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he electronic structures of KMPO4 (M ¼ Sr, Ba) were calculated by the density functional theory with the local-density approximation. The calculated result shows that KSrPO4 and KBaPO4 are direct-band gap materials with direct energy gaps of 4.52 and 4.35 eV, respectively. Meanwhile, by analyzing the valence band structures of KMPO4 (M ¼ Sr, Ba), the strength of binding of valence band electrons of KBaPO4 is stronger than that of KSrPO4. In addition, the photoluminescence (PL) properties of the intense red-emitting phosphors KM1–xPO4:Eu3þx (M ¼ Sr, Ba) were investigated. The PL emission spectra excited at 393 nm are dominated by the peak at 611 nm due to the forced electric dipole 5D0–7F2 transition of Eu3þ ions, which is attributed to low local symmetry sites occupied by Eu3þ ions in these hosts. And the optimum integrated intensities for KSr1–xPO4:Eu3þx and KBa1–xPO4:Eu3þx are 1.3 times and 1.1 times of that for commercial Y2O3:Eu3þ, respectively.

I. INTRODUCTION

The white light-emitting diodes (LEDs) have attracted much attention as an excellent solid-state lighting. This is because LED has several advantages in comparison with conventional incandescent and fluorescent lamps, such as high brightness, reliability, low power consumption, and long lifetime.1–3 The first white light was generated by combining a 465-nm blue light from the GaN-based LED and yellow light from the phosphor (YAG):Ce [(Y, Gd)3(Al, Ga)5O12:Ce3þ].4 However, this type of white light has several problems, for instance, blue-yellow color separation and low color rendering index caused by lack of red component in the spectrum. To solve these problems, the combination of near-ultraviolet (n-UV) LEDs peaking at 370–410 nm with a red, green, and blue phosphors blend has been proposed. This kind of white LEDs can offer superior color uniformity and a high color-rendering index (Ra > 90). In the tri-color LED phosphors, the present commercially available red-emitting phosphors are sulfide phosphors such as Y2O2S:Eu3þ and (Sr, Ca)S:Eu2þ for n-UV LEDs, whereas these phosphors are chemically unstable and inefficient.5 Thus, searching for a stable and superior red-emitting phosphor that absorbs n-UV light and emits red light is an urgent need. Recently, KMPO4 (M ¼ Sr, Ba) has drawn increasing attention due to their excellent thermal, chemical, and hydrolytic stability. The luminescence properties of Eu2þ a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0125

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http://journals.cambridge.org

J. Mater. Res., Vol. 25, No. 5, May 2010 Downloaded: 18 Mar 2015

in KMPO4 (M ¼ Sr, Ba) hosts were first investigated in 1997 by Poort et al.6 Afterwards, KSrPO4 and KBaPO4 as appropriate phosphor hosts were investigated by many authors.7–9 These results indicate that it is possible to develop efficient phosphors from rare-earth ions doped KMPO4 (M ¼ Sr, Ba). KMPO4 (M ¼ Sr, Ba) have a bK2SO4 structure with a space group of Pnma.10,11 The structures are formed by regular PO43 tetrahedral sur˚ ) and divalent rounding the m