Introduction
VUV-excited phosphors in applications such as mercury-free fluorescent tubes, plasma display panels (PDP) and three-dimensional (3D) displays has become more and more important. In these field, luminescent materials are required for the efficient conversion of vacuum ultraviolet radiation to visible light. Much work has been devoted to improve the efficiency of phosphors, and most important approach is by quantum cutting (QC) processes. Another way is by sensitization, which is a traditional manner for enhancing the luminescence efficiency. It has been reported that the VUV sensitization effect in host-Tb3+ systems paves the way for the creation of efficient VUV phosphors.

The visible quantum cutting via cross-relaxation between Tb3+ ions in
Na3Gd0.9-xYxSi3O9: 0.1Tb3+ phosphors are identified for the first time. The emission intensity of band at 312 nm decreases with the increase Tb3+ concentration, which indicates that Gd3+–Tb3+ ET efficiency increases with the x value in series samples of Na3Gd0.5TbxY0.5-x(SiO4)3.

It seems to suggest that the transition of 5D4 is stronger than that of 5D3, which indicates there could be QC occurrence in all these phosphors.

The emission from 5D4 under 231 nm excitation is higher than that under 274 and 352 nm excitation, which indicates that the QC process via cross relaxation occurs in Na3Gd0.9-xYxSi3O9:0.1Tb3+ (x = 0, 0.2, 0.6) sample.

It is proved that during the QC process, Gd3+ plays an important
intermediate role in ET to convert VUV light to visible light and reinforces the cross relaxation efficiency.

Phosphors GdBO3: Tb3+ were prepared using a high-temperature solid –state reaction technique. Upon 235 nm excitation, two photons in the visible range can be emitted through a two-step energy transfer process from one Tb3+-Tb3+ and/or Tb3+-Gd3+. Upon 164 nm excitation, the excitation energy either directly transfers to a Tb3+ in its 4f75d1 state or first transfer to the 6Gl evel of a Gd3+ then to a Tb3+ in its 4f75d1 state, then relaxes via cross relaxation energy transfer as that excited at Tb3+ 4f75d1 state.

Undoped and Tb3+-doped borates KBaY(BO3)2 phosphors were synthesized through the conventional high-temperature solid state method. The PLE spectrum of KBYB:0.001Tb3+ exhibits similar feature with a broad absorption band below 210 nm resulting from the host absorption, which gives an evidence to the host-Tb3+ ET.

KBYB exhibits a broad ultraviolet emission due to the recombination of self-trapped excitations. In KBYB: Tb3+, an efficient ET from the host to Tb3+ was observed. The strong quenching of STE emission and decay curves monitoring the STE emission gave conclusive evidence to such ET.