Site-selective time resolved laser fluorescence spectroscopy of Eu and Cm doped LaPO₄

Abstract

Samples of LaPO₄ doped with Eu³⁺ or Cm³⁺ were synthesized by a hydrothermal process which resulted in a solid solution at temperatures less than conventional processing. Time resolved laser fluorescence spectroscopy was used to probe the incorporated Eu³⁺ or Cm³⁺ in order to gain structural information on its local environment. This revealed that Eu³⁺ and Cm³⁺ incorporate on the La site as expected. The emission spectrum of Eu³⁺ resolves the fully degenerate 5-fold splitting of the peaks in the F₂ transition due to the low symmetry of the site, confirming previous calculations. A minor site in the Eu³⁺ doped sample is identified as coordinated with hydroxide contamination. Direct excitation of Cm³⁺ doped samples show the presence of “satellite” species. Although these spectral features have been observed in Cm³⁺ doped LuPO₄ and YPO₄, this is the first time that these satellites are resolved into their individual species. These are hypothesized to be due to a disturbance in the ideal structure which creates a break in the equivalence of the four lanthanum sites within a unit cell. The 4-fold ground state splitting of all species is identical, although slightly shifted, indicating similar environments. The fluorescence lifetimes were long (1.2 ms for Cm and 3.6 ms for Eu) indicating an absence of water in the immediate coordination sphere due to the incorporation of the doping ion.