller CO2– ion, the SO2– radical is assumed to substitute for only one halide ion. The monovacancy model for the SO2– ion is discussed. Such a model is in accordance with that for the smaller ozonide O3– ion. On the other hand, the larger S3 and Se3– ions occupy a trivacancy site.

ENDOR study of RbCl: S2–

A recent EPR study revealed an exceptionally well resolved superhyperfine structure in the spectrum of RbCl : S2– for some specific orientations of the magnetic field. In this work a complete angular variation of two sets of 85Rb and 87Rb ENDOR transitions is presented. The corresponding hyperfine and nuclear quadrupole coupling tensors were determined. The orientation of the principal axes of these tensors is in agreement with the overall D2h symmetry of the S2– ion in the crystal. The ENDOR results can be explained only by assuming a monovacancy model, i.e. S2– replacing a single Cl–. The earlier EPR results are in perfect agreement with the present ENDOR analysis.

EPR of S2- in RbCl and RbBr

An EPR study of a new S2- defect in RbCl is presented. A comparison with the S2- centre in RbBr, previously found by Vannotti et al. is made. For both salts a clearly resolved superhyperfine structure was observed, which has not yet been reported. The interpretation of the spectra was complicated by the simultaneous presence of the 85Rb (I=5/2) and 87Rb (I=3/2) nuclei. An excellent quantitative agreement between simulation and the experimental data was obtained, using only two parameters, i.e. the linewidth for a single superhyperfine component and the 85Rb (I=5/2) splitting. The analysis is based on the interaction of the unpaired electron on the S2- ion with four equivalent Rb nuclei. The model is consistent with an S2- ion replacing a single Cl-, respectively Br-, ion.

An EPR study and spin-Hamiltonian analysis of a new SSe- defect in NaCl

An EPR study of a new S-Se defect in NaCl is presented. The paramagnetic centre is formed after appropriate doping and exposure to X-rays at room temperature. By enriching the SSe- centre with either 33S or 77Se it is shown that the defect involves one S and one Se nucleus. Theoretical expressions for the g and hyperfine tensors are derived and compared to the experimental results.

An ENDOR study of RbI:O2-

In this work, an ENDOR analysis of the O2- defect in RbI is presented. A complete angular variation of one set of 85Rb and 87Rb ENDOR transitions is shown. The corresponding hyperfine and nuclear quadrupole coupling tensors are determined. The orientation of the principal axes of these tensors is in good agreement with the overall D2h symmetry of the O2- ion in the crystal. The ENDOR results can only be explained assuming a monovacancy model, wherein the O2- molecule is replacing a single I- ion. A comparison with a previous ENDOR study of the S2- centre in RbCl is presented. The linewidth of the EPR signals can be explained using the ENDOR results.