${}^{151}\text{E}\text{u}$ Mössbauer spectra in zero magnetic field of highly dilute (0.1 mol%) ${\text{Eu}}^{2+}$ ions in ${\text{CaF}}_2$ showed an almost temperature-independent asymmetrically split pattern, arising from the paramagnetic hyperfine interaction $A\mathbf{S}.\mathbf{I}$ in a cubic crystal field with slow electron spin relaxation; in a small external magnetic field $B$ of 0.2 T such that $g{\mu }_{B}B>A$ an almost symmetrical pattern was observed. Both the spectra with and without external field are well described using the spin Hamiltonian and previous electron paramagnetic resonance data. A more concentrated (2 mol% ${\text{Eu}}^{2+}$) sample exhibited a strongly broadened symmetrical resonance line due to an increased Eu-Eu spin relaxation rate; in an external magnetic field of 0.2 T the Mössbauer spectra exhibited further broadening and additional magnetic structures due to the reduced relaxation rate. When a large field of 6 T was applied such that $g{\mu }_{B}B$ is much larger than the crystal field splitting, a fully resolved hyperfine pattern was observed at 2.5 K, with an effective field at the Eu nuclei of $-33.7\text{T}$; at higher temperatures superimposed patterns originating from excited electronic states were observed in the spectra. The present results on the highly dilute ${\text{CaF}}_2:0.1\%{\text{Eu}}^{2+}$ sample deliver a straightforward explanation for previous observations of a seemingly large dependence of the ${\text{Eu}}^{2+}$ isomer shift on europium concentration.

• Received 19 March 2008

DOI:https://doi.org/10.1103/PhysRevB.77.224442