Structure and composition-dependent photoluminescence for Ca8MgGd1-xTbx(PO4)7 (0 ≤ x ≤ 1)

Complete solid solutions of Ca8MgGd1-xTbx(PO4)(7) (0 <= x <= 1) were prepared by high temperature solid state reactions and the phase purity was confirmed by careful analyses on powder X-ray diffraction (PXRD) data. For the first time, the detailed structures for Ca8MgGd(PO4)(7) and Ca8MgTb(PO4)(7) were determined according to Rietveld refinements on high quality PXRD. In both cases, Ca2+ and Gd3+ (or Tb3+) co-occupy M1, M2 and M3 sites and all in an eight-fold coordination by oxygen atoms. M4 is vacant. Mg2+ resides at the relatively small M5 position with a typical octahedral coordination. By monitoring the Tb3+ emission, strong absorptions due to the Tb3+ f-d and f-f transitions in ultraviolet (UV) and near UV ranges can be observed as well as the typical Gd 3+ absorption, indicating the Gd3+ to Tb3+ energy transfer. Under excitation at 222 nm, typical D-5(4) -> F-7(J) (J = 6-3) and D-5(3) -> F-7(J) (J = 6-2) emissions were observed, and their different variations against the Tb3+ content led to the tunable emission from blue (0.216, 0.194) to green (0.287, 0.595). Moreover, inverse energy transfer from Gd3+ to Tb3+ was also detected.