Local Structural Effects of Eu³⁺ Incorporation into Xenotime-type Solid Solutions with Different Host Cations

In this study, the effect of host cations on the local structure around the dopant site of materials from the xenotime family is systematically studied on the molecular level. A series of six Eu³⁺ -doped xenotime-type single crystals (Tb, Y, Ho, Er, Yb, and LuPO₄ ) have been grown and spectroscopically analyzed by using polarization-dependent laser-induced luminescence spectroscopy (p-TRLFS). Our results demonstrate that the structural disorder changes in a non-linear manner with a structural break between Yb³⁺ and Lu³⁺ . Despite adopting identical crystal structures, the solid solutions of these materials vary significantly, and differ from monazite solid solutions. Similar Eu³⁺ incorporation behavior with a strongly distorted dopant site is found for the early members of the xenotime family, whereas LuPO₄ with the largest host versus dopant radii mismatch is anomalous in that it contains the most symmetrical lattice site. This goes along with a significantly stronger crystal field, indicating a shorter Eu-O bond length, as well as a strong vibronic coupling to external translational lattice vibrations. The p-TRLFS analysis confirms the breakdown of the crystallographic site symmetry from D_2d to C₁ in YPO₄ , whereas a small distortion of the crystallographic site in LuPO₄ results in an S₄ point symmetry for the Eu³⁺ cation. The lattice with the smallest cation host site is no longer sufficiently flexible to make room for Eu³⁺ and instead "forces" the guest ion to occupy a less distorted Lu³⁺ site.