Isomer shift of Eu²⁺ in fluorites

Abstract

Mossbauer studies are carried out for divalent europium as dopant in the fluorites CaF,, SrF,, and BaF,. The lslEu Mossbauer isomer shift in these materials increases, increases and decreases, respectively, as the Eu2+ concentration decreases below a few mole percent. Previous explanations of these effects as due to lattice pressure or host covalency are found t o be wrong. Instead these trends are interpreted as primarily due to variations in the admixture of t,he prevalent defect structure of E U~+ O : -F~-~~ (i = 1, 2 , 3 , or 4) which stabilizes the Eu2+ in these crystals. Mossbauerstudien werden fur zweiwertiges Europium als Dotierung in den Fluoriten

Paramagnetic hyperfine splitting in theE151uMössbauer spectra ofCaF2:Selling1, Bielemeier2, Wortmann3 et al. 2008Phys. Rev. B3020View full textAdd to dashboardCite

“…10,11,13,14 This can be unambiguously attributed to the fact that the resolved paramagnetic hyperfine splitting, observable even at room temperature (as demonstrated in the present study), has not been recognized in the earlier studies and therefore not considered in the analysis of the spectra. The present study solves a longstanding puzzle in 151 Eu Mössbauer spectroscopy.…”

“…Nitsche et al 12 found a very unusual temperature dependence of the Eu 2+ isomer shift in CaF 2 :1%Eu. Lambe and Schroeer 13 and Schroeer et al 14 extended the Mössbauer studies to lower concentration (0.1% to 10%) and observed an even stronger anomalous variation of the isomer shift with Eu 2+ concentration from 11.3 mm/s (0.1%) to approximately 13.4 mm/s (1%-10%). In addition, these authors reported on a very broad linewidth (up to ≈8 mm/s compared to a common experimental line-width of ≈2.5 mm/s) of the Eu 2+ absorption lines, which they attributed to spin-spin relaxation effects of paramagnetic splitting (known from the above EPR and ENDOR studies).…”

“…Such small Eu 3+ contents, eventually stabilized by oxygen neighbors, are difficult to avoid and were also observed in other studies of CaF 2 :Eu systems. 14 Figure 4 shows the Mössbauer spectra of the CaF 2 :2%Eu sample at 295, 90, and 10 K in a magnetic field of 0.2 T, applied by a permanent magnet parallel to the γ-ray direction. The spectra change considerably and consist now of one dominant (≈77%) line at −13.4 mm/s, the same isomer shift as without an external field, but somewhat more broadened (FWHM=8.2 mm/s at 10 K), together with another broad feature (21%) in the wings, indicating a magnetic splitting.…”

“…[10][11][12][13][14] The paramagnetic splitting of dilute Eu 2+ (0.01%) has been determined at low temperature by EPR by Baker, Bleaney, and Hayes, 6 and the hyperfine interaction has been measured by electron nuclear double resonance (ENDOR) by Baker and Williams. 7 Later the temperature and pressure dependence of these parameters have been determined.…”

“…The anomalous shifts were speculatively attributed to oxygen neighbors of the Eu 2+ ions, with a detailed model for possible O 2− configurations. 13,14 In order to clarify the contradicting results and interpretations in these previous Mössbauer studies of CaF 2 :Eu 2+ systems, we performed the following series of 151 Eu Mössbauer studies: (i) CaF 2 :Eu 2+ with an Eu doping level of only 0.1% was studied at different temperatures between 4.2 and 295 K without an external field; (ii) CaF 2 : 0.1%Eu 2+ was studied in a relatively small external magnetic field of 0.2 T at 295 K; (iii) CaF 2 :Eu 2+ with 2% Eu was studied between 4.2 and 295 K without an external magnetic field; (iv) CaF 2 :2%Eu 2+ was studied in an external magnetic field of 0.2 T between 10 and 295 K (to elucidate the impact of an external magnetic field on the observed paramagnetic line broadening) and, in addition, in a large external field of 6 T between 2.5 and 10 K.…”