Orderly-Layered Tetravalent Manganese-Doped Strontium Aluminate Sr 4 Al 14 O 25 :Mn 4+ : An Efficient Red Phosphor for Warm White Light Emitting Diodes Mingying Peng, ‡,1,† Xuewen Yin, ‡,1 Peter A. Tanner, § Chuqi Liang, ‡ Pengfei Li, ‡ Qinyuan Zhang, ‡ and Jianrong Qiu ‡ ‡ State Key Laboratory of Luminescent Materials and Devices, Institute of Optical Communication Materials, School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China § Department of Science and Environmental Studies, The Hong Kong Institute of Education, 10 Lo Ping Road, Tai Po, New Territories, Hong Kong, China Searching for an efficient non rare earth-based oxide red phos- phor, particularly excitable by light in the wavelength from 380 to 480 nm and unexcitable by green light, is essential for the development of warm white light emitting diodes (WLEDs). Here, we report a promising and orderly-layered candidate: Sr 4 Al 14 O 25 :Mn 4+ with CIE color coordinates (0.722, 0.278). It has higher luminescence efficiency particularly upon blue excitation and is much cheaper than the commercial red phos- phor 3.5MgO0.5MgF 2 GeO 2 :Mn 4+ (MMG:Mn 4+ ). In sharp contrast to Eu 2+ -doped (oxy)nitrides, the phosphor can be synthesized by a standard solid-state reaction at 1200°C in air. The effects of flux boron content, environment, and preparation temperature, sintering dwelling time as well as Mn concentra- tion have been systematically investigated for establishing the optimal synthesis conditions. The low temperature emission spectra reveal that there are at least three types of Mn 4+ ions in Sr 4 Al 14 O 25 :Mn 4+ due to the substitution for the distorted octahedral Al 3+ sites. The AlO 6 layers where Mn 4+ prefers to reside are well separated from one another by AlO 4 tetrahedra in one dimension parallel to axis a. This scenario can efficiently isolate Mn 4+ ions from local perturbations, thereby enabling the high efficiency of luminescence. The energy transfer rates and mechanism are discussed. I. Introduction E VER since the emergence of civilization, lighting has been a subject of perennial interest for mankind. In recent years, considerable attention has been paid to white lighting based upon the technique of LEDs. This is primarily because of the overwhelming advantages compared with incandescent and fluorescent lightings, such as high luminous efficiency, long lifetime, and small size. 1–5 The high efficiency confers energy saving and environmental benefits upon WLED, and thus promotes the widespread applications, such as in domes- tic and commercial lighting, automobiles, communications, imaging, agriculture, and in medicine. 1–5 The mainstream technology for WLED is based upon the combination of a single LED chip with one or more conversion phosphors. Nowadays, the commercial and most popular approach is the conjunction of an InGaN blue LED (typically with a wavelength between 450 and 480 nm) with Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce 3+ ). 1–5 However, YAG:Ce 3+ exhibits weak emission in the red spectral region, and this results in poor color rendering ability and a higher color temperature of the dichromatic system. 1–5 The addition of a red phosphor especially with strong blue absorption is one of best strategies for generating white light with the warm perception similar to incandescent light. Another alternative approach involves blending the emis- sions from blue, green, and red phosphors with a UV chip (typically with a wavelength in the range from 380 to 420 nm). 2,6 This scheme can improve the color rendering index and simultaneously keep the light color stable, especially when the driving current changes. So, creating a warm WLED with either of the above two approaches necessitates the development of an efficient red phosphor. The challenge has stimulated researchers to revisit commercial lamp and cathode ray tube phosphors. This has proved to be unfruitful because of the absence of effective absorption in the spectral range from 380 to 480 nm, or because of poor resistance of the phosphor against thermal and environmental attack, for example Eu 3+ -based phosphors. 7–9 The high demand also provoked the quest for new phos- phors, which mainly focus on rare earth-doped schemes, for instance Eu 2+ -doped oxynitrides, nitrides, aluminates, and aluminosilicates. 1,2,10 Among these systems, nitrides and oxynitrides have been the subjects of intensive studies because of their outstanding performances, such as high ther- mal and chemical stability. 1,2,10 However, the synthesis of these hosts has to be performed at high temperature and high pressure, for example at 1900°C and 10 atm N 2 atmo- sphere for Eu 2+ -doped b-SiAlON. 10 The harsh preparation conditions and high price of raw materials destine these phosphors to be costly. Also, for Eu 2+ -doped red phosphors with blue absorption, the reabsorption of the coexistent green emission of WLEDs will be unavoidable. 2 In recent years non rare earth-based eco-friendly phos- phors, which can be prepared under milder conditions have received burgeoning interest. This is driven by the soaring price of rare earths and the strong desire to minimize the cost of the LED device, one of the main obstacles for wide- spread adoption. Lin et al. found that doping Ba 2+ into BPO 4 can create oxygen-related defects, which emit a bluish white light with a quantum yield as high as 31%. 6 Further- more, the color of light can be tuned simply by the concen- tration of Ba 2+ . Peng et al. recently reported a new family of phosphors activated by bismuth, one of the less-toxic heavy metals, which can absorb near-UV or blue light and emit brilliant red light. 5,11–13 A. Srivastava—contributing editor Manuscript No. 32724. Received February 19, 2013; approved April 18, 2013. 1 Equal contribution to this work. † Author to whom correspondence should be addressed. e-mail: pengmingying@scut. edu.cn 2870 J. Am. Ceram. Soc., 96 [9] 2870–2876 (2013) DOI: 10.1111/jace.12391 © 2013 The American Ceramic Society J ournal