ELSEVIER Materials Chemistry and Physics 38 (1994) 98-101

MATERIALS CHEM;rSTJlsND

Materials Science Communication

A study of site occupation of Eu3+ in Me,Y,(Si04)60, (Me=Mg, Ca, Sr)

Jun Lin and Qiang Su* Laboratory of Rare Earth Chemistry and Physics,

Changchun Institute of Applied Chemistry, Academia Sinica,

Changchun 130022 (People’s Republic of China)

(Received October 27,1993; accepted January 26.1994)

Abstract

Alkaline earth (Mg, Ca. Sr) yttrium silicate oxyapatites doped with Eu3+ show red luminescence with comparable intensity. In this system

of phosphors, the Eu3+ ions enter 4f sites and 6h sites simultaneously according to the fluorescence spectra, in which two SDo-7Fo lines of Eu3+ are

observed. The assignment of 5Do-7Fo lines from Eu3+(4f) and Eu3+(6h), as well as their different properties has been discussed in the context.

Introduction

The location of activators in haloapatites has been the object of great interest because of their use as fluor- escent lamp phosphors and as potential hosts for laser materials. Silicate oxyapatites are excellent matrices for luminescence of various lanthanide ions. For ex- ample, the luminescence of Eu3+ in Me2RE8 (Si0&02 (Me=Mg, Ca, RE=Y, Gd, La) [l], Ce3+ and Tb3+ in Gd9.33(Si04)h02 [2], Pr3+ in Ba2La8(Si0&02 [3] and Nd3+ in Sr2La8(SiO&02 [4] has been investigated. There exist two sites available for cations in these compounds, viz. 4f(C3) with nine-coordination and 6h(C,) with seven- coordination. A large difference between these two sites is that the 6h site has a O(4) ion (free oxygen ion) to coordinate, which does not belong to any silicate group, so that the binding strength of the O(4) ion is not satu- rated, whereas the nine oxygen ions coordinated to the 4f site all belong to the silicate group. This results in the average covalency on the 6h site higher than that on the

* To whom correspondence should be addressed.

4f site. Two methods, i.e. the variation of the lattice parameters of the apatite unit cell with the ionic radius of the constituting lanthanides, and local charge com- pensation were utilized by Felsche [5] and Blasse [6] respectively to study the problem of cation site occupa- tion in these compounds. For the silicate oxyapatites Me2RE8(Si0&Oz(Me=Mg, Ca, Sr, Ba, RE=rare earths, not in all cases), the Mgz+, Ca*+ ions are supposed to be in (6h), and Sr *+, Ba*+ in (4f) by Felsche; whereas the alkaline-earth ions are expected to be in (4f) by Blasse, except that RE3+ ions are relatively large (e.g. La3+) and Me*+ ions are small (e.g. Mg*+), Me*+ ions are also in (6h). Doping Eu3+ ions in these compounds, Blasse [6] has predicted that the Eu 3+ ions enter the 6h sites in Me2La8(Si0&02 and 4f sites in Me2Y8(Si0&02(Me= Mg, Ca), respectively, based on the fact that the Eu3+ is smaller than La3+ and larger than Y3+. In later reports [2, 31 the doping rare earth ions (Ce3+, Pr3+) have been found to enter the 4f sites and 6h sites simultaneously. This tempted us to study the site occupation of Eu3+ in Me2Y8(Si0&02 (Me=Mg, Ca, Sr) directly by high resolution fluorescence spectrum. Some new results have been obtained.

Elsevier Science S.A. SSDI 02S4-0584(94)01355-K

J. Lin, Q. Su i Materials Chemistty and F’hysics 38 (1994) 98-101 99

Experimental

Powder samples for the series Me2Y7,86Eu0.,4 (SiO&02 were prepared by the sol/gel technique. The stoichiometric amounts of the starting materials MgO, CaC03. SrCO, (all purified from A.R grade [7]), YZ03(99.99%), Eu203(99.99%) were dissolved in diluted HN0,tG.R.) and subsequently mixed with ethanol and Si(OC2H5)J(C.P.). The mixtures were heated under reflux in a 70°C water bath until homoge- neous gels formed. The gels were dried at 120°C for 24 hours and subsequently fired at 800°C for 4 hours, fully ground, fired again at 1000°C for 6 hours and the final products were obtained. This synthesis tempe- rature is about 300°C lower than that of the solid state reaction reported by T. J. Isaacs [l], indicating the ad- vantages of the sol/gel method.

All samples were checked by X-ray powder diffr- action and appeared to be single phase (oxyapatite). The high resolution fluorescence spectra were recorded at room temperature on a Spex-1403 spectrophoto- meter using Ar+ 476Snm laser as excitation source (power=50mw).

Results and discussion

The excitation and emission spectra of Eu’+ -acti- vated MezY8(Si0&02(Me=Mg, Ca) have been reported byT.J. Isaacs[l], whose resultsshow that EU3+ luminesces in red region with comparable intensity, but no analysis was given.

The fluorescence spectra of Me~Y,,*~E~.,~ (SiO,),OZ (Me=Mg, Ca, Sr) excited by Ar+ 476Snm laser are present in Fig. 1, related data of which are compiled in Table 1 together with their assignment and theore- tical number. Independent of Me2+ ion, these spectra all consist of sD0-7F,, (J=O, 1, 2, 3, 4) lines of E$+. No emission from higher SDr (J>O) levels is observed. This is ascribed to the fact that the energy gaps between SD2 and SD, and between SD1 and SD0 of Eu~+ are 2.500 cm-’ and 175Ocm-1 respectively, the silicate groups (with v,,,, = 9.50 cm-‘) are able to bridge the gaps be- tween the higher lying levels of the E$+ ion and the SD0 level. In agreement with the low local symmetry for Eu’+(C, or C,), the forced electric dipole hypersensi- tive transition SD0 -7F2 is the most prominent emission.

Generally speaking, the energy levels of Eui+ 7FJ split under the effect of crystal field of the surrounding ions, which depends on the site symmetry of Eu’+ and the number of J. C3 and C, symmetry fields cause SDo-7FI splitting into 2 lines and 3 lines, “Do-7FZ into 3 lines and 5 lines, respectively. 5Do-7F0 can not be split because J=O is a singlet state. So the number of

Ca

2

,i

0 1

570 639

Sr

2

,.1 1

0’

570 639

wavelength (nm)

Fig. 1. Fluorescence spectra of Me2Y7.sbEuu.t4 (SiO&O: excited with Ar’476Snm laser. Me and SDrr-7F~ (J=O. 1.2) are indicated in the figure(J=3.4 omitted).

TABLE 1. Fluorescence spectrum data for Me2Y78hEU0,t4(Si04)602 (wavenumher. cm-‘)

Me Mg Ca Sr assignment theoretical number

sDo-7Fr, 17288 17285 17253 17221

Ao-o 3.5 5x

sD0-7F, 17087 17082 17047 17022 17022 16927 16915 16855 16830 167.52 167.55 16677

GCo_, * 16943 16885

5D(j-7F2 16383 16357 16360 16305 16340 16272 16245 16095 16162 15975 16110 15842 16062 15962

GCu.: 16203 16144

17287 4f 2 17227 6h

60

17075 17025 16875 4f+6h 16752 16675

16880

16360 16312 16272 16080 15990 15842

4f+6h

16142

*GC=gravity center

xD0-7Fo lines denotes the number of different Et@+ lumi- nescent centers. It is known from Fig. 1 that there exist two 5Do-7F0 lines for all Me2+, indicating that the Eu3+ ions simultaneously occupy two different sites in these host lattices, viz. 4f(C3) sites and 6h(C,) sites. This result can also be confirmed further by the number of 5Do-‘F, lines and SD0-7F2 lines. The enlarged sD0-7FZ group of Eu-l+ in MgzY8(SiOd)OZ shows eight lines clearly (Fig. 2) which exactly equals the sum of splitting

100 J. Lin, Q. Su I Materials Chemishy and Physics 38 (1994) 98-101

, 1 I I I 1 I I 1 I I

609 619 629

wavelength (nm)

Fig.2. The enlarged 5Do-7F2 emission group of Eu3+ in MgzYs

(sio4) 6%

number of 5D,,-7F2 from Eu3+(4f) and Eu3+(6h). So it is with the number of 5D,,-7F1 lines (five) for Me=Sr. As for Me=Mg, Ca, six Do-7F1 lines have been observed, which exceeds the theoretical number due to the ex- istence of vibration lines or structural disorder in the host lattices [8] (e.g. different orientation of SiO, groups). Obviously, this result differs from literature in which the Eu3+ ions are supposed to enter 4f sites only in these hosts[6]. Our conclusion can be explained as follows. The 4f site with nine-coordination needs large cations for its space, on the other hand the 6h site with seven-coordination is favourable for high charge cations due to the fact that it has a O(4) ion to coordinate. The Eu3+ has a larger ionic radius (0.112 nm for CN=9) and higher charge (+3), which are favourable for 4f site and 6h site, respectively. It is the competition of these two trends that makes Eu3+ ions enter 4f sites and 6h sites simultaneously.

As mentioned above, the 4f site has no free oxygen ion to coordinate while the 6h site has one (at a very short distance). Consequently, the Eu3+ (6h) is expected to be more covalently bonded than the Eu3+(4f). There- fore, the 5D,,-7F,, line at the lower energy is assigned to Eu3+(6h), and the other at higher energy to Eu3+(4f). The assignment of 5D,,-7F1 and 5D0-7F2 from Eu3+(4f) and Eu3+(6h) needs site-selective excitation which is not available at present in our laboratory. It is quite obvious that the 5D0-7F0 line from Eu3+(4f) is broader than that from Eu3+(6h) for all Me2+. This is due to mixed occupation of the 4f sites, as shown in Table 2. The crystal fields at the 4f site cations vary slightly, so that

TABLE 2. The distribution of cations in Me2Ys(Si0&02

4f 2Mg2+, 2Y3+ 2Ca2+, 2Y3+ 2Sr2+, 2Y3’

6h 6Y3+ 6Y3+ 6Y3+

TABLE 3. Ionic radius [9] and electronegativity [lo] of Me2+

Mg2+

Ca2+

Sr2+

r(nm, CN=9)

0.089

0.118

0.131

E

1.31

1.00

0.95

Ar AE

0.029 0.31

0.013 0.05

an inhomogeneous broadening of the spectral lines will result.

Additionally, the 5D0-7F0 line from Eu3+(6h) is very narrow, meaning that the 6h sites are occupied ex- clusively by Y3+. This is different from Felsche’s deduc- tion that Mg2+, Ca2+ also enter the 6h sites in these compounds [5].

Form Table 1 it can be seen that the energy position of Eu3+(4f) 5D,,-7F0 is almost identical for all Me2+, while the energy difference of 5D,,-7F0(Ao_o) between Eu3+(4f) and Eu3+(6h) increases in the se- quence of A&Mg) < .&,(Ca)< &(Sr), pointing to the red shift of Eu3+(6h) 5D0-7Fo increases with the increas- ing of the radius of Me2+ ions. In view of the crystal structure, this effect is not surprising. The 4f sites and 6h sites alternate in the host lattices, i.e. 4f(MeZ+, Y3+)- 6h(Y3+)-4f(Me2+, Y3+)-6h(Y3+). So it can be expected the formation of Eu3+(4f)-02.-Y3+(6h) and Eu3+(6h)-02.- Me2+(4f) in the host lattices. The next neighbour of Eu3+(4f)-02. is Y3+ in all the three hosts, so the energy position of Eu3+(4f) 5D0-7F0 is almost similar. As far as Eu3+(6h)-02. is concerned, its next neighbour is Me2+ ion which will influence the degree of covalency of Eu3+(6h)-02. bond. With increasing of the radius of Me2+ ion its electronegativity decreases (Table 3), so that the electronic cloud of 02. attracted by Me2+ decreases and the nephelauxetic effect between Eu3+(6h) and 02. becomes more prominent. This causes the degree of covalency of Eu3+(6h)-02. bond to increase and makes the red shift of Eu3+(6h) 5D0-7Fo increase from Mg to Sr. The same tendency holds for the gravity centers of 5D,-/Fi and 5D0-7F2, i.e. shift to lower energy from Mg to Sr, as shown in Table 1. It should be mentioned that the above tendency is prominent from Mg to Ca and weak from Ca to Sr, because the electronegativity (or ionic radius) difference between Mg2+ and Ca2+ is much larger than that between Cal+ and Sr2+ (Table 3).

Conclusions

It is shown that the Eu3+ ions enter 4f sites and 6h sites simultaneously in the silicate oxyapatites Me2Y8(Sio&j02. The 5D,,-7F0 line from Eu3+(4f) is

J. Lin, Q. Su i ~a~e~a~ Chemistry and Physics 38 (19943 98-101 101

broader than that from Eu3+(4h> due to mixed occupation of the 4f sites. With increasing of the radius of Me2+ ion, the positions of Eu3+(6h) SDOJFO, together with the gravity centers of sD,,JF, and sDo-7F2 shift to lower energy.

Acknowledgments

This work is supported by National Nature Science Foundation of China.

References

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.I. Electrochem. Sot., I18 (1971) 637. 5. J. Felsche. J. Solid State Chem., 5 (1972) 266. 6. G. Blasse. J. Solid Stale Chem., 14 (1975) 181. 7. B. Li and Y. Bai, Chinese Sci. Bull., 30 (1985) 186.

8. J. H&i. K. Jyrk%s and M. L.eske1l.J. Less-Cwwnon Met.,126 (1986) 215.

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