Crystal-field analysis of Eu3+ energy levels in the new rare-earth R BiY1-xRxGeO5 oxide

Pale colored BiY1-xRxGeO5 (R=rare-earth from Pr to Yb) polycrystalline samples exhibit a crystalline phase isostructural with the orthorhombic Pbca (No. 61) structure-type established for BiYGeO5 and BiYbGeO5. R occupies a single point site in the host, with the lowest C1 symmetry. While for Pr and Nd x must be ≤0.35, for smaller R ions, Sm to Yb, the phase appears for any x content. Detailed crystallographic data for BiErGeO5 have been determined from the structure refinement of its neutron diffraction profile at room temperature. Optical absorption and photoluminescence measurements at 10 K have been performed for BiEuGeO5. An initial approach to the parametrization of crystal-field effects on this new host has been provided by results of the semi-empirical Simple Overlap Model, which considers the crystallographic positions of the nearest neighbors around R. Furthermore, the strongly reduced 7FJM set of levels of the 4f6 configuration has been taken into account for a trustworthy phenomenological determination of crystal-field parameters of the observed optical center for the Eu3+ sample. In spite of difficulties imposed by the low symmetry of Eu3+, a very good root mean squares deviation σ=5.6 cm-1 between experimental and simulated energy level schemes has been obtained considering the approximate C2(Cs) symmetry for the Eu3+ in the host