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EPR has beenused to study yttrium-stabilised electron-hole centres of natural anhydrites. Geometrical models have been constructed by measuring the main directions of the tensors of the spectroscopic constants accurately and by correlating them with the crystal structure. Hyperfine splitting constant and thermal stability of the centres have been shown to vary as the localization of the unpaired electron density at the yttrium ions which replace calcium of the initial lattice.Mittels EPR wurden Yttrium-stabilisierte Elektron-Loch-Zentren in natiirlichem Anhydrit untersucht. Geometrische Modelle werden durch genaue Messung der Hauptwerte der Tensoren der spektroskopischen Konstanten und durch Korrelation mit der Kristallstruktur konstruiert. Es wird gezeigt, daB die Hyperfeinaufspaltungskonshnte und die thermische Stabilitiit der Zentren wie die Lokalisierung des ungepaarten Elektrons am Yttriumatom variiert, das das Kalziumatom des urspriinglichen Gitters ersetzt.

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Abstract. Electron paramagnetic resonance (EPR) measurements of natural barite BaSO4 have revealed the presence of PO~-and SO~ ionic radicals. For the latter a hyperfine structure from the 33S isotope has been detected and measured for the first time. The temperature dependence of the hyperfine interaction (HFI) constant of the PO22-and SO~ radicals has been investigated in the 100400 K range.

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For the first time, an EPR spectrum of Mo3+ ions in yttrium-aluminium garnet has been obtained. The ions isomorphously replace A13+ ions in the trigonally distorted oxygen octahedra of the garnet. The spectrum may be described by an axial spin-Hamiltonian with the parameters S = 3/2, I = 5/2,g11 = g 1 = 1.967, bi = 26.0 GHz, A = (131 f 2) MRz, and B = (139 f 2) MHz. The results have been correlated with the data reported earlier for other compounds, and the parameters of the fine structure of C P and Mo3+ ions have been compared. This allows to discuss certain regularities which govern the variation in the spectroscopical parameters of the ground state of similar ions in going from the 3d3 to the 4ds configuration. Erstmalig wurde ein EPR-Spektrum von Mo3+-Ionen in