Effect of uniaxial stress on the fluorine-transferred hyperfine parameters and lattice distortions in alkaline-earth fluorides withTm2+impurities

Abstract

We have measured the effect of uniaxial stress on the fluorine-transferred hyperfine interaction for ${\mathrm{Tm}}^{2+}$:Ca${\mathrm{F}}_2$ and ${\mathrm{Tm}}^{2+}$:Sr${\mathrm{F}}_2$ using electron-nuclear double-resonance (ENDOR) techniques. We obtain for $\frac{\partial (A_s-A_p)}{\partial P}$ the values 4.3(4) kHz ${\mathrm{kg}}^{-1\mathrm{}}$ ${\mathrm{mm}}^2$ and 4.2(3) kHz ${\mathrm{kg}}^{-1\mathrm{}}$ ${\mathrm{mm}}^2$, respectively. These results, as well as those obtained for the same hosts without applied stress, are discussed within the framework of an improved model on the lattice distortions around the impurity and its change with uniaxial stress. The positive conclusion of this work is the remarkable agreement between the local distortion predicted by the model and that derived from ENDOR experiments. The same agreement is also found for ${\mathrm{Eu}}^{2+}$ in the fluorides as compared with data derived from measurements of the crystal field. Differences between the experimental data on the stressed samples and the predictions of the model are attributed to stress-induced electric dipoles. We suggest that present uncertainties in the estimate of this contribution can be partially solved by further experiments on trivalent rare-earth metals in the fluorides.