Mn-Doped Heterodialkaline Fluorogermanate Red Phosphor with High Quantum Yield and Spectral Luminous Efficacy for Warm-White-Light-Emitting Device Application.

Narrow band red-emitting Mn-doped fluoride phosphor is an essential red component of modern white-light-emitting-diode (WLED) devices. Its luminescence has sensitivity to structure and influences the performance of WLED. In this paper, we report a high-performance Mn phosphor based on a new heterodialkaline fluorogermanate, CsNaGeF:Mn. As determined by the single-crystal X-ray diffraction analysis, the CsNaGeF compound crystallizes in the orthorhombic crystal system with space group Pbcm (No. 57). Under excitation by 360 and 470 nm photons, CsNaGeF:Mn emits intense red light near 630 nm with a high quantum yield of 95.6%. The electronic energy levels of the Mn ion in CsGeF, NaGeF, and CsNaGeF are calculated using the exchange charge model of crystal-field theory. The local Mn environment inducing different zero-phonon-line emissions in the structures is probed by electron paramagnetic resonance. The Mn-doped heterodialkaline fluorogermanate CsNaGeF:Mn exhibits broader emission as a result of the lowest symmetry. It has higher quantum yield than NaGeF:Mn and higher spectral luminous efficacy than CsGeF:Mn. Given the good thermal stability and efficient luminescence, a prototype warm-WLED device with a color rendering index of 92.5, a correlated color temperature of 3783 K, and a luminous efficacy of 176.3 lm/W has been fabricated by employing the CsNaGeF:Mn phosphor as the red component. Our results not only reveal that a high-performance Mn red phosphor is achieved through cationic substitutions but also construct a relationship of performance-structure to guide the design of Mn phosphors in the future.