Elsevier

Advanced Powder Technology

Volume 31, Issue 9, September 2020, Pages 4045-4052
Advanced Powder Technology

Original Research Paper
Far-red emitting Mg2La3NbO9:Mn4+ powder phosphor: Synthesis and luminescence properties

https://doi.org/10.1016/j.apt.2020.08.009Get rights and content

Highlights:

  • Novel Mg2La3NbO9:Mn4+ phosphor is prepared by the solid-state reaction method in air.

  • Mg2La3NbO9:Mn4+ phosphor emits the far-red emission with the PL band peaking at ~708 nm.

  • Mg2La3NbO9:Mn4+ phosphor has the good thermal stability.

  • The luminous mechanism is explained by the Tanabe Sugano energy level diagram of Mn4+ ion.

Abstract

Novel Mg2La3NbO9:Mn4+ powder phosphors are successfully synthesized by the high-temperature solid-state reaction method under ambient air. The crystal structure, morphology, decay lifetimes, time-resolved emission spectra, and luminescence properties are investigated. The excitation spectrum of Mg2La3NbO9:Mn4+ phosphor monitored at 708 nm is observed in the region from 210 nm to 600 nm. Mg2La3NbO9:Mn4+ phosphor with excitation at the ultraviolet (UV) and blue light can emit the far-red emission with the emission band peaking at ~708 nm because of the 2E → 4A2 transition of Mn4+ and the corresponding chromaticity coordinates (0.7322, 0.2678). The optimal doping concentration of Mn4+ ion is ~0.6 mol%. The decay lifetime of Mg2La3NbO9:xMn4+ phosphors decrease from 1.143 ms to 1.099 ms with increasing the Mn4+ ion concentration from 0.2 mol% to 1.0 mol%. The good thermal stability of Mg2La3NbO9:Mn4+ phosphor is confirmed by the temperature-dependent emission spectra. The luminous mechanism of Mg2La3NbO9:Mn4+ phosphor is explained by using the Tanabe Sugano energy level diagram of Mn4+ ion. All the experimental results imply that Mg2La3NbO9:Mn4+ phosphor has a potential application in the white light-emitting diodes (LEDs) field.

Introduction

Red-emitting phosphors as an important component in solid-state lighting have been investigated widely [1], [2]. The white light-emitting diodes (LEDs) derived from the combination of the blue InGaN chip and yellow-emitting phosphor (YAG:Ce) emit the “cool” white light because of the lack of red component that can be efficiently excited by blue light [3]. Red-emitting phosphors are one of the components in the plant growth red LEDs and the white LEDs fabricated by the tri-phosphor + near ultraviolet (UV). At present, some ions (Eu2+, Eu3+, Pr3+, Sm3+, Bi2+, Mn2+, and Mn4+) can be used as red activator for phosphors. The sharp absorption peaks of Eu3+, Pr3+, and Sm3+ ions in the UV/near UV region [4], [5], [6], the strict synthesis conditions of Eu2+-doped nitrides and oxynitrides [7], [8], the finite doped hosts for Bi2+ ion [9], [10], no very strong red emission of Mn2+ ion [11], [12], and the expensive price of rare earth materials etc affect the further practical applications of Eu3+, Pr3+, Sm3+, Bi2+, Mn2+, and Eu2+-doped red-emitting phosphors. Thus, the Mn4+ used as an activator for red phosphors have attracted much attention due to the broad absorption bands and stronger emission [13], [14], [15], [16], [17].

Mn4+ ion has 3d3 electronic configuration and can usually be stabilized in octahedron environment after replacing the site of Al3+ [18], Ga3+ [19], Sc3+ [20], Si4+ [21], Sn4+ [22], Ge4+ [23], Ti4+ [24], Zr4+ [25], Nb5+ [26], Sb5+ [27], Ta5+ [28], Te6+ [29], Mo6+ [30], and W6+ [31] ions in many host materials. The naked 3d3 electron configuration causes that the optical properties of Mn4+ ion are seriously affected by the crystal field environment of host materials [32]. In octahedron environment, the 3d3 configuration of Mn4+ ion includes the quartet terms (4F and 4P) and doublet terms (2G and 2F). Mn4+ activated phosphors has broad excitation bands covering the spectral range from the UV to the blue region attributed to the O2− - Mn4+ charge transfer band (CTB) and the 4A24T1, 4A22T2, and 4A24T2 transitions of Mn4+ ion [33], [34]. Mn4+ activated phosphors can efficiently emit red or deep-red light in the range of 600–760 nm due to the 2E → 4A2 transition of Mn4+ ion. Recently, Mn4+-doped phosphors as the potential red phosphor have been widely researched because of their good luminescence properties. As is known, two main kinds of host materials including fluorides and oxides are being investigated in parallel for Mn4+ doping. Mn4+ doped fluoride shows typical narrow band emission peaking at ~630 nm because of the different crystal field and nephelauxetic effect, such as Li3Na3Ga2F12:Mn4+ [35], (NH4)3AlF6:Mn4+ [36], BaTiOF4:Mn4+ [37], (NH4)2NaInF6:Mn4+ [38], K2xBa1-xTiF6:Mn4+ [39], and K3TaO2F4:Mn4+ [40]. Mn4+ doped fluorides have an excellent luminescence properties and physical/chemistry stability expect the thermal stability. However, the application of toxic HF acid used as raw material in the synthesis process is harmful to the environments and human health. Thus, Mn4+-doped oxides with an environmentally friendly synthesis process and good chemical/thermal stability are attracting more interest, such as Ba2TiGe2O8:Mn4+ [41], La(MgTi)1/2O3:Mn4+ [42], SrLaMgTaO6:Mn4+ [43], Mg2La2SnO7:Mn4+ [44], Y3Al4GaO12:Mn4+,Dy3+ [45], Sr9Y2W4O24:Mn4+ [46], Li1.5La1.5WO6:Mn4+,Nd3+,Yb3+ [47], and Ca2LaTaO6:Mn4+ [48]. In Mn4+-doped oxides, the coordination environment of the hosts can affect the emission properties, and the emission peak position shifts in the range of 640–730 nm. So, the choosing a suitable host for Mn4+ ion are an important issue. Among the oxides, niobates, which include abundant [NbO6] octahedra that provides an alternative environment for Mn4+ ion, have attracted much attention because they have a good electrical/optical properties and thermal stability [49], [50]. Mn4+-doped niobate phosphors have been recently investigated widely, such as Li3Mg2NbO6:Mn4+ [51], Sr2YNbO6:Mn4+ [52], LiLa2NbO6:Mn4+ [53], A2LaNbO6:Mn4+, Eu3+ (A = Ba, Ca) [54], and BaLaMgNbO6:Mn4+, Dy3+ [26]. However, Mg2La3NbO9 is one of niobates and is not used as the host for Mn4+ ion.

In this work, a series of Mg2La3NbO9:Mn4+ phosphors are synthesized by the high temperature solid state reaction method under ambient air. We investigate their crystal structure, morphology, luminescence properties, the influence of Mn4+ concentration and temperature on emission intensity, lifetime, and the time-resolved emission spectra in detail. The concentration quenching (CQ) and thermal quenching (TQ) properties are explored by the emission spectral properties. The luminous mechanism is explained by Tanabe–Sugano energy level diagram of Mn4+ ion in octahedron.

Section snippets

Experimental process

In this work, the high purity chemicals with aladdin brand (e.g., MgO (A.R. 99.9%), La2O3 (99.99%), Nb2O5 (99.99%), and MnCO3 (A.R. 99.9%)) are directly used as the raw materials. All phosphors with the composition of Mg2La3Nb(1-x)O9:xMn4+ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol%) are synthesized by using the high temperature solid state reaction method under ambient air. At first, the raw materials are weighed accurately on the basis of the composition of Mg2La3Nb(1-x)O9:xMn4+ phosphors, and

Crystal structure and morphology

On the basis of the inorganic crystal structure database (ICSD) #88931 [55], the crystal structure schematic diagram of Mg2La3NbO9 can be depicted and displayed in Fig. 1. The crystal system of the host Mg2La3NbO9 is orthorhombic with space group Pbnm (62) and lattice constants (a = 5.6126(3) Å, b = 5.6549(3) Å, c = 7.9497(4) Å, V = 252.31(2) Å3, and Z = 4) [55]. In the crystal structure of the host Mg2La3NbO9, Mg2+ and Nb5+ ions share a position and are coordinated by six O2− ions to form [MgO6

Conclusions

In summary, the XRD patterns, EDS, and luminescence properties confirm that a novel Mg2La3NbO9:Mn4+ phosphors are successfully synthesized by using the high-temperature solid-state reaction method under ambient air. Mn4+ ions are successfully doped into the host (Mg2La3NbO9) lattice and tend to replace the Nb5+ sites in [NbO6] octahedron. Mg2La3NbO9:Mn4+ phosphor shows two broad excitation bands in the region from 210 nm to 600 nm and can be efficiently excited by the light from UV to blue. The

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work is financially supported by the National Natural Science Foundation of China (Nos. 51862015), Foundation of Jiang’xi Educational Committee (No. GJJ180564), Natural Science Foundation of Jiangxi Province (20181BAB206028), and National Undergraduate Training Program for Innovation and Entrepreneurship of China (No. 201910419014).

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