Comparison of the ligand HFI data for some rare-earth ions in various alkaline-earth ions in various alkaline-earth fluorides is useful from many points of view. Particularly, in the case of cubic symmetry magnetic centers, this comparison allows to estimate the distortion of the immediate vicinity of the magnetic ion, which cannot be determined directly due to the considerable covalent contribution in the first fluorine shell HFI constants.The a i m of our note was the measurement of ligand HFI constants for cubic Er3+ centers in BaF in order to compare them with those measured for cubic Er3+ in CaF2 /1/.BaF2: Er3+ single crystals were studied by the radio-frequency discrete saturation (RFDS) method / 2 / (pulse analogue of ENDOR). The experiments were carried out on the X-band RFDS spectrometer at 4.2 K . The samples (impurity concentration about 0.01%) were grown by the BridgmanStockbarger technique. Most of the impurity ions were found in trigonal sites /31. Only 2% of Er3+ form the cubic EPR spectrum with g = 6.761 (21.
2The angular dependence of the ligand RFDS spectrum was studied (Fig. 1). Only the first fluorine shell frequencies were observed. The resonance frequencies were described by the standard axial spin-Hamiltonian .
The ligand pseudonuclearZeeman interaction and the second-order corrections, quadratic in ligand HFI, have been taken into account. Measured values of the ligand HFI constants and the pseudonuclear corrections for BaFZ:Er3+ /1/ are given in Table 1.In order to determine the location of neighbouring fluorine ions, we have used Baberschke's suggestion 141, that for certain rare-earth ions in different lattices the covalent contribution A' in A linearly depends on A P P
s 'A' = A -A~ = const A~ ; P P 1) Prospekt Chavchawadze 1, SU-380028 Tbilisi, U S S R .
Substitution of a trivalent rare-earth ion for the divalent cations of the lattice in alkaline-earth fluorides gives rise to a variety of magnetic centers. Exposing these samples to ionizing radiation, it is possible t o reduce rare-earth ions t o the divalent state. The symmetry of such reduced magnetic centers generally is cubic. It is rather surprising t o find non-cubic sites for divalent ions, as charge compensation is unnecessary in this case. However, thermally non-stable trigonal centers of Tm2+ in alkaline-earth fluorides were reported in /1, 2/. Particularly, Sabisky and Anderson /l?) have reported EPR results for room temperature r-irradiated thulium-doped BaF2 single crystals. These samples generally exhibit the EPR spectrum of Tm2+ in stable cubic sites. It was found that about 3% of the Tm2+ ions can be switched t o trigonal sites by illumination with blue light (X< 500 nm) at or below 77 K.Cycling t o room temperature o r irradiation with red light ( W 5 5 0 nm) all trigonal sites convert back to cubic ones.The nature of these centers has remained uncertain. Authors mentioned
2+only that this light-generated (LG) trigonal spectrum can be attributed to T m ions with nearby defect and that the defect can be ionized (non-ionized) upon application of light.In order t o determine a reliable model of thermally non-stable trigonal centers of Tm2+ in BaF29 we have investigated a-irradiated BaF2:Tm single crystals by EPR and radiofrequency discrete saturation (RFDS) /3/ methods (the last one is the pulsed analogue of ENDOR). The impurity concentration was about 0.01%. The irradiation dose from a 6oCo source was about 10 R. Unlike /l/, r-irradiation has been carried out at 77 K. After that the samples were moved to the resonator and were cooled t o liquid helium temperature 7 1) Prospekt Chavchavadxe 1, SU-380028 Tbilisi, USSR.2) The given value of 2.23 for g, given in /1/ is wrong as any calculations including this t e r m show, it has t o be 2.32.
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