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

Abstract

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 Eu -doped xenotime-type single crystals (Tb, Y, Ho, Er, Yb, and LuPO ) 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 Yb and Lu . Despite adopting identical crystal structures, the solid solutions of these materials vary significantly, and differ from monazite solid solutions. Similar Eu incorporation behavior with a strongly distorted dopant site is found for the early members of the xenotime family, whereas LuPO 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 to C in YPO , whereas a small distortion of the crystallographic site in LuPO results in an S point symmetry for the Eu cation. The lattice with the smallest cation host site is no longer sufficiently flexible to make room for Eu and instead "forces" the guest ion to occupy a less distorted Lu site.

A Spectroscopic Investigation of Eu3+ Incorporation in LnPO4 (Ln = Tb, Gd1-xLux, X = 0.3, 0.5, 0.7, 1) Ceramics

“…In our previous study (Xiao et al, 2018) we synthesized Eu 3+ –doped LuPO 4 single crystals based on a flux–growth method. These single crystals were also stored under ambient conditions for a time period of 9 months.…”

“…In contrast to the powder samples, the aging process has not induced any visible changes in the Eu 3+ luminescence signal collected from the single crystal sample, a feature that will be discussed in more detail in the discussion section below. For a detailed description of Eu 3+ incorporation in the single crystal material the reader is referred to Xiao et al (2018). At both times a lifetime measurement was performed (Supplementary Information 7).…”

“…In black: (A) Excitation and (B) emission spectra of Eu 3+ –doped LuPO 4 single crystals after 9 months aging. In gray: Corresponding luminescence data measured directly after synthesis [see (Xiao et al, 2018)].…”

“…Selected xenotime single crystals synthesized in our previous study were investigated with the TRLFS method after an aging time of ~9 months to complement the aging investigations of the powder samples. A detailed description of the synthesis is given in Xiao et al (2018). Briefly, LuPO 4 single crystals were obtained by a flux-growth method (Li et al, 2014, 2017).…”

“…The PXRD and spectroscopic investigations were conducted directly after synthesis as well as after an aging period of 1 year, to account for the stability of the initially formed local coordination environment of the dopant. The aging studies were complemented with spectroscopic results obtained for xenotime single crystals, previously synthesized and studied by our group (Xiao et al, 2018) after a similar aging period of ~1 year.…”