Li8CaRE2Ta2O13 (RE=La3+, Eu3+, Gd3+, Dy3+, Er3+, Y3+, Yb3+): A highly lithium-stuffed tantalate garnet with versatile luminescence characteristics

Abstract

Highly lithium-stuffed tantalates Li8CaRE2Ta2O13 (RE = La3+, Eu3+, Gd3+, Dy, Er3+, Y3+, Yb3+) were prepared by incorporating Ca2+ ions in the lattices of the well-known garnet Li5La3Ta2O12. The well-crystallized ceramic samples were synthesized via high-temperature solid-state reaction. Rietveld structural refinements were applied to verify the phase formations. The detailed luminescence properties of these rare-earth ions (RE)-concentrated tantalates were investigated. The phosphors show versatile luminescence phenomena without concentration quenching due to the Ca2+ ions incorporation. Under excitation with UV radiation, the luminescence band peaking at 435 nm was detected in Li8CaRE2Ta2O13 (RE = La3+, Gd3+, Y3+, Yb3+) even at room temperature, which is from the electronic transitions of self-activation centers. Especially, Li8CaRE2Ta2O13 (RE = La3+) shows scintillating characteristics under X-ray irradiation. In Li8CaRE2Ta2O13 (RE = Eu3+, Dy3+, Er3+, Yb3+), UV-excitations produce characteristic f-f transitions of RE ions caused by energy transfers from TaO6 centers. Li8CaRE2Ta2O13 (RE = Eu3+) presents the strong red emission under both near-UV and blue excitation, which shows potential phosphor for fabrications of white LEDs. The experimental lamps were fabricated by packaging the prepared phosphors with near-UV LED-chips. The intense green upconversion is observed in Li8CaRE2Ta2O13 (RE = Er3+) under near-IR excitation at 980 nm. The luminescence mechanism was investigated via applying the site-selective excitation and emission of the spectral probe Eu3+ ions and discussed on basis of the structural characteristic.