A distorted octahedron-dependent red-emitting Li2K4TiOGe4O12:Mn4+ phosphor for white LEDs

Abstract

An Mn⁴⁺-doped narrow-band red phosphor is a promising material to improve the color rendering and luminous efficiency of white LEDs (w-LEDs). Usually, the luminescence of Mn⁴⁺ originates from the octahedral crystallographic sites. To our knowledge, the luminescence of Mn⁴⁺ in the TiO₅ square pyramid sites is rarely reported. In this work, a distorted octahedron was formed by sharing oxygen atoms near two TiO₅ square pyramids, and Mn⁴⁺ was introduced into Li₂K₄TiOGe₄O₁₂ (LKTGO) with distorted octahedral crystallographic sites. Replacing Ti⁴⁺ with Mn⁴⁺ to form the Mn⁴⁺ luminescence center with a distorted octahedron coordination environment will be beneficial to the Mn⁴⁺ luminescence in the LKTGO host material and the enhancement of luminescence intensity. Herein, a new type of narrow-band red phosphor LKTGO:Mn⁴⁺ was synthesized. Interestingly, the LKTGO:Mn⁴⁺ phosphor has a wide excitation band in the spectral range of 250–570 nm and two peaks at 331 and 466 nm, indicating that they can be effectively excited by near ultraviolet (n-UV) and blue light. Under the strongest excitation at 466 nm, the LKTGO:Mn⁴⁺ phosphor shows a series of sharp characteristic emission bands of Mn⁴⁺ in the red region of the emission spectrum, and exhibits the strongest red emission at 665 nm corresponding to the ²E_g → ⁴A_2g transition of Mn⁴⁺. Moreover, the band in the blue region is much stronger than that in the n-UV region, indicating that the LKTGO:Mn⁴⁺ phosphor can be used as a narrow-band red phosphor for blue InGaN chip excitation. In addition, the as-prepared LKTGO:0.003Mn⁴⁺ phosphor and the YAG:Ce³⁺ yellow phosphor were coated on a 460 nm blue LED chip to fabricate a warm w-LED, which emits white light with a high color rendering index of 90.2 and a low color temperature of 2938 K. The results show that the LKTGO:0.003Mn⁴⁺ phosphor has potential applications in warm w-LEDs, providing a new method for optimizing phosphor materials for solid-state lighting.