Cation composition sensitive visible quantum cutting behavior of high efficiency green phosphors Ca9Ln(PO4)7:Tb3+ (Ln = Y, La, Gd)

Abstract

Visible quantum cutting (QC) has the potential to overcome the still low efficiency of the presently used vacuum ultraviolet (VUV) phosphors. The understanding of this process will provide important guidance for exploring VUV phosphors with high efficiency. The QC process through cross-relaxation energy transfer between Tb ions of Tb³⁺ doped Ca₉Ln(PO₄)₇ (Ln = Y, La, Gd) phosphors is observed upon host absorption transition and 4f⁸–4f⁷5d¹ excitation. Based on the location of host absorption and the levels of Tb ions in the hosts, QC models and the corresponding ideal quantum efficiency (QE) calculation formula obtained by an indirect method for the phosphors with and without Gd³⁺ are proposed. By comparing the values of three kinds of phosphors, the results represent the relationship of η_CYP:Tb³⁺ > η_CGdP:Tb³⁺ > η_CLaP:Tb³⁺. Furthermore, the critical concentration for QC in the three kinds of phosphors exhibits some difference. The phenomena have been discussed combining the crystal structures, Raman spectra, PL/PLE spectra, and decay curves, and have been confirmed to mainly involve the occupied sites of Tb³⁺ in the host rather than the phonon energies.