Optical properties and energy transfers of Ce³⁺ and Mn²⁺ in Ba₉Sc₂(SiO₄)₆

Highlights

• Analysis of XRD patterns indicates that Ce³⁺ and Mn²⁺ in Ba₉Sc₂(SiO₄)₆ (BSS) are located at Sc³⁺ and Ba²⁺ sites, respectively.

• BSS:Ce³⁺ exhibits an intense ultraviolet emission band peaking at 383 nm.

• BSS:Ce³⁺, Mn²⁺ exhibits an intense red emission band at 615 nm due to the effective Ce³⁺→Mn²⁺ energy transfers.

• The energy transfer mechanisms of Ce³⁺ and Mn²⁺ in BSS are investigated based on Inokuti–Hirayama model.

Abstract

Analysis of XRD patterns indicates that Ce³⁺ and Mn²⁺ in Ba₉Sc₂(SiO₄)₆ (BSS) presents preferably at the Sc³⁺ and the Ba²⁺ site, respectively. Ba₉Sc₂(SiO₄)₆:Ce³⁺ (BSS:Ce³⁺) exhibits an intense emission band peaked at 383 nm. Codoping Mn²⁺ into this material can generate a red emission band at 615 nm of Mn²⁺ through Ce³⁺–Mn²⁺ energy transfers (ETs). To achieve the ET mechanisms, critical distance and coefficient, the dynamical processes of the ET are investigated based on Inokuti–Hirayama model. For Ce³⁺ concentration of 5 mol%, the corresponding rate constant and the critical distance are evaluated to be 8.4×10⁻³⁷ cm⁶ s⁻¹ and 0.52 nm, respectively. The ET efficiency can reach 48%. The emission intensity ratio of the Mn²⁺ to the Ce³⁺ is also studied to understand color-tunable luminescence as the blue Ce³⁺ is excited by UV light. These results show the promising application of this phosphor for white LEDs.

Introduction

Phosphor-converted white light emitting diode (pc-WLED) is regarded as a new lighting source for the next generation. The most current pc-WLEDs employ blue InGaN LEDs combined with yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) garnet phosphors [1], [2]. However, since lacking in a red component, the mixed light emitted by this type has a poor color rendering index (R_a<80) and a high correlated color temperature (CCT>5000 K). One of the alternative ways to achieve white light with a suitable CCT and a high R_a is the combination of ultraviolet (UV) LEDs with the red, green and blue (RGB) multi-phased phosphors [3]. However, one of the significant drawbacks of this way is the lacking in high efficiency red phosphors.

Mn²⁺ is a kind of commonly used UV-excited red light emission ion. Depending on the crystal field of the host materials the emission spectrum of the Mn²⁺ ranges from green to deep red (500–700 nm) [4], [5]. However, the biggest obstacle for the application of the Mn²⁺ used as good candidates for red phosphors is that the d–d absorption transition is difficult to be pumped. Since this transition is strongly parity forbidden, and therefore, seldom Mn²⁺ singly doped hosts can act as applicable phosphors [6]. In order to improve the emission intensity of Mn²⁺, Ce³⁺ or Eu²⁺ is widely used as sensitizers in many Mn²⁺-doped hosts such as KCaY(PO₄)₂:Eu²⁺, Mn²⁺ [7], SrAl₂O₄:Ce³⁺, Mn²⁺ [8], and Ca₃Sc₂Si₃O₁₂:Ce³⁺, Mn²⁺ [9].

In 2009, a novel green-emitting phosphor of Eu²⁺ doped Ba₉Sc₂(SiO₄)₆ (BSS) was reported by Nakano et al., which can be used in pc-WLED [10]. In this paper, we report a strong UV-blue emission in the range of 350–450 nm by doping Ce³⁺ into the BSS host. By codoping Mn²⁺ into BSS:Ce³⁺, furthermore, an intense Mn²⁺ red emission band at 615 nm is achieved due to the effective energy transfers (ETs) from Ce³⁺ to Mn²⁺, which indicates that BSS:Ce³⁺, Mn²⁺ can also be used as red phosphors for white LEDs.