Quadrupole interactions at divalent and trivalent europium sites in several europium oxides

The electric-quadrupole interactions at the Eu sites in ${\mathrm{Eu}}_2$ ${\mathrm{CuO}}_4$, ${\mathrm{EuBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$, ${\mathrm{Eu}}_{0.6}$ ${\mathrm{Sr}}_{0.4}$ ${\mathrm{VO}}_{2.96}$, ${\mathrm{Eu}}_3$ ${\mathrm{V}}_2$ ${\mathrm{O}}_7$, and ${\mathrm{Eu}}_2$ ${\mathrm{VO}}_4$ have been studied at room temperature with ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ Mössbauer spectroscopy. Whereas Eu ions are found to be in trivalent oxidation state in all these oxides, they also occur concurrently in the divalent state in ${\mathrm{Eu}}_3$ ${\mathrm{V}}_2$ ${\mathrm{O}}_7$ and ${\mathrm{Eu}}_2$ ${\mathrm{VO}}_4$. It is found that the quadrupole coupling constant ${\mathit{eV}}_{\mathit{z}\mathit{z}}$ ${\mathrm{Q}}_{\mathit{g}}$ is negative in all oxides studied here and in the literature, except ${\mathrm{Eu}}_2$ ${\mathrm{CuO}}_4$. This is discussed in terms of different contributions to the electric-field-gradient tensor. The value ${\mathit{eV}}_{\mathit{z}\mathit{z}}$ ${\mathit{Q}}_{\mathit{g}}$=-18.032(134) mm/s found in ${\mathrm{Eu}}_2$ ${\mathrm{VO}}_4$ is, to our knowledge, the largest ever reported for divalent Eu ions on any Eu oxide system.