An EPR study of F$+ centres in single crystals of KTiOP04 (KTP) has been performed. Twocrystalsgrownunderidenticalconditionshave been usedintheexperiment carried out at the Q-band frequency: the first was imn doped and the other was undoped. Four Fe3+ centres have been observed. All spin-Hamiltonian parameters Were calculated for the four centres. Comparison of the pseudosymmetries obtained from fourthirrder constants of the spin Hamiltonian and from a fourth-order crystal-field calculation for the W O Ti octahedra allowed us to locale two Fe3* centres at each Ti site in KIP. The absence of astrongaxialdeviationfromcubicsymmetryshowsthdt thechargecompensationmechanism is not at short distance for the four centres. All four cenres denoted as ST1, ST2, ST3 and ST4 are present in the iron-doped crystal but only one (ST4) is observable in the undoped crystal. In the undopedcrystal,alsoCr" wasobserved. Resultsfor r h e C P c e n t r e s a r e g n and compared with those already reported by Hasanova and co-workers.

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Abstract--The Fe 3+ substituted for AP + at the 2 octahedral positions is one of the most common impurities in the kaolinite structure detected by electron paramagnetic resonance (EPR). Evidence has been provided for a relationship between the shape of EPR spectra for structural Fe and the structural disorder in kaolinite. It is proposed that the structural Fe be used as a sensitive probe for the degree of disorder of natural kaolinites. With this aim in view, an EPR disorder index (E) is defined from the width of selected EPR lines. Using reference kaolinites, it is shown that this index can account as well for longrange disorder detected by means of X-ray diffraction (XRD) as for local perturbations such as radiationinduced defects (RID). It is shown that the disorder observed through EPR has some points in common with the XRD-measured one. The influence on E of the presence of RID is shown by the study of artificially and naturally irradiated kaolinites.

A study of the EPR spectrum of Fe3+ in diopside CaMg(SiO3)2 and spodumene LiAl(SiO3)2 has been made. It permits the calculation of all the constants of the spin Hamiltonian. We show that the pseudosymmetry axes of the fourth order tensor of the spin Hamiltonian are the same as the axes of the immediate environment of the Fe3+. From the fourth order constants of the spin Hamiltonian it is possible to obtain qualitative information about the Fe3+ environment. In both crystals, the Fe3+ is in the M 1 site. We show that the M 1 site is smaller and more distorted in spodumene than in diopside, and that in spodumene there is large tetragonal distortion along the M 1–01′ direction. It is seen that the information we obtain agrees very well with the crystallographic results.

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Electron paramagnetic resonance of a forsterite Mg2SiO4 crystal doped with 57Fe revealed two strong Fe3+ centers which can be unambiguously assigned to the M2 and Si sites. All the fourth order terms of the spin Hamiltonian were determined for the two centers. It is shown that the local environment of Fe3+ at M2 is distorted without any apparent pseudoaxial symmetry, while the environment of Fe3+ at the Si position exhibits a strong axial distortion along the crystallographic a direction. There is no immediate local charge compensation for the Mg2+ and Si4+ substitution by Fe3+ in the crystal.

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Site-selective investigation of site symmetry and site occupation of iron in Fe-doped lithium niobate crystalsA new method of site assignment of Fe3+ in low symmetry crystals is presented. This method is based on the fact that the fourth order spin-Hamiltonian constants of Fe3+ are sensitive mainly to the immediate surroundings of the Fe3+. It is shown how it is possible to find the pseudosymmetry of the fourth order term of the spin Hamiltonian, while from the pseudosymmetry it is possible to define the orientation of the Fe-On complex in the structure and to localize Fe3+ in the crystal. To use this method the spin-Hamiltianian constants must be well known. The determination of the 20 constants of a general spin Hamiltonian is described. To apply our method in a low symmetry case, we studied Fe3+ in NaAJSiaOs, which has a triclinic lattice. Experiments were done on a single crystal at Q band frequency. The spin-Hamiltonian constants are given. It is shown that Fe 3 + is in a T1(0) site and substitutes for AJ3+.