Optical absorption spectra of the Yb 2 Ti 2 O 7 single crystals and luminescence spectra of the Y 2 Ti 2 O 7 :Yb (1%) polycrystalline samples were registered at temperatures 4.2-300 K. These spectra and earlier published data on magnetic properties of Yb 3+ ions and on the temperature dependence of the electric field gradient at Yb nuclei in Yb 2 Ti 2 O 7 were used to analyze the crystal-field parameters in rare-earth titanates with the pyrochlore structure. The self-consistent sets of crystal-field parameters for rare-earth ions in the 16d sites with the D 3d symmetry that describe satisfactory all known single-ion magnetic properties and low-energy excitations in R 2 Ti 2 O 7 crystals (R = Tb, Ho, Er, Tm, Yb) are presented.

We report high-resolution optical absorption spectra for NdFe 3 (BO 3 ) 4 trigonal single crystal which is known to exhibit a giant magnetoelectric effect below the temperature of magnetic ordering T N = 33 K. The analysis of the temperature-dependent polarized spectra reveals the energies and, in some cases, symmetries and exchange splittings of Nd 3+ 84 Kramers doublets. We perform crystalfield calculations starting from the exchange-charge model, obtain a set of six real crystal-field parameters, and calculate wave functions and magnetic g-factors. In particular, the values g ⊥ = 2.385, g || = 1.376 were found for the Nd 3+ ground-state doublet. We obtain B loc =7.88 T and |J FN |= 0.48 K for the values of the local effective magnetic field at liquid helium temperatures at the Nd 3+ site and the Nd -Fe exchange integral, respectively, using the experimentally measured Nd 3+ ground-state splitting of 8.8 cm -1 . To check reliability of our set of crystal field parameters we model the magnetic susceptibility data from literature. A dimer containing two nearest-neighbor iron ions in the spiral chain is considered to partly account for quasi-one-dimensional properties of iron borates, and then the mean-field approximation is used. The results of calculations with the exchange parameters for Fe 3+ ions J nn = -6.25 K (intrachain interactions) and J nnn = -1.92 K (inter-chain interactions) obtained from fitting agree well with the experimental data.

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