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

Abstract

The electric-quadrupole interactions at the Eu sites in ${\mathrm{Eu}}_{2}$${\mathrm{CuO}}_{4}$, ${\mathrm{EuBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$, ${\mathrm{Eu}}_{0.6}$${\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 $^{151}\mathrm{Eu}$ M\"ossbauer 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.