Abstract
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 Gd3+ absorption, indicating the Gd3+ to Tb3+ energy transfer. Under excitation at 222 nm, typical 5D4→7FJ (J = 6–3) and 5D3→7FJ (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.
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