Crystal structure of ABPO₅ and optical study of Pr³⁺ embedded in these compounds

The crystal structure of borophosphates ABPO₅ (A = alkaline earth or Pb) was resolved on a polycrystalline sample using the Rietveld method. The x-ray diffraction patterns data show that ABPO₅ crystallize in a centrosymmetric space group P3₁21 and their structure is related to the borogermanates REBGeO₅ with a stillwellite-type structure. Pr³⁺ ion was used as a local structural probe to corroborate the structural resolution results. Absorption and fluorescence spectra of A_1−xPr_xBP_1−xGe_xO₅ (A = alkaline earth or Pb; x = 0.05) have been investigated at different temperatures. At 9 K the ³H₄ → ³P₀ transition of trivalent praseodymium ion (4f² configuration) is observed as a single line. This indicates a unique crystallographic site for the rare earth ion in these compounds replacing the divalent cation. Energy level schemes were deduced from the low-temperature spectroscopic measurements. Comparing the electronic level splittings of studied compounds with the already reported data on REBGeO₅ doped with Pr³⁺ ion, it is possible to dispel the doubt existing about structural determination. Moreover, under 460 nm excitation, intense red emission of trivalent praseodymium is observed corresponding to ¹D₂ → ³H₄ transition. The lifetime measurements of ¹D₂ level have been performed for all the title compounds.