Vacuum ultraviolet spectroscopic properties of rare-earth (RE³⁺ = Sm³⁺, Tb³⁺, Dy³⁺)-activated zirconium-based phosphates MZr₄(PO₄)₆ (M²⁺ = Ca²⁺, Sr²⁺)

Vacuum ultraviolet (VUV) spectroscopic properties of Sm³⁺, Tb³⁺ and Dy³⁺-activated MZr₄(PO₄)₆ (M²⁺ = Ca²⁺, Sr²⁺) phosphates, which have been prepared by solid-state reaction method, are presented. The strong absorption bands in the VUV range of undoped and rare-earth activated MZr₄(PO₄)₆ were observed. The band range from 7.76-9.56 eV is due to the host lattice absorption, and the band at 6.21-7.76 eV is attributed to the charge transfer transition of O^2--Zr⁴⁺. Doped with Sm³⁺ resulted in bright red emission originating from ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2 and 9/2) transitions. For Tb³⁺-activated samples, the broad band around 5.52 and 4.76 eV are identified as the spin-allowed and spin-forbidden f-d transitions of Tb³⁺. Typical f-f transitions of Dy³⁺ were observed in Dy³⁺-activated samples. Because the 2p electrons of O^2- are tightly bound to Zr⁴⁺ rather than RE³⁺, making the O^2--Sm³⁺ and O^2--Dy³⁺ charge transfer bands too weak to be observed. It is demonstrated that there is energy transfer from the host to the luminescent activators.