Mössbauer effect studies of europium pyrochlores

The axial electric field gradient (EFG) at the Eu site of cubic pyrochlore ${\mathrm{Eu}}_2$ ${\mathrm{Z}}_2$ ${\mathrm{O}}_7$ (where $Z=\mathrm{T}\mathrm{i},\mathrm{R}\mathrm{u},\mathrm{I}\mathrm{r},\mathrm{M}\mathrm{o},\mathrm{S}\mathrm{n},\mathrm{Z}\mathrm{r},\mathrm{P}\mathrm{b},\mathrm{P}\mathrm{t},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{H}\mathrm{f}$) has been measured by the nuclear $\gamma$-ray resonance of the 21.6-keV state of ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$. The EFG is largest in ${\mathrm{Eu}}_2$ ${\mathrm{Ti}}_2$ ${\mathrm{O}}_7$ and smallest in ${\mathrm{Eu}}_2$ ${\mathrm{Pb}}_2$ ${\mathrm{O}}_7$. This variation of EFG's can be correlated with the structural parameters of pyrochlore. Using the results measured in ${\mathrm{Eu}}_2$ ${\mathrm{Z}}_2$ ${\mathrm{O}}_7$ as well as those reported for ${\mathrm{Gd}}_2$ ${\mathrm{Z}}_2$ ${\mathrm{O}}_7$ and ${\mathrm{Dy}}_2$ ${\mathrm{Ti}}_2$ ${\mathrm{O}}_7$, the lattice and the second-order $4f$ contribution of EFG in ${\mathrm{Eu}}^{3+}$ have been deduced; the latter is about 40% of the former. A convergent point-charge calculation of the EFG does not satisfactorily account for the observed variation of EFG's. This is attributed to covalency and bonding effects, which are manifested by the various values of the measured isomer shift. The large isomer shifts observed in the semiconducting ${\mathrm{Eu}}_2$ ${\mathrm{Z}}_2$ ${\mathrm{O}}_7$ (where $Z=\mathrm{R}\mathrm{u},\mathrm{I}\mathrm{r},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{P}\mathrm{t}$) are attributed to the open $d$ shell of these $Z^{4+}$ ions. A similar effect on the EFG has also been observed. All samples of ${\mathrm{Eu}}_2$ ${\mathrm{Z}}_2$ ${\mathrm{O}}_7$ studied in this work are nonmagnetic at 4.2 K.