Complex crystal structure and photoluminescence of Bi3+-doped and Bi3+/Eu3+ co-doped Ca7Mg2Ga6O18

Recently, broad-band white-light-emitting using Eu2+, Ce3+, or Bi3+ in multi-sites of a single-phase phosphor have drawn extensive attention due to their potential to realize high-quality indoor lighting. This work reports a novel oxide, Ca7Mg2Ga6O18 (CMG), possessing a complex crystal structure in the space group F432. Rietveld refinements of high-resolution synchrotron X-ray diffraction data were performed to determine the accurate atomic positions and occupancy factors, giving a reasonable composition of Ca7Mg1.91(4)Ga6.09(4)O18. Due to the multiple sites of Ca2+, which are suitable for the doping of Bi3+ activators, CMG is a promising host to achieve broad-band white-light emission. Detailed structural and spectroscopic analyses revealed that the Bi3+-activator shows multiple and site-selective occupancy, which is the origin of the red-shifts in both broad-band excitation and emission spectra upon increasing the Bi3+ content. A series of Bi3+ and Eu3+ codoped phosphors with tunable blue-pink-red emission were prepared due to the Bi3+-to-Eu3+ energy transfer. Due to the distinctive thermal behaviors of Bi3+ and Eu3+ emissions, CMG:Bi3+,Eu3+ is a candidate for optical thermometry.