Ba(1-x)SrxMg3SiN4:Eu narrowband red phosphor

Abstract

A new narrowband red phosphor, Ba0.8Sr0.2Mg3SiN4:Eu, suitable for use in LED lighting, has been identified as the optimal composition from the series of Ba(1-x)SrxMg3SiN4:Eu including x = 0, 0.2, 0.5, 0.8, and 1. Experimental X-ray diffraction measurements and structures calculated via density functional theory reveal that Ba(1-x)SrxMg3SiN4:Eu stabilizes in a tetragonal crystal structure, changing to the triclinic structure for Ba >0.8. Narrow band Eu2+ emission is observed in the tetragonal phase, but as a function of Ba substitution onto the Sr site, several properties change in concert: the emission maximum shifts to longer wavelengths, room temperature quantum efficiency improves, Eu2+ decay time increases and thermal stability of emission improves. Concomitant with the structure change to triclinic, the narrow Eu2+ emission band becomes broadened, the bandgap increases, and decay time slows to a typical Eu2+ decay of ∼1 μs. Although significantly moisture-sensitive, the strong absorption of Ba0.8Sr0.2Mg3SiN4:Eu at 450 nm, its emission peak at 635 nm and width of 51 nm FWHM provide efficient color rendering in the red, potentially achieving CRI and R9 values in excess of 95, along with good efficacy, in combination with other phosphors. Excitation intensity droop is comparable to other similar Eu-doped nitrides, but while thermally induced quenching is reduced with Ba substitution, it is still significant. The measured photoluminescence decay for Ba0.8Sr0.2Mg3SiN4:Eu of ∼200 ns is faster than of related Eu-doped nitrides, and together with a Q.E. of only ∼40% suggests the presence of competing non-radiative decay routes, thought to be due to thermal ionization of the Eu2+ excited state to the conduction band. Improved Q.E. for SrMg3SiN4:Eu partially substituted with Ba is likely due to the cation disorder this introduces, suppressing nonradiative processes correlated with lattice vibrations and increasing the population of shallow traps that release electrons on the emission lifetime timescale, consistent with the emission decay time lengthening and Q.E. improvement as a function of Ba content.