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 .
BYThe ligand hyperfine interaction (HFI) of the trigonal fluorine centre of Er3+ in B a F 2 single crystals has been investigated by the method of radiofrequency discrete saturation (RFDS) /1/9 which is the pulsed analogue of the ENDOR meth- od.The experiments have been carried out using an X-band superheterodyne F o r the c a s e of ligand nuclei with low symmetry of the ligand HFI tensor and nuclear g-tensor the spin-Hamiltonian is = > -:n=x, y, z n, m=x, y, z n, m=x, y, z are the components of the ligand HFI tensor, g?;Anm components of the nuclear g-tensor, Ag(") are pseudonuclear corrections, nm @(n) is the nuclear magneton. The symmetry of the ligand HFL and pseudo-1) Prospekt Chavchavadze 1, 380028 Tbilisi, LESR.
The ligand hyperfine interaction (HFI) of the trigonal fluorine centre of Er3+ in BaF2 single crystals has been investigated by the method of radiofrequency discrete saturation (RFDS) /l/, which is the pulsed analogue of the ENDOR method.The experiments have been carried out using an X-band superheterodyne spectrometer at liquid helium temperatures, on the EPR lines of the erbium isotopes with zero nuclear spin. The impurity concentration was about 0.003 to 0.05%. The angular dependence of the RFDS spectrum of eight fluorine nuclei in the nearest neighbourhood of the magnetic ion has been studied. Further we discuss the results of the investigation of the ligand HFI with nuclei 1 and 8, located along the trigonal Z-axis (Fig. 1).For the Er3+ ions in BaF2 the distance from the ground doublet up to the -1 two nearest excited ones is merely 70 cm and 76 cm-l /2/. In this case the mixed parts of the Hamiltonian, containing products of the matrix elements of electron Zeeman and magnetic hyperfine interactions may give a significant contribution, having the form of the non-isotropic nuclear Zeeman interaction (pseudonuclear Zeeman effect) /3/. This effect may take place not only for the nucleus of the magnetic ion itself, but for the ligand nuclei, too /4/, though in the latter case the magnitude of the effect is significantly less.As it came out, for the right interpretation of our experimental data it is necessary to take into account the ligand pseudonuclear Zeeman effect (contribution to resonance frequency ~1 0 0 to 200 kHz).The Hamiltonian of the magnetic ion and ligand nucleus can be written in the form
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.
Exposing the rare-earth doped alkaline-earth fluorides (CaF2, S r F 2 , BaF2) to ionizing radiation, it is possible to convert the p a r t of the trivalent i mpurity ions to the divalent state. Reduced RE2+ ions form mainly the stable cubic symmetry centers, thoroughly studied both by optical /1 to 3/ and magnetic resonance /4, 5/ methods. In some c a s e s /6/ the non-cubic sites for reduced RE2+ ions were registered in the thermoluminescence spectra of 2+ irradiated samples, but EPR spectra have been observed only for Tm ions.Particularly, the EPR results for the thernially non-stable trigonal Tm centers i n r-irradiated S r F 2 and BaF2 were reported in /7, 8 , ' .In the previous paper /9/ we have reported the model of trigonal Tm centers i n BaF2. The fluorine ion, located in the nearest to the Tm2+ interstitial position along the trigonal symmetry axis was found to lower the cubic symmetry. This "charge compensator", non-necessary in the case of divalent ions, when asaociated with the Tm3+ ion during the crystal growing process, stays a t this position a f t e r reduction of Tm3+ to the divalent state by r-irradiation a t 77 K. 2+ 2+ 2+Here we report complete results of a n investigation of trigonal Tm centers i n SrFZ and BaF2 by EPR and radiofrequency discrete saturation (RFDS) /lo/ methods (the l a s t one is the pulse analogue of ENDOR). The experiments w e r e carried out on an X-band RFDS spectrometer a t 4 , 2 K. The irradiation technique allows to move the samples from the 6oCo source to the microwave resonator and to cool them to liquid helium temperature without intermediate heating. The irradiation dose was about 1 0 R, the impurity concentration was within 0.01 to 0.05 %.served: the cubic spectrum of Tm2+, the thermally non-stable spectrum of 7In both cases, S r F 2 and BaF2, three different EPR spectra have been ob-1) Prospekt Chavchavadze 1, SU-380028 Tbilisi, USSR. 4 physica (b)
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