Rare-earth free self-activated and rare-earth activated Ca2NaZn2V3O12 vanadate phosphors and their color-tunable luminescence properties

doi:10.1016/j.jpcs.2013.05.002

Journal of Physics and Chemistry of Solids

Volume 74, Issue 10, October 2013, Pages 1439-1443

https://doi.org/10.1016/j.jpcs.2013.05.002 Get rights and content

Highlights

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Rare-earth free self-activated and Eu³⁺ or Sm³⁺ doped vanadate phosphors Ca₂NaZn₂V₃O₁₂ were synthesized.
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The pure-phase garnet structure was verified by the XRD Rietveld refinement.
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The energy transfer behaviors from VO₄³⁻ to Eu³⁺/Sm³⁺ ions have been demonstrated.

Abstract

Novel rare-earth free self-activated and Eu³⁺ or Sm³⁺ doped vanadate phosphors Ca₂NaZn₂V₃O₁₂ were synthesized via the solid-state reaction route. The pure-phase garnet structures in this system were verified by the X-ray diffraction (XRD) and Rietveld refinement. The self-activated luminescence and rare earth doped luminescence behaviors have been studied in detail. The broad-band green emission can be found from the as-prepared Ca₂NaZn₂V₃O₁₂ compound, originating from the VO₄³⁻ emission. The energy transfer behaviors from VO₄³⁻ to Eu³⁺/Sm³⁺ ions in Ca₂NaZn₂V₃O₁₂:A (A=Eu³⁺, Sm³⁺) phosphors have been demonstrated by photoluminescence (PL) and decay time measurement. All the results indicate that self-activated Ca₂NaZn₂V₃O₁₂ and the Eu³⁺ or Sm³⁺ doped Ca₂NaZn₂V₃O₁₂ phosphors show great potential for the application in the near-UV excited white LEDs.

Introduction

White light-emitting diodes (w-LEDs) are considered to be one of the most important solid-state light sources, which can be used to take place of conventional light sources such as incandescent lamp or fluorescent lamp, because of their high efficiency, long lifetime, low power requirement, and so on [1], [2]. With the appearance of the first commercial w-LED in 1997, made by the combination of the blue LEDs chip and the yellow-emitting phosphor, typically Ce³⁺-doped yttrium aluminum garnet (YAG:Ce³⁺) [3]. However, low color-rendering index (Ra<80) and high correlated color temperature due to the deficiency of red emission seriously affects the quality of the present w-LEDs [4]. Therefore, the development of the new phosphors has attracted more and more attentions. In recent years, vanadates are widely studied as a kind of new phosphor hosts from the commercial red-emitting YVO₄:Eu³⁺ phosphors. Moreover, in the near-UV region of the excitation spectrum, vanadates exhibit a broad absorption band in some hosts owing to the intense charge-transfer (CT) transition, which also show the broad-band emission from 400 to 700 nm in some hosts [5]. Generally, the majority of the commercially available phosphors require radiation to excite the rare-earth activator in the phosphor system to generate multi-color light. However, the luminescent centers (dopants) of the phosphors used in fluorescent lamps or light emitting diodes (LEDs) are rare-earth elements that are typically expensive. But some recent works indicate that rare-earth free self-activated luminescence can be realized in some vanadates hosts [6]. Furthermore, Eu³⁺ ions were widely used as the activators of red luminescent materials. Sm³⁺ ions were also very important red-emitting lanthanide ions, which have the 4f⁵ electronic configuration exhibiting a strong orange–red fluorescence in the visible region [7]. It is believed that color-tunable luminescence can be obtained in some rare-earth free self-activated and Eu³⁺ or Sm³⁺ doped vanadates systems.

Rare-earth aluminate garnets (REAG, RE₃Al₅O₁₂) have been widely studied for optical applications [8]. However, they belong to the rare earth-containing compounds, which possess the high cost during the practical applications. Therefore, Bayer firstly reported LiCa₃M₂V₃O₁₂ (M=Mg, Cu, Zn, Co and Ni) phase in 1965, which is of the same garnet structure as aluminate garnet YAG [9]. Recently, Dhobale et. al. proved that efficient energy transfer takes place in Ca₂NaMg₂V₃O₁₂ from the host vanadate to the dopant RE ions without any cross transfer between the RE ions [10]. In 2004, single crystals of the vanadate garnet Ca₂NaZn₂V₃O₁₂ were synthesized by the floating zone method, and its crystal structure was investigated [6]. However, the intrinsic photoluminescence properties and rare earth ions doped luminescence behavior have not yet been reported in the Ca₂NaZn₂V₃O₁₂ system. In this paper, we firstly report the self-activated and Eu³⁺ or Sm³⁺ doped Ca₂NaZn₂V₃O₁₂ phosphor. We have demonstrated the efficient energy transfer from the host lattice to the RE ions in the present vanadate garnet matrix. By fine controlling of the rare earth ion content, color-tunable emission from bluish green to yellow can be realized in the Ca₂NaZn₂V₃O₁₂ system upon excitation of the broad-band near-UV light.

Section snippets

Synthesis

Ca₂NaZn₂V₃O₁₂ host, Ca₂NaZn₂V₃O₁₂:Eu³⁺ and Ca₂NaZn₂V₃O₁₂:Sm³⁺ phosphors were all synthesized via the high temperature solid state reaction route. The raw materials were 99.5% pure calcium carbonate (CaCO₃), sodium carbonate (Na₂CO₃), zinc oxide (ZnO), and ammonium metavanadate (NH₄VO₃) powders, and the 99.995% pure europium oxide (Eu₂O₃) and samarium oxide (Sm₂O₃). Firstly, stoichiometric amounts of starting materials were mixed and ground thoroughly in an agate mortar vibration milled for 1 h.

Structural characterizations

Rietveld analysis was performed to ensure the phase purity and to obtain the detailed crystal structure information of Ca₂NaZn₂V₃O₁₂. Powder diffraction patterns for as-prepared Ca₂NaZn₂V₃O₁₂ host were performed, as shown in Fig. 1. In this work, Ca₂NaZn₂V₃O₁₂ served as an initial structural model. Structural refinements of the Ca₂NaZn₂V₃O₁₂ were performed by a cubic space group of Ia-3d. Crystallographic data and details in the data collection and refinement parameters are summarized in Table 1

Conclusions

In summary, we have successfully synthesized the self-activated and Eu³⁺ or Sm³⁺ doped vanadate Ca₂NaZn₂V₃O₁₂ phosphors via the solid state reaction. XRD results indicate the formation of single phase compound with garnet structure. Through the study of luminescence properties and decay curves, we can find that energy transfer takes place from the VO₄³⁻ to the dopant RE ions. Ca₂NaZn₂V₃O₁₂ consists of a broad emission band covering 400–700 nm with emission peak near 497 nm. For the Eu³⁺ or Sm³⁺

Acknowledgments

This present work was supported by the National Natural Science Foundations of China (Grant Nos. 51002146 and 51272242), the Natural Science Foundations of Beijing (2132050), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0950), the Fundamental Research Funds for the Central Universities (2011YYL131) and the College Student Research Innovation Program of China University of Geosciences, Beijing (2012AG0025).

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Journal of Physics and Chemistry of Solids

Highlights

Abstract

Introduction

Section snippets

Synthesis

Structural characterizations

Conclusions

Acknowledgments

References (16)

Mater. Chem. Phys.

Chem. Phys. Lett.

J. Lumin.

J. Alloys Compd.

J. Lumin.

Mater. Res. Bull.

Spectrochim. Acta A

Mater. Res. Bull.

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Concentration dependent luminescence, energy transfer and Judd- Ofelt analysis of single composition white light emitting NaCa<inf>2</inf>Mg<inf>2</inf>V<inf>3</inf>O<inf>12</inf>: XEu<sup>3+</sup> phosphor