Up/down conversion luminescence and charge compensation investigation of Ca0.5Y1−x(WO4)2:xLn3+ (Ln = Pr, Sm, Eu, Tb, Dy, Yb/Er) phosphors

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Highlights

  • Phosphors Ca0.5Y(WO4)2:Ln3+ (Ln = Pr, Sm, Eu, Tb, Dy, Yb/Er) were synthesized.

  • Structural and morphological investigations were carried out.

  • Photoluminescence properties of as-synthesized phosphors were investigated.

  • Photometric characteristics of the phosphors were studied.

  • Enhancement of PL emission was found due to the addition of alkali chlorides.

Abstract

Microstructures of Ca0.5Y(1−x)(WO4)2:xLn3+ (Ln = Pr, Sm, Eu, Tb, Dy, Yb/Er) phosphors were prepared via the solid-state reaction method. X-ray diffraction, scanning electron microscopy and photoluminescence were used to characterize the prepared phosphor samples. The results reveal that the phosphor samples have single phase scheelite structures with tetragonal symmetry of I41/a. The down/up conversion photoluminescence of the Ca0.5Y(1−x)(WO4)2:xLn3+ (Ln = Pr, Sm, Eu, Tb, Dy, Yb/Er) phosphors properties reveal characteristic visible emissions. The energy transfer process, fluorescence lifetime and color coordinates are discussed in detail. Furthermore, the phosphor Ca0.5Y(1−x)(WO4)2:xPr3+ co-doped with alkali chlorides shows the enhancement of luminescence, which was found in the sodium chloride co-doped powder phosphor. The photometric characteristics indicate the suitability of the inorganic powder phosphors for solid-state lighting and display applications.

Introduction

Tungstates have their own importance due to their unique structural, electrical, thermal and spectroscopic properties, which have prospective applications in the field of opto-electronics. Tungstates can be used as promising hosts for luminescence due to their excellent properties, such as chemical and thermal stability [1]. Rare-earth elements are well known by their luminescence behavior due to their various energy levels, and rare-earth tungstates are among the realm of luminescent materials [2]. As efficient luminescence centers, rare-earth ions play a crucial role in the field of modern lighting displays with varying emission colors arising from 4f–4f or 5d–4f transitions. To achieve the best luminescent performance, the doping of lanthanide with a suitable host material plays a crucial role. Rare-earth tungstates are vital functional materials and show tremendous performance in the fields of lasers, catalysis and ionic conductors and serve as potential host materials for doping with rare-earth ions due to the presence of the WO42− group [3]. Tungstates doped with rare-earth elements show self-emission in the blue-green region [1]. Rare-earth tungstates doped with trivalent lanthanide ions could generate white light which has significance in the solid-state lighting applications [1].

Falling into an important class of inorganic compounds, complex tungstates with the formula ARE(WO4)2 (A = Li, Na, K; RE = Rare-earth) have attracted the scientific interest due to their excellent opto-electronic properties and chemical stability. The extensively studied scheelite type compounds are AWO4 (A = divalent cations). Compared to AWO4 compounds, ARE(WO4)2 compounds are more suitable for doping rare-earth ions because they have remarkable acceptance for dopant ions. Additionally, the structural diversities (such as tetragonal and monoclinic symmetries) allow for ARE(WO4)2 doping with lanthanide ions without any fluorescence quenching [4]. Alkaline double rare-earth tungstates possess scheelite structures with a reasonable quantum yield and critical concentration [5]. Rare-earth double tungstate families possess excellent thermal and chemical stability with a broad near-UV charge transfer band (CTB) [6]. A wide variety of rare-earth compounds such as metal oxides, vanadates, phosphates, borates, molybdates and tungstates have been reported [3], [4], [5], [6]. Among these compounds, tungstates with the formula AMO4 [A = Cation; M = W] have received significant attention due to their excellent properties and they have potential applications in the realm of negative expansion materials and quantum electronics [7], [8], [9], [10]. Among these, alkaline double rare-earth tungstate compounds have been little studied. To control the shape and size of the crystalline phosphors, there are various synthesis techniques reported as solid-state reaction, co-precipitation, combustion, sol–gel and hydrothermal synthesis. Among these techniques, solid-state reaction is a familiar and simple technique for the synthesis of rare-earth doped tungstate phosphors. In this method, solid powders are obtained by grinding into fine powders to facilitate the maximum surface area, blended together and heated at an appropriate temperature. For the preparation of simple and complex inorganic phosphor materials, this is the easiest method [11], [12], [13], [14].

In this context, we report the preparation of novel Ca0.5Y(1−x)(WO4)2:xLn3+ (Ln = Pr, Sm, Eu, Tb, Dy, Er/Yb) down/up-conversion phosphors by the solid-state reaction technique. The aim of this study is to identify a suitable inorganic phosphor for solid-state lighting applications. Information regarding the crystal structure and phase purity was gathered by X-ray diffraction (XRD) study. The surface morphology and elemental analysis were conducted by scanning electron microscopy and energy dispersive X-ray (EDX) analysis. The photoluminescence (PL) was used to study the luminescence characteristics of the down/up conversion phosphors. Furthermore, the photometric characteristics such as color coordinate, color correlated temperature and luminous efficacies of radiation were estimated. The energy transfer process and decay profiles were discussed.

Section snippets

Syntheses of Ca0.5Y1−x(WO4)2:xLn3+ (Ln = Pr, Sm, Eu, Tb, Dy, Yb/Er)

All chemicals were analytical grade and were used without further purification. The synthesis of these compounds was performed by solid-state reaction using the precursors CaCO3, Y2O3, WO3 and Ln2O3. For a classical synthesis of inorganic phosphors, stoichiometric amounts of the starting reagents were mixed thoroughly in an agate mortar and ground for 1 h. These mixtures were transferred into an alumina crucible and annealed in a furnace at 900 °C for 3 h [14]. The annealed samples were used for

Structural and morphological studies

The crystal structure and phase purity of the samples were determined using powder X-ray diffraction. The XRD patterns of Ca0.5Y0.84(WO4)2:0.16Ln3+ (Ln = Pr, Sm, Eu, Tb, Dy and Yb/Er) as a representative for all of the other concentrations are shown in Fig. 1. The diffraction peaks are indexed, and the peak positions agree well with the standard JCPDS card No. 48-0886 of NaY(WO4)2. The strongest intensity peak is observed at 2θ = 28.98°. The XRD patterns show the scheelite structure with a space

Down Conversion luminescence of Ca0.5Y(1−x)(WO4)2:xLn3+ (Ln = Pr, Sm, Eu, Tb and Dy)

Rare-earth doped tungstates are good hosts for the luminescence of lanthanide ions. Yttrium forms the best host because of its empty 4f shells, which makes the f–f transition impossible unless it is doped with lanthanide ions. Similarly, under UV excitation, tungstates efficiently transfer energy to the doped lanthanide ions [1].

Conclusion

Microstructures of Ca0.5Y(1−x)(WO4)2:Ln3+ (Ln = Pr, Sm, Eu, Tb, Dy, Yb/Er) phosphors were synthesized by the conventional solid-state reaction method. The phase purity and the crystal structure were determined from the XRD patterns, and no impurity peaks were found. The results reveal that the phosphor samples are single phase scheelite structures with tetragonal symmetry of I41/a. The particle size is approximately 6.2 μm. The EDX spectrum confirms the presence of Ca, Y, Mo, O and Ln (Ln = Pr, Sm,

Acknowledgements

The authors gratefully acknowledge Alagappa University, Karaikudi and SAIF-IIT Madras for providing their instrumentation facilities for characterization.

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