Low Temperature Spectroscopic Properties of Divalent Bismuth Doped Ba2P2O7 for White Light LEDs

For application in white light LEDs, the temperature-dependence of photoluminescence (PL) is an essential property of a phosphor. For divalent bismuth doped Ba2P2O7, this has not yet been reported, although this class of materials exhibits very interesting spectroscopic properties as a red-emitting phosphors for near-UV (NUV) to blue excitation. This work examines the low temperature spectroscopic properties of Bi2+ doped Ba2P2O7. We report the resistance of luminescence to thermal quenching in the low-to-ambient temperature range. That is, for NUV excitation at similar to 386 nm, the emission at 300 K retains an 83% of the intensity observed at 10 K. At the same time, emission lifetime decreases only slightly. The latter is related to the gradually enhanced interaction between Bi2+ and local crystal field at rising temperature, which manifests in the disappearing phonon side band of 2P(1/2) -> 2P(3/2) and the increased splitting of P-2(3/2) (1) and P-2(3/2) (2) in the excitation spectrum. Static and dynamic PL spectra reveal two types of Bi2+ emission centers in the compound, consistent with crystallographic data, where two distinct Ba-sites are available for hosting Bi2+. From dielectric chemical bond theory, we obtained a mean bond covalency of 0.1025 for Ba(1)-O and 0.1358 for Ba(2)-O. When substituting Bi2+ (1) and Bi2+ (2) for Ba(1) and Ba(2), they also exhibit distinct emission behavior, i. e., with emission band maxima at 716 and 733 nm, respectively.