Site Occupancy Preference and Antithermal Quenching of the Bi2+ Deep Red Emission in β‑Ca2P2O7:Bi2

The resistance to thermal quenching is an essential factor in evaluating the performance of luminescent materials for application in white light emitting diodes (WLEDs). In this work, we studied the site occupancy preference and thermal quenching of luminescence in β-Ca2P2O7:Bi2+ red phosphor at low (10–300 K) and high temperatures (303–573 K). In β-Ca2P2O7, the host lattice has four different calcium sites, at which Bi2+dopant can be located. After comparing the change of bond energy when the Bi2+ ions are incorporated into the four calcium sites, we found out that Bi2+ would preferentially occupy the smaller energy variation sites Ci(2) and Ci(1) in this compound, which can be assigned to Bi(2) and Bi(1), respectively. Surprisingly, we noticed that the variation of emission intensity is different under different excitations when the temperature changes from 10 to 300 K. When exciting into the typical absorption of Bi(1) sites at 419 nm, the emission intensity at 300 K remains only 38% as compared to that at 10 K, while exciting into typical Bi(2) absorption at 460 nm, the emission intensity increases to 110%. When further increasing the temperature from 303 to 573 K, we observed a similar phenomenon, and the emission at 460 nm excitation starts to quench at 453 K. The emission intensity at 573 K still remains 86.1% of that at 303 K. This might be attributed to the Bi(2) → Bi(1) energy transfer. It is also evidenced by the time-resolved emission spectra and lifetime values. This work gives new insights into better understanding luminescent behaviors of Bi2+-doped materials with multiple cation sites. This should be helpful in the future when designing the bismuth doped phosphor for WLEDs with better resistance to thermal quenching.