J. M. Gaite

Characterization and origin of two Fe3+ EPR spectra in kaolinite

The electron paramagnetic resonance (EPR) spectra of Fe3+ in a well cristallized kaolinite from Decazeville in France are well resolved. It is shown that in this sample there are mainly two slightly different spectra, well separated at low temperature and characterized at -150° C by the constants B20= 0.112 cm−1, B22= 0.0688 cm−1 for one and B20= 0.116 cm−1, B22= 0.0766 cm−1 for the second. These two spectra arise from Fe3+ substituted for Al3+ at the two octahedral positions in equal amounts. The temperature dependence of EPR spectra was studied and was explained by a modification of the octahedral sites.

Electron paramagnetic resonance and ENDOR studies of Cr3+ - Al3+ pairs in forsterite

The electron paramagnetic resonance (EPR) spectrum of Cr3+ in synthetic crystals of forsterite consists primarily of lines of Cr3+ “isolated” at the M1 and M2 positions in a “perfect” crystal environment without local charge compensation. In addition it shows two nonequivalent superhyperfine-split sextets with different intensities which are due to strong interaction of the Cr3+ electron spin S (S=3/2) with an adjacent nuclear spin I(I=5/2). Electron nuclear double resonance (ENDOR) experiments revealed that the sextets result from Cr3+ (M1) - Al3+ and Cr3+ (M2) - Al3+ pairs, Al being located at adjacent tetrahedral Si sites. The g, D, A, and A′ tensor components of the Cr3+ - Al3+ pairs have been determined at room temperature. The values of the pairs are distinct although they are not very different from the corresponding data of “isolated” Cr3+. From the intensities of the EPR spectra the relative concentration of the Cr3+ - Al3+ pairs with respect to “isolated” Cr3+ has been estimated.

Al-O--Al paramagnetic defects in kaolinite

Trapped holes located on Al-O-Al bonds in kaolinite were studied by electron paramagnetic resonance spectroscopy (EPR) at 9.3 and 35 GHz applied to well-crystallized, X-ray irradiated and oriented samples. The Q-band EPR spectrum is characterized by three clearly separated groups of 11 quasi-equidistant superhyperfine lines centered at gxx=2.040±0.0005, gyy=2.020±0.0005 and gzz=2.002±0.001. In each of these groups, the 11 superhyperfine lines exhibit intensities according to the ratios 1∶2∶3∶4∶5∶6∶5∶4∶3∶2∶1. An angular dependence of the Q-band EPR spectrum with respect to the magnetic field is demonstrated by measurements on oriented films of kaolinite. An appropriate numerical treatment of the EPR spectra is described, which allowed extraction of the SuperHyperfine Structures (SHFS). X-and Q-band spectra have also been simulated. It is concluded from these experiments that only one type of center is present. This center, labelled the B-center in the literature, is very probably a hole trapped on oxygen (O- center) atoms coupled to two octahedral aluminium.

Study of the symmetry of Fe3+ sites in SnO2 by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) of Fe3+ in SnO2 has been realized in a natural single crystal of cassiterite at 9.55 GHz (X-band) and at 34.40 GHz (Q-band). Spectra show the simultaneous presence of four groups of independent signals, each one typical of the immediate environment of a specific paramagnetic iron. Fe3+ always substitutes Sn4+ in an octahedral site. The four paramagnetic centers are due to four different charge compensation mechanisms. The spin Hamiltonian constant values for the SN center and I1 center confirm the former results of the authors about for these two centers. SN and I1 present a weak deviation from axial symmetry. The first preserves the crystallographic local symmetry of the tin site and the second shows a symmetry deviation of 0.6° probably due to the presence of an OH group in the coordination polyhedron. On the other hand, for the Sd1 center and mostly for the Sd2 center, never previously subjected to single crystal EPR measurements, the study of spectra symmetry and the determination of B20and B22constants produced new data. The Sd1 center could be due to a relaxation of the lattice together with a non local charge compensation mechanism. The Sd2 center presents a strong deviation from axial symmetry with mm local symmetry coordination due to coupling of Fe3+ and Nb5+. This coupling is proven by EPR studies of synthetic cassiterites doped with iron and niobium.

Pseudo-symmetries of crystallographic coordination polyhedra. Application to forsterite and comparison with some EPR results

A quantitative analysis of the distortion of octahedral and tetrahedral coordination polyhedra with respect to perfect symmetry in crystal structures is presented. The distortion of a polyhedron is defined by all the fourfold and threefold pseudo-symmetries. Tests on the significance of the pseudo-symmetries are presented. In forsterite the results are compared to ones previously proposed, based on the pseudo-symmetry of fourth order expansion of the crystal field, and also compared to the pseudo-symmetries of the environment of Mn2+ and Gd3+ that were determined by electron paramagnetic resonance (EPR) measurements. Agreements and differences between these results are given.

Local Distortion of the Spodumene Structure Around Isolated Cations Using E.P.R. and the Superposition Model

AbstractThe superposition model is used to attempt to explain the EPR spectrum of Fe3+ in spodumene (LiAlSi2O6).Two different models for the distortion of the local environment of Fe3+ are considered.If the local structure around Fe3+ is distorted toward that of its isomorphic compound LiFeSi2O6, it is possible to obtain an agreement between experimental data and calculated constants, only if it is considered that the values of

Study of the symmetry of Fe 3+ sites in SnO 2 by electron paramagnetic resonance

\bar B_2

Measurements of iron concentration in kaolinites considering disorder broadening of EPR lines

are different for the two non equivalent oxygens O1 and O2.A second model of local distortion of the structure, based on crystal chemical considerations, is proposed. With this model, it is possible to explain the EPR spectrum with a single value of