Materials Research Bulletin 45 (2010) 1614–1617

Novel Dy3+-doped Ca2Gd8(SiO4)6O2 white light phosphors for Hg-freelamps application

Yuhua Wang *, Yan Wen, Feng Zhang

Department of Material Science, School of Physical Science and Technology, Lanzhou University, Tianshui South Road No. 222, Lanzhou, Gansu 730000, PR China

A R T I C L E I N F O

Article history:

Received 20 December 2009

Received in revised form 8 July 2010

Accepted 19 July 2010

Available online 24 July 2010

Keywords:

A. Optical materials

A. Composites

D. Optical properties

D. Luminescence

A B S T R A C T

The luminescent properties of Ca2Gd8(1�x)(SiO4)6O2:xDy3+ (1% � x � 5%) powder crystals with

oxyapatite structure were investigated under vacuum ultraviolet excitation. In the excitation spectrum,

the peaks at 166 nm and 191 nm of the vacuum ultraviolet region can be assigned to the O2� ! Gd3+, and

O2� ! Dy3+ charge transfer band respectively, which is consistent with the theoretical calculated value

using Jwrgensen’s empirical formula. While the peaks at 183 nm and 289 nm are attributed to the f–d

spin-allowed transitions and the f–d spin-forbidden transitions of Dy3+ in the host lattice with

Dorenbos’s expression. According to the emission spectra, all the samples exhibited excellent white

emission under 172 nm excitation and the best calculated chromaticity coordinate was 0.335, 0.338,

which indicates that the Ca2Gd8(SiO4)6O2:Dy3+ phosphor could be considered as a potential candidate

for Hg-free lamps application.

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1. Introduction

Recently, phosphors matching with vacuum ultraviolet (VUVwavelength l < 200 nm and energy E > 50,000 cm�1) excitationfor Hg-free lamps fundamental research and the applications haveattracted much attention [1,2] due to the avoidance of harmful Hglamps (lex = 254 nm) [3,4]. Generally, the phosphors currentlyused to convert VUV photons generated by the discharge of Xe(with wavelength 147 nm) and Xe2 (172 nm) to blue, green, andred light have low luminous efficiency and other drawbacks suchas instability and low lifetime like commercial blue phosphorBaMgAl10O17:Eu2+ (BAM), and poor color purity like commercialred phosphor (Y,Gd)BO3:Eu3+ (YGB:Eu3+). It leads to a lessefficiency and costs more rare earth consumption throughblending tricolor phosphors to obtain white light. So thedevelopment of single-doped efficient white phosphor seemsmuch meaningful.

As one of activator ions in phosphors, Dy3+ ion has beeninvestigated in many hosts under VUV excitation [5–7]. There aretwo dominant bands in the emission spectrum, the yellow band(around 575 nm) corresponds to the hypersensitive transition of4F9/2–6H13/2 (DL = 2, DJ = 2), and the blue band (around 480 nm)corresponds to the 4F9/2–6H15/2 transition. Furthermore, the linelinking the yellow and blue wavelengths in the CIE 1931

* Corresponding author. Tel.: +86 931 8912772, fax: +86 931 8913554.

E-mail address: wyh@lzu.edu.cn (Y. Wang).

0025-5408/$ – see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2010.07.013

chromaticity diagram usually passes through the white lightregion. Therefore, it is possible to obtain near-white emission withonly Dy3+-activated luminescence materials. As far as host materialconcerned, silicates are of interest host materials because of theirvarious crystal structures and high physical–chemical stability [8–10]. As one kind of silicate, Ca2Gd8(SiO4)6O2 has been investigatedunder ultraviolet (UV) excitation [11–13]. However, to ourknowledge, photoluminescence (PL) properties of Ca2Gd8

(SiO4)6O2:Dy3+ under VUV excitation has not been reported sofar. According to some references [14–16], it is well known that[SiO4]4� tetrahedral has strong absorption in VUV region.Additionally, Gd3+ always acts as sensitizer to transfer energy toDy3+ under VUV excitation in many hosts, such as in CsGd2F7 andNaGdF4 systems [5,7]. Thus for obtaining optimal white lightphosphors for Hg-free lamp applications, a series samples ofCa2Gd8(1�x)(SiO4)6O2:xDy3+(CGSO:xDy) (1% � x � 5%) were syn-thesized and the luminescent properties under VUV excitationwere evaluated in detail in this paper.

2. Experimental

A series samples of Ca2Gd8(1�x)(SiO4)6O2:xDy3+ were preparedby conventional solid-state reaction. CaCO3 (99.8%), Gd2O3

(99.99%), H2SiO3 (99%) and Dy2O3 (99.99%) were employed asthe raw materials. The doping amount x varied from 1% to 5%.Stoichiometric amounts of the starting reagents CaCO3, H2SiO3,Gd2O3 and Dy2O3 were weighed and blended in agate morta andthen sintered at 1360 8C for 6 h.

Fig. 1. Experimental (crosses), calculated (red solid line), and difference (green solid

line) results of XRD refinement of CGSO:4%Dy. The inset part shows the XRD

patterns of all samples. (For interpretation of the references to color in this figure

legend, the reader is referred to the web version of the article.)

Fig. 2. The excitation spectrum of CGSO:4%Dy (lem = 575 nm), the inset illustrates

the excitation spectrum of CGSO:2% Ce3+ (lem = 388 nm).

Y. Wang et al. / Materials Research Bulletin 45 (2010) 1614–1617 1615

The X-ray diffraction (XRD) patterns were obtained on a RigakuD/max-2400 X-ray diffractometer, which was operated at 40 kV/60 mA with Ni-filtered Cu Ka radiation (l = 1.54056 A). The UVluminescent spectra were measured by FLS920T fluorescencespectrophotometer (Edinburgh Instrument, Britain). The VUVluminescent spectra were obtained on VM504 VUV monochroma-tor (Acton Research Corporation, ARC) with a 150 W Deuteriumlamp (Cathodeon Incorporated) and the resolution of grating was0.06 nm with standard 1200 g/mm. The VUV excitation spectrawere corrected by dividing the excitation intensity of sodiumsalicylate at the same measurement conditions. All measurementswere performed at room temperature.

3. Results and discussion

Fig. 1 plots experimental, calculated, and difference results ofthe XRD refinement of CGSO:4%Dy as a representative at roomtemperature, obtained using GSAS program [17]. All of theobserved peaks satisfy the reflection condition, Rp = 3.37%, andRwp = 5.94%. On analyzing the result, it was found that thesynthesized CGSO:4%Dy had hexagonal structure with P63/m spacegroup and lattice constants were a = 9.4192 (4) A, b = 9.4192 (4) A,c = 6.8793(5) A, respectively, which are nearly the same with thatof Ca2Gd8(SiO4)6O2. The XRD patterns of series of samples weresimilar. The result indicates that the crystal structure does not bechanged obviously by doping Dy3+ in Ca2Gd8(SiO4)6O2.

The VUV excitation spectra (lem = 575 nm) of a series ofCGSO:xDy (1% � x � 5%) samples were essentially the same exceptthe intensity. For instance, the excitation spectrum of CGSO:4%Dyin the range of 130–300 nm is presented in Fig. 2. In the excitationspectrum of CGSO:4%Dy, a broad band with several shoulder peaksat 166 nm, 183 nm, 191 nm, 277 nm, and 289 nm.

It was reported that host excitation band of BaGd4Si3O13:Tb,which has the same crystal structure with the Ca2Gd8(SiO4)6O2, isbetween 130 nm and 220 nm [18]. Thus the broad band from160 nm to 200 nm is considered to include the host-relatedabsorption. Additionally, O2�! Gd3+ charge transfer band (CTB),O2� ! Dy3+ CTB and the f–d spin-allowed transition of Dy3+

usually locate in the VUV region [19,20]. Combining theexperimental results with the calculation results below, thecorroboration for these peaks is as follows.

According to the empirical formula (1) proposed by Reisfeld andJwrgensen [21], the position of CTB of the rare earths ions can be

calculated:

Ect ðcm�1Þ ¼ ½xoptðXÞ � xoptðMÞ� � 30;000 cm�1 (1)

xopt (X) and xopt (M) are the optical electronegativities of the anionand central metal ion, respectively. The electronegativity of O2� isvariational in different hosts, thus according to the reportedposition of O2�! Eu3+ CTB which is situated at 280 nm inCa2Gd7.86Eu0.14(SiO4)6O2 [11] and xopt (Eu) which is reported tobe 1.74 [22], the calculated optical electronegativity of O2� is 2.93.By putting xopt (O) = 2.93 and xopt (Gd) = 0.91 [22] into the Eq. (1),the position of O2� ! Gd3+ CTB can be calculated to be 165 nm. Sothe peak at 166 nm should be assigned to the O2�! Gd3+ CTB.Similarly, the peak at 191 nm should be ascribed to O2� ! Dy3+

CTB by replacing xopt (Gd) = 0.91 with xopt (Dy) = 1.21 [22].For Dy3+ ions, when one electron is promoted from ground

states of 4f9 to excited states of 4f85d1, it can give rise to two groupsof f–d transitions, spin-allowed (SA) transitions are stronger andwith higher energies than spin-forbidden (SF) transitions. Theenergies of the lowest SA and the lowest SF f–d transitions can beevaluated according to Eq. (2) proposed by Dorenbos [23,24] withthe spectroscopic data of Ce3+ ions in the host lattice:

EðLn;AÞ cm�1 ¼ 49;340 cm�1 � DðAÞ þDELn;Ce (2)

where E(Ln, A) is the f–d energy difference (the energy needed toexcite an electron from the 4fn multiplet ground-state to the lowest4fn�15d state) in the lanthanide ion Ln3+ doped in host A.49,340 cm�1 is the energy of the first f–d transition of Ce3+ as afree ion, which is usually described as E(Ce, free). D(A) is defined asthe average value of the crystal field depression D(Ln, A), thelowing of E(Ln, free) when Ln3+ is doped in host A. DELn, Ce isdefined as the difference in f–d energy of Ln3+ with that of the firstelectric dipole allowed transition in Ce3+. The inset of Fig. 2 showsthe excitation spectrum of CGSO:2% Ce3+, and the first f–dtransition energy of Ce3+ ions was observed around 29,412 cm�1

(�340 nm). Thus by inserting this value and DECe, Ce = 0, theD(Ca2Gd8(SiO4)6O2) can be obtained as 19,928 cm�1, which isin good agreement with the reported D(Ca2Gd8(SiO4)6O2) =19,947 cm�1 calculated with Tb3+ in this compound [24]. Becausethe influence of the crystal field and covalency of the host lattice onthe red shift of 5d levels are approximately equal for all rare-earthions, so the D value calculated above is adopted by Dy3+ inCa2Gd8(SiO4)6O2. The value of DEDy, Ce has been reported to be25,100 � 610 cm�1 (spin allowed) and 7400 cm�1 (spin forbidden)[24]. So E(Dy3+, Ca2Gd8(SiO4)6O2) can be calculated to be54,362 � 886 cm�1 (184 � 3 nm) and 34,602 cm�1 (289 nm) accord-

Fig. 3. The emission spectra of the samples Ca2Gd8(SiO4)6O2:0.1% Dy3+ and

Ca2Gd8(SiO4)6O2 (lex = 172 nm).

Fig. 5. The chromaticity coordinates (x, y) of CGSO:xDy (1% � x � 5%) under 172 nm

excitation in the CIE 1931 chromaticity diagram.

Table 1The chromaticity coordination of CGSO:xDy (1%� x�5%) and color temperature.

The doped

concentration

of Dy3+ (x)

The chromaticity coordinate Color

temperature

(K)x y

x = 0.01 0.297 0.308 7774

x = 0.02 0.353 0.382 4830

x = 0.03 0.34 0.369 5229

x = 0.04 0.335 0.338 5387

x = 0.05 0.312 0.315 6663

Y. Wang et al. / Materials Research Bulletin 45 (2010) 1614–16171616

ing to Eq. (2). Accordingly, the excitation peaks located at 183 nm and289 nm should be, respectively, due to the SA and SF f–d transitions ofDy3+. At the same time, the other peak observed at 277 nm in UVregion is attributed to 8S7/2! 6IJ of Gd3+, which demonstrates that inthis compound the energy transfer exists from Gd3+ to Dy3+.

In Fig. 3, the emission spectra of the samples Ca2Gd8

(SiO4)6O2:0.1% Dy3+ and Ca2Gd8(SiO4)6O2 under the excitation of172 nm are shown as curve (a) and (b), respectively. It is observedthat the intensity of the peak at 313 nm (6P7/2! 8S7/2 transition ofGd3+ ion) in curve (a) is much weaker than that in curve (b),indicating that the energy transfer process from Gd3+ to Dy3+ isefficient, and hence luminescence of CGSO:xDy under VUVexcitation is effective. The peaks centered at 486 nm, 575 nmand 665 nm were observed in curve (a), which are attributed to 4F9/

2–6H15/2, 4F9/2–6H13/2 and 4F9/2–6H11/2 transitions of Dy3+, respec-tively.

Fig. 4 shows the emission spectra of a series of samplesCGSO:xDy (1% � x � 5%) under 172 nm excitation. It was obviousthat the emission intensities of the peaks were parabolicallyincreased with increasing x but decreased when x > 0.03. So thequenching concentration of CGSO:Dy under 172 nm excitation hasbeen determined to be x = 0.03.

The chromaticity coordination (x, y) of CGSO:xDy (1% � x � 5%)excited under 172 nm in the CIE 1931 chromaticity diagram ispresented in Fig. 5. The blackbody radiation locus is indicated bythe solid curve. Color temperatures are also calculated according to

Fig. 4. The emission spectra of a series samples CGSO:xDy (1% � x � 5%) under

172 nm excitation.

the formula proposed by McCamy [25]. Detailed color coordinatesand corresponding color temperature are shown in Table 1. Itis obvious that the chromaticity coordinate of CGSO:xDy(1% � x � 5%) gradually moves to the warm white side andapproaches the yellow region along with increasing x, whenx > 0.02, it moves to the cold white side in contrary orientationwhich could arise from the ratio of luminescence intensity of thepeaks at 486 nm and 575 nm. When x = 0.04, the chromaticitycoordinate calculated as (0.335, 0.338) is closest to the applicationstandard of white lamp plotted as point E (0.333, 0.333) in Fig. 5. Sothe white light phosphor of CGSO:4%Dy has the most potentialapplication in efficient Hg-free lamps.

4. Conclusions

The Ca2Gd8(SiO4)6O2 polycrystalline doped with1–5 at.% Dy3+

has been investigated from the viewpoint of obtaining a white lightphosphor for Hg-free lamps application. All the samples arehexagonal single-phase with P63/m space group according to theanalysis of XRD refinement, and the excitation and emissionspectra have been assigned in detail. The results show suitablespectroscopic characteristics such as broad and strong absorptionaround 172 nm, white light emission with the best chromaticitycoordinates (0.335, 0.338) for Hg-free lamps application. It can beconcluded that Ca2Gd8(SiO4)6O2:xDy is an effective white lightphosphor under vacuum ultraviolet excitation, and have thepotential application of being applied to a white lamp-house forHg-free lamps.

Y. Wang et al. / Materials Research Bulletin 45 (2010) 1614–1617 1617

Acknowledgments

This work is supported by National Science Foundation forDistinguished Young Scholars (Grant no. 50925206); NationalNatural Science Foundation of China (Grant no. 10874061) and theDoctoral Program of Higher Education (Grant no. 200807300010).In addition, the authors appreciate Doctor Yezhou Li for hervaluable comments and suggestions on the original manuscript.

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