Luminescence of Eu³⁺ and Pr³⁺ in the weberite, Ca₂La₃Sb₃O₁₄

The spectroscopic properties of the Eu³⁺ and Pr³⁺ ions in the weberite, Ca₂La₃Sb₃O₁₄ are examined in this paper. The Eu³⁺ is used as a structural probe ion to elucidate the local site symmetry of the La³⁺ ion in Ca₂La₃Sb₃O₁₄. The interpretation of the Eu³⁺ emission spectrum is not consistent with the C_2h and S₂ (C_i) point symmetry that are prescribed for the La³⁺ sites by the I2/m11 space group of Ca₂La₃Sb₃O₁₄. The emission spectrum of Ca₂La₃Sb₃O₁₄:Pr³⁺ is dominated by emission transitions emanating from the ¹D₂ state. The low energy of the Pr³⁺4f² → 4f¹5d¹ interconfiguration excitation transition in Ca₂La₃Sb₃O₁₄ (3.83 eV or 32 258 cm^-1) is attributed to the high covalence which is induced by short Pr³⁺O^2- bond distances within the coordination polyhedral units in the lattice. The high covalence of chemical bonding appears to be the hallmark of the weberite structure. A study that compares the optical properties of Eu³⁺ and Pr³⁺ in the weberites, Ca₂La₃Sb₃O₁₄ and NaGdSb₂O₇ is presented.