Article pubs.acs.org/cm
Efficient Yellow-Orange Phosphor Lu4Ba2[Si9ON16]O:Eu2+ and Orange-Red Emitting Y4Ba2[Si9ON16]O:Eu2+: Two Oxonitridosilicate Oxides with Outstanding Structural Variety Christian Maak,† Robin Niklaus,† Franziska Friedrich,† Andre Maḧ ringer,† Peter J. Schmidt,‡ and Wolfgang Schnick*,† †
Department of Chemistry, University of Munich (LMU), Butenandtstrasse 5-13, 81377 Munich, Germany Lumileds Germany GmbH, Lumileds Phosphor Center Aachen, Philipsstrasse 8, 52068 Aachen, Germany
‡
S Supporting Information *
ABSTRACT: The oxonitridosilicate oxides Y4Ba2[Si9ON16]O:Eu2+ and Lu4Ba2[Si9ON16]O:Eu2+ have been synthesized starting from REF3, RE2O3 (RE = Y, Lu), BaH2, Si(NH)2, and EuF3 in a radiofrequency furnace at 1550 °C. The crystal structures were solved and refined from single-crystal X-ray data supported with Rietveld refinement on X-ray powder diffraction data. Both compounds are isotypic and crystallize in monoclinic space group P21/c (no. 14) with Z = 4 and a = 6.0756(2), b = 27.0606(9), c = 9.9471(3) Å, and β = 91.0008(8)° for RE = Y and a = 6.0290(3), b = 26.7385(12), c = 9.8503(5) Å, and β = 90.7270(30)° for RE = Lu. The unique crystal structure exhibits a three-dimensional network made up from Q4-type SiN4 and Q3-type SiON3 tetrahedra. Containing 4-fold bridging N[4] atoms in star-shaped units [N[4](SiN3)4] next to N[3], N[2], O[1], and noncondensed oxide ions, the title compounds illustrate the vast structural variety in (oxo)nitridosilicates. Under excitation with UV to blue light, Y4Ba2[Si9ON16]O:Eu2+ shows emission in the orange-red spectral range (λmax = 622 nm, full width at half-maximum (fwhm) ≈ 2875 cm−1). Yellow-orange emitting Lu4Ba2[Si9ON16]O:Eu2+ (λmax = 586 nm, fwhm ≈ 2530 cm−1) exhibits high internal quantum efficiency (IQE) ≈ 85%. This makes Lu4Ba2[Si9ON16]O:Eu2+ a promising phosphor for low color rendering index (CRI) warm white phosphor converted light emitting diodes (pcLEDs).
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INTRODUCTION In recent years, light emitting diodes (LEDs) have been replacing traditional incandescent light bulbs and fluorescent tubes due to longer lifetime, higher energy efficiency, and improved sustainability. White LED light is most commonly generated by down-conversion using green and red emitting phosphors on top of a blue emitting (In,Ga)N-LED.1−4 Red components used in pcLEDs (phosphor converted LEDs) are, for example, the nitridosilicates (Ba,Sr) 2Si5N8:Eu2+ and (Sr,Ca)AlSiN3:Eu2+.5,6 However, these phosphors show broad emission bands (90−100 nm) leading to energy loss caused by emission in the infrared range. Therefore, narrow-band red emitting phosphors with reduced emission in the infrared range are of crucial importance in developing energy saving high CRI (color rendering index) pcLEDs.7,8 Compounds with narrowband red emission are nitridolithoaluminates such as Sr[LiAl3N4]:Eu2+ and Ca18.75Li10.5[Al39N55]:Eu2+ and nitridomagnesoaluminates like Sr[Mg2Al2N4]:Eu2+, as well as the nitridomagnesosilicates Li2(Ca1−xSrx)2[Mg2Si2N6]:Eu2+ and Sr[Mg3SiN4]:Eu2+ (SMS).9−13 To the best of our knowledge, SMS is the most narrow red emitting Eu2+ doped nitride phosphor (λem = 615 nm, full width at half-maximum (fwhm) = 1170 cm−1) reported so far. Because of the intriguing emission © 2017 American Chemical Society
properties of known phosphors, novel (oxo)nitridosilicate compounds are promising materials for investigation and application in pcLEDs. A number of materials of this class of compounds also combine crucial properties for state-of-the-art phosphors like optical transparency, high thermal and chemical stability, low thermal quenching, and high conversion efficiency.14,15 Recently, we reported on several novel (oxo)nitridosilicates(chlorides) synthesized by reaction of LaX3 (X = F, Cl), AEH2 (AE = Sr, Ba), and different silicon sources (e.g., “Si(NH)2”, “Si2(NH)3·6 NH4Cl”) in a radiofrequency furnace. The formation of highly stable binary salts AEX2 from LaX3 and AEH2 is discussed to be the thermodynamic driving force of this reaction type.16−18 Besides an intriguing structural chemistry of the formed silicates, La3BaSi5N9O2:Ce3+ is a promising phosphor for single phosphor luminescence conversion due to its quite broad yellow emission.16 In this contribution, we used the reaction type described above to synthesize the two oxonitridosilicates Y4Ba2[Si9ON16]Received: July 11, 2017 Revised: September 11, 2017 Published: September 11, 2017 8377
DOI: 10.1021/acs.chemmater.7b02900 Chem. Mater. 2017, 29, 8377−8384
Article
Chemistry of Materials O:Eu2+ and Lu4Ba2[Si9ON16]O:Eu2+. The materials crystallize in a unique structure type and show strong orange-red and yellow-orange luminescence, respectively, when excited with UV to blue light. Especially the highly efficient Lu compound with emission in the yellow-orange spectral range seems promising for application in low CRI warm white pcLEDs.
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UV/Vis Spectroscopy. A diffuse reflectance spectrum of Y4Ba2[Si9ON16]O was recorded on a Jasco V-650 UV/vis spectrophotometer with a deuterium and a halogen lamp (Czerny-Turner monochromator with 1200 lines/mm concave grating, photomultiplier tube detector). The spectrum was obtained in a range of 200 to 800 nm with 1 nm step size. DFT Calculations. Structural relaxations were carried out with the Vienna ab initio simulation package (VASP).28−30 Convergence of total energies of the unit cell (
