Mn⁴⁺-Doped Heterodialkaline Fluorogermanate Red Phosphor with High Quantum Yield and Spectral Luminous Efficacy for...

Narrow band red-emitting Mn⁴⁺-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 Mn⁴⁺ phosphor based on a new heterodialkaline fluorogermanate, CsNaGeF₆:Mn⁴⁺. As determined by the single-crystal X-ray diffraction analysis, the CsNaGeF₆ compound crystallizes in the orthorhombic crystal system with space group Pbcm (No. 57). Under excitation by 360 and 470 nm photons, CsNaGeF₆:Mn⁴⁺ emits intense red light near 630 nm with a high quantum yield of 95.6%. The electronic energy levels of the Mn⁴⁺ ion in Cs₂GeF₆, Na₂GeF₆, and CsNaGeF₆ are calculated using the exchange charge model of crystal-field theory. The local Mn⁴⁺ environment inducing different zero-phonon-line emissions in the structures is probed by electron paramagnetic resonance. The Mn⁴⁺-doped heterodialkaline fluorogermanate CsNaGeF₆:Mn⁴⁺ exhibits broader emission as a result of the lowest symmetry. It has higher quantum yield than Na₂GeF₆:Mn⁴⁺ and higher spectral luminous efficacy than Cs₂GeF₆:Mn⁴⁺. 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 CsNaGeF₆:Mn⁴⁺ phosphor as the red component. Our results not only reveal that a high-performance Mn⁴⁺ red phosphor is achieved through cationic substitutions but also construct a relationship of performance-structure to guide the design of Mn⁴⁺ phosphors in the future.