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Using Eu3+ as an atomic probe to investigate the local environment in LaPO4–GdPO4 monazite end-members

Huittinen, Nina, Arinicheva, Yulia, Schmidt, Moritz, Neumeier, Stefan, Stumpf, Thorsten
Journal of colloid and interface science 2016 v.483 pp. 139-145
absorption, cations, energy transfer, europium, ligands, luminescence, phosphates, spectral analysis
In the present study, we have investigated the luminescent properties of Eu³⁺ as a dopant in a series of synthetic lanthanide phosphates from the monazite group. Systematic trends in the spectroscopic properties of Eu³⁺ depending on the size of the host cation and the dopant to ligand distance have been observed. Our results show that the increasing match between host and dopant radii when going from Eu³⁺-doped LaPO4 toward the smaller GdPO4 monazite decreases both the full width at half maximum of the Eu³⁺ excitation peak, as well as the ⁷F2/⁷F1 emission band intensity ratio. The decreasing Ln⋯O bond distance within the LnPO4 series causes a systematic bathochromic shift of the Eu³⁺ excitation peak, showing a linear dependence of both the host cation size and the Ln⋯O distance. The linear relationship can be used to predict the energy band gap for Eu³⁺-doped monazites for which no Eu³⁺ luminescent data is available. Finally, mechanisms for metal-metal energy transfer between host and dopant lanthanides have been explored based on recorded luminescence lifetime data. Luminescence lifetime data for Eu³⁺ incorporated in the various monazite hosts clearly indicated that the energy band gap between the guest ion emission transition and the host ion absorption transition can be correlated to the degree of quenching observed in these materials with otherwise identical geometries and chemistries.