High-temperature EPR in superionic fluorites

The EPR of the ${\mathrm{Mn}}^{2+}$-doped superionic conducting fluorites Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$ has been studied at high temperatures at 9.3 GHz, and that of Pb${\mathrm{F}}_2$ and Sr${\mathrm{Cl}}_2$ at 22 GHz as well. In the fluoride compounds, the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ transferred hyperfine structure (hfs) that is superimposed upon each ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$ hfs component is observed to collapse when the ${\mathrm{F}}^{-}$ hopping rate $w^{-}$ exceeds the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ hfs coupling $\frac{A^{\mathrm{}\left(19\right)\mathrm{}}}{\hslash }$. The integrated intensity, $m^{\mathrm{}\left(55\right)\mathrm{}}$, and frequency dependence of the line profiles have been used to identify (in addition to static dipolar broadening) three distinct line-broadening mechanisms: (1) a quasistatic, nonlocal distortion of the cubic crystal field that results from the growth of ion disorder with increasing $T$, (2) a local, dynamic broadening related to enhanced anharmonic phonon effects, and (3) a combined ${\mathrm{Mn}}^{2+}$-concentration— and $T$—dependent "collision" broadening which proceeds via dipolar encounters between diffusing ${\mathrm{Mn}}^{2+}$ ions and is proportional to their hopping rate, $w^{+}$. The latter mechanism is closely related to the effects of "magnetic tagging" on the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ NMR relaxation in Mn-doped Pb${\mathrm{F}}_2$.