Color-tuning of Eu2+/3+ ions doped in phosphate using crystal field modulation

Abstract

Abstract The discovery and design of new single-phased phosphors that possess high efficiency and good thermal stability with absorption in the near-UV region is an industry-wide research hotspot. The strategy to find new phosphors involves alternating their chemical composition based on the solid solution design or energy transfer among the activators. For example, Ji et al. revealed that the Sr2+ ion substitution for the Ca2+ ion in β-Ca3(PO4)2: Eu2+ induced a dramatic red shift of the broad emission peak from 493 to 532 nm. The other strategy is by co-doping sensitizer and an activator in the host, using the energy transfer from sensitizer to activator, such as Eu2+/Mn2+, Ce3+/Eu2+, Eu2+/Mn2+/Tb3+, and Ce3+/Mn2+/Tb3+. Since the host’s excitation and emission characteristics are sensitive to the covalence and the crystal field environment, adjusting the chemical composition of the host can lead to an efficient technique for controlling the emission color. Here, the aim for the target host is a whitlockite-based structure, the primary phosphate minerals that can be easily found in most planetary materials including rocks from the Earth, Moon, Mars, and asteroids, from which a strong thermal stability can be predicted by the excellent structural rigidity. The oxy-whitlockite-based phosphors have drawn considerable attention as a host lattice because of their promising properties such as excellent thermal and chemical stability, high luminescence efficiency, and the ability to produce plenty of crystal field environments imposed on emission centers upon activation using rare earth ions. Moreover, as the luminescence center is isolated by the PO43- group in phosphate based hosts, M3M*(PO4)2 (M= Na, K; M*= Gd, Y, La, Lu) compounds were spotlighted as good host materials because of the host absorption edge located at a rather short wavelength (about 140-180 nm), which is suitable as host for rare earth ions. Moreover, phosphates based host series have significant advantages such as low sintering temperature, low cost, chemical stability, broad band gap and structural diversity, and they can produce plenty of crystal field environments.