Nonresonant energy transfer in a system with two different rare-earth sites: β’-${\mathrm{Gd}}_2$(${\mathrm{MoO}}_4$${)}_3$:${\mathrm{Eu}}^{3+}$ and β’${\mathrm{Eu}}_2$(${\mathrm{MoO}}_4$${)}_3$

In β’-${\mathrm{Gd}}_{2\mathrm{-}\mathit{X}}$ ${\mathrm{Eu}}_{\mathit{X}}$(${\mathrm{MoO}}_4$ ${)}_3$ the ${\mathrm{Eu}}^{3+}$ ion occupies two different crystallographic sites, leading to small differences in the respective energy-level schemes. At very low temperatures a one-phonon-assisted energy transfer and at higher temperatures a two-site nonresonant two-phonon-assisted energy transfer occurs from a ${\mathrm{Eu}}^{3+}$ ion on one site to a neighboring ${\mathrm{Eu}}^{3+}$ ion on the other site over a distance of 6.3 Å. For x=2, there no longer is a distinction possible between the two sites because of inhomogeneous line broadening. Energy migration occurs over the ${\mathrm{Eu}}^{3+}$ sublattice with quasi-two-dimensional character, via a one-site resonant two-phonon-assisted process.