SrEr 2 O 4 is a geometrically frustrated magnet which demonstrates rather unusual properties at low temperatures including a coexistence of long-and short-range magnetic order, characterized by two different propagation vectors. In the present work, the effects of crystal fields (CFs) in this compound containing four magnetically inequivalent erbium sublattices are investigated experimentally and theoretically. We combine the measurements of the CF levels of the Er 3+ ions made on a powder sample of SrEr 2 O 4 using neutron spectroscopy with site-selective optical and electron paramagnetic resonance measurements performed on single-crystal samples of the lightly Er-doped nonmagnetic analog, SrY 2 O 4 . Two sets of CF parameters corresponding to the Er 3+ ions at the crystallographically inequivalent lattice sites are derived which fit all the available experimental data well, including the magnetization and dc susceptibility data for both lightly doped and concentrated samples.

The absorption spectra of KZnF3:Tl+ and KMgF3:Tl+ crystals have been measured over the energy range 1.5-6.4 eV at temperatures of 10-300 K. A wide absorption band with a clear doublet structure identified as an absorption A band of Tl+ impurity centres is observed for both crystals. The position of the band, its bandwidth and the distribution of absorption intensity between the two components of the band are temperature dependent in the range T>65 K. All features of the absorption spectra of the crystals studied are explained within the frameworks of the conventional theory on the basis of the Frank-Condon principle and the semiclassical picture of crystal lattice vibrations. The band shapes calculated by the Monte Carlo method are in satisfactory agreement with observed ones.

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Electron paramagnetic resonance (EPR) of |1o 3 § single ions and llo3'-Mg-"-vacancy-llo 3' associates in holmium-doped forsterite single crystals are studied at 9.4, 37.3 and 65-250 Gtlz. Crystals were grown from melt by the Czochralski technique in slightly oxidizing atmosphere. For both centers, directions of the principal magnetic axes and parameters of the effective spin Hamiltonians describing dependences of electron-nuclear levels on applied magnetic field are obtained. For [lo s' substituting Mg"" in the M2 site as the single ion and for Ho "~" ions in dimer centers, values of crystal field parameters related to a real crystal lattice structure are estimated in the framework of the exchange charge model. The calculated crystal field energies, values of the g-factors of the ground I-[o 3~ quasi-doublet and the directions of the corresponding magnetic moments agree satisfactorily with the data obtained from measurements of EPR and optical absorption and site-selective luminescence spectra. I IntroductionForsterite (Mg2SO4) is a member of family of olivines (Mg.~ej_x)2SiO 4, one of the widespread rock-forming minerals. Nowadays the interest to forsterite single crystals is related also with the efficient tunable laser oscillation of chromiumdoped forsterite, obtained to the moment in different regimes, including continious-wave oscillation [1, 2] and femtosecond pulse generation [3].A unir cell of forsterite crystalline structure (the lattice constants equal a = 0.4753, b = 1.019, c = 0.5988 nm) contains 4 formula units and has an orthorhombic symmetry with a space group Pbnm. The idealized olivine structure consists of a hexagonal close-packed array of oxygen anions, between which the octahedral and tetralaedral holes are located. One hall of those octahedral holes are occupied by Mg -'+ cations and 1/4 of tetrahedral holes are occupied