Optical Spectroscopy of Ca9Tb1–xEux(PO4)7 (x = 0, 0.1, 1): Weak Donor Energy Migration in the Whitlockite Structure

The luminescence spectroscopy of Tb3+ and Eu3+ has been studied in the Ca9Tb(PO4)7, Ca9Eu(PO4)7, and Ca9Tb0.9Eu0.1(PO4)7 materials having a whitlockite structure, by using excitation in the near UV, vacuum UV and X-ray regions. The Eu3+ ion in Ca9Eu(PO4)7 is located mainly in two cationic sites, as evidenced by the fine structure of the 5D0 → 7F0 transition at 5 K. In the case of Ca9Tb0.9Eu0.1(PO4)7, weak Tb3+ → Eu3+ energy transfer is observed upon excitation in the UV bands of Tb3+. The low efficiency of the transfer appears to be due to slow energy migration in the 5D4 subset of the Tb3+ ions. The overall behavior is strongly affected by the multisite and disordered nature of the Tb-based whitlockite host.