Optical Spectroscopy of Ca₉Tb_1–<i>x</i>Eu_<i>x</i>(PO₄)₇ (<i>x</i> = 0, 0.1, 1): Weak Donor Energy Migration in the Whitlockite Structure

Abstract

The luminescence spectroscopy of Tb 3+ and Eu 3+ has been studied in the Ca 9 Tb(PO 4 ) 7, Ca 9 Eu(PO 4 ) 7, and Ca 9 Tb 0.9 Eu 0.1 (PO 4 ) 7 materials having a whitlockite structure, by using excitation in the near UV, vacuum UV and X-ray regions. The Eu 3+ ion in Ca 9 Eu(PO 4 ) 7 is located mainly in two cationic sites, as evidenced by the fine structure of the 5 D 0 → 7 F 0 transition at 5 K. In the case of Ca 9 Tb 0.9 Eu 0.1 (PO 4 ) 7, weak Tb 3+ → Eu 3+ energy transfer is observed upon excitation in the UV bands of Tb 3+ . The low efficiency of the transfer appears to be due to slow energy migration in the 5 D 4 subset of the Tb 3+ ions. The overall behavior is strongly affected by the multisite and disordered nature of the Tb-based whitlockite host.