Mössbauer effect studies of europium pyrochlores

The axial electric field gradient (EFG) at the Eu site of cubic pyrochlore Eu₂{Z}₂O₇ (where $Z=Ti,{{0ex}{0ex}}Ru,{{0ex}{0ex}}Ir,{{0ex}{0ex}}Mo,{{0ex}{0ex}}Sn,{{0ex}{0ex}}Zr,{{0ex}{0ex}}Pb,{{0ex}{0ex}}Pt,{{0ex}{0ex}}and{{0ex}{0ex}}Hf$) has been measured by the nuclear ${γ}$-ray resonance of the 21.6-keV state of $¹⁵¹Eu$. The EFG is largest in ${Eu}₂Ti₂{O}₇$ and smallest in ${Eu}₂Pb₂{O}₇$. This variation of EFG's can be correlated with the structural parameters of pyrochlore. Using the results measured in ${Eu}₂Z₂{O}₇$ as well as those reported for ${Gd}₂Z₂{O}₇$ and ${Dy}₂Ti₂{O}₇$, the lattice and the second-order $4f$ contribution of EFG in ${Eu}³⁺$ 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 Eu₂{Z}₂O₇ (where Z=Ru,0ex0exIr,0ex0exand0ex0exPt) are attributed to the open d shell of these Z⁴⁺ ions. A similar effect on the EFG has also been observed. All samples of Eu₂{Z}₂O₇ studied in this work are nonmagnetic at 4.2 K.