Synthesis and luminescence enhancement of Eu³⁺, Sm³⁺ co-doped Li_1.11Ta_0.89Ti_0.11O₃ phosphor

Abstract

Phosphors based on the Li_1+xTa_1−xTi_xO₃ solid solutions (0.05 ≤ x ≤ 0.25) as host materials were investigated. The optimal composition of the host for red-light emitting phosphor was found to be Li_1.11Ta_0.89Ti_0.11O₃ (x = 0.11) when activated with Eu³⁺. In this work, in order to further enhance the emission intensity, a series of Eu³⁺- and Sm³⁺-doped Li_1.11Ta_0.89Ti_0.11O₃ phosphors was synthesized by solid-state reaction. The phosphor doped with 2.5 wt% Eu₂O₃ and 0.1 wt% Sm₂O₃ showed the highly efficient red-light emission upon excitation at 399 nm, with the internal quantum efficiency being 98%. Its PL intensity was ca. 1.4 times higher than that of the 2.5 wt% Eu³⁺-doped Li_1.11Ta_0.89Ti_0.11O₃ phosphor, indicating that the small amount of Sm³⁺ acted as an effective sensitizer.

Introduction

In the ternary Li₂O–Nb₂O₅–TiO₂ system, the compounds with the general composition Li_1+x−yNb_1−x−3yTi_x+4yO₃ (LNT), known as the “M-phase” solid solutions, form superstructures, where M = Nb or Ta with 0.06 ≤ x ≤ 0.33 and 0 ≤ y ≤ 0.17. Since the discovery of the M-phase for LNT by Castrejon et al. [1], [2], the physical properties as well as the crystal structures of the solid solutions have been investigated in detail [3], [4], [5], [6]. The M-phase superstructures have been found to be formed by the periodical insertion of intergrowth layers in the matrix having a trigonal structure. The relationship between dielectric properties and periods of the intergrowth layers of the M-phase have been studied successively [7], [8].

The LNT compounds have been applied to the host materials of new phosphors [9], [10]. The photoluminescence (PL) intensities at 625 nm were much higher for LNT:Eu³⁺ than for LiNbO₃:Eu³⁺ [11], [12]. The RE-doped LiTaO₃ phosphors have also been reported by other groups, where RE = Pr³⁺ [13], Er³⁺ [14], Tb³⁺ [15] Eu³⁺ [16] and Tm³⁺ [17]. Recently, we have succeeded in synthesizing new red phosphors based on the quaternary Li_1+x(Ta_1−zNb_z)_1−xTi_xO₃ (LTNT, 0 ≤ x ≤ 0.25, 0 ≤ z ≤ 1.0) solid solutions as the host materials [18]. We have confirmed that the superstructures are actually formed in the higher Ti content regions of 0.06 ≤ x ≤ 0.2 in the Ta-containing system (0.05 ≤ z ≤ 0.175). The PL intensities of the LTNT:Eu³⁺ phosphors were not affected by the structural distortions induced by the superstructure formation, but dependent on the x- and z-values as well as the concentration of Eu³⁺.

The RE (= Sm³⁺, Er³⁺, Dy³⁺ and/or Tm³⁺) doped Li_1+xTa_1−xTi_xO₃ (LTT) phosphors, showing various emission colors, have been synthesized by the conventional electric furnace in air to compared their PL properties to those of the LNT phosphors [19]. We have concluded that the small differences in environment of the dopant sites between the host materials of LNT and LTT eventually affect the emission energy of the RE³⁺ ions. In the LTT host materials, the most effective activator was Eu³⁺. The internal quantum efficiency attained the value of over 84% for the host composition of Li_1.11Ta_0.89Ti_0.11O₃ (x = 0.11) [19]. The Li_1.11Ta_0.89Ti_0.11O₃:Eu³⁺ phosphor, when excited by the near ultraviolet (UV) light, emits strong red light, hence it could be suitable for the backlight of projectors; the backlight contains an array of red, green, and blue LEDs whose combined light forms white light [20]. To apply the LTT-host phosphors to the backlight, much higher PL intensities are required. Because the Sm³⁺ ions have been reported to act as sensitizers for various types of Eu-doped phosphors [21], [22], [23], [24], we have tried to enhance the emission intensities of the LTT phosphors by co-doping of Eu³⁺ and Sm³⁺. We have successfully prepared the red-light emitting Li_1.11Ta_0.89Ti_0.11O₃:Eu³⁺, Sm³⁺ phosphors with the improved PL properties.