A Quantum‐Splitting Phosphor Exploiting the Energy Transfer from Anion Excitons to Tb 3+ in CaSO 4 :Tb,Na

Wide‐bandgap materials doped with rare‐earth ions are currently of great interest as new vacuum ultraviolet (VUV) phosphors for lighting and displays. This paper reports the development of a highly sensitive green phosphor, CaSO 4 :Tb,Na, which exhibits a quantum efficiency higher than 100 % by exploiting the energy‐transfer mechanism from anion excitons to the activator ions, Tb 3+ . The VUV excitation spectra of CaSO 4 :Tb 3+ with Na + as a charge compensator show two prominent excitation bands at 147 and 216 nm. The former band is attributed to the charge‐transfer excitations within SO 4 2– complexes while the latter was assigned to the 4f 8  → 4f 7 5d transitions on Tb 3+ . The energy‐transfer mechanism from anion excitons to Tb 3+ strongly raises the possibility of two‐photon emission via a second‐order down‐conversion under the VUV excitation, which is basically a new approach in the goal of achieving a quantum‐splitting phosphor.