Mat. Res. Bul l . , Vol. 17, pp . 451-457, 1982. P n n t e d in the USA. 0025-5408/82/040451-07503.00/0 C o p y ~ h t (e) 1982 Pergamon Press Ltd.

LUMINESCENCE PROPERTIES FOR THE HIGH-PRESSURE POLYMORPHS OF C a S i O 3 : p b 2 + AND S r S i O 3 : P b 2+

K. Machida, G. Adachi,* N. Ito, and J. Shiokawa Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565, Japan

and M. Shimada and M. Koizumi

Institute of Scientific and Industrial Research Osaka University, Yamadaoka, Suita, Osaka 565, Japan

(Received J a n u a r y 18, 1982; Communicated by W. B. White)

ABSTRACT High-pressure phases of CaSiO3:pb2+ and SrSiO3:Pb 2+ phosphors were synthesized at 40-55 kbar and 1000°C, viz. 6-CaSiO3:Pb 2+, 6-SrSiO3:Pb 2+, and 6~SrSiO3:pb2+ , and their luminescence properties were investigated. Among them, 6- CaSiO3:Pb 2+ was found to give a strong violet-blue emission (ca. 341 nm) as well as 8-CaSiO3:pb2+ (an atmospheric p~se), and the emission intensity of SrSiO3:Pb 2+ drasti- cally increased when the host lattice transformed into high-pressure phases (6 and ~' forms). These results were discussed by considering their structures and quenching temperatures of luminescences.

Introduction

Some of silicates activated by Pb 2+ ions, e.~. BaSi205:pb2+ and CaSiO3:pb2+ , give violet-blue emissions and are useful as black light phosphors (1,2). The luminescence of CaSiO3:pb2+,Mn 2+ has been found to vary with the crystal structure of the matrix, e- or 8-CaSiO 3 (3). Alkaline earth metasilicates show various high- temperature and high-pressure polymorphism. For CaSiO3, the 8 form (wollastonite or parawollastonite) transforms into the ~ form (pseudowollastonite) above about I125°C under atmospheric pressure (4), and 6-CaSi03 (a high-pressure phase) can be obtained at 30 kbar and 900°C (5). Similarly, e-SrSiO 3 transforms into 6 and 6' forms at about 34 and 59 kbar around 1000°C, respectively (6). In this paper, we report the luminescence properties for the high- pressure phases of CaSiO3:pb2+ and SrSiO3:Pb 2+.

To whom correspondence should be addressed.

451

452 K. MACHIDA, et al . Vol. 17, No. 4

Experimental

The atmospheric phases of pb2+-activated phosphors, e- CaSiO3:pb2+ , B-CaSiO3:pb2+ , and ~-SrSiO3:Pb 2+, were prepared by the following standard ceramic technique: appropriate amounts of CaCO 3 and SrCO 3 (luminescence grade), SiO 2 (99.999 %), and PbO (99.9 %) were fully mixed, pelletized, and heated at various temperatures (1050-II00°C for 8-CaSiO3:pb2+ and 1250-1350°C for e-CaSiO3:Pb 2+ and e-SrSiO3:pb2+ ) for 3x2 h (two times) in Ar. The high-pressure treatments of ~-CaSiO3:pb2+ and ~-SrSiO3:Pb 2+ were carried out with a cubic anvil type apparatus according to the method described else- where (7).

Ultraviolet luminescence spectra were measured with a Shimadzu recording absolute spectrofluorophotometer and the measurement tech- nique is described in ref. 8.

Results and Discussion

Crystallography of host lattices. In Table i, we summarize the crystallographic properties of the CaSiO 3 and SrSiO 3 polymorphs.

TABLE 1.

Crystallographic classification for the polymorphs of CaSiO 3 and SrSiO 3

Phase Symmetry C.N.(Ca,Sr) Silicate anion Ref.

~-CaSiO 3 Triclinic 8 (Si309)6- ring 9 a

8_CaSiO3 Triclinic, 67% 6 (SiO3) ~ chain 10 Monoclinic 33% 7

67% 6 (Si309)6- ring ii ~-CaSiO 3 Triclinic 33% 8

~-SrSiO 3 Monoclinic 8 (Si309)6- ring 12

67% 6 (Si309)6- ring 13 ~-SrSiO 3 Triclinic 33% 8

6tSrSiO 3 Monoclinic 8 (Si4012)8- ring 13

aThe detailed structural analysis of e-CaSiO 3 has not been performed, but the structure is approximately isostructural with e-SrSiO 3.

Among them, ~- and ~-CaSiO 3 are approximately or entirely iso- structural with the corresponding ~ and ~ forms of SrSiO 3, re- spectively. However, the B or 6' form is a characteristic phase of CaSiO 3 or SrSiO 3.

Luminescence properties. In Figs. 1 and 2, we show the relative

Vol . 17, No. 4 CaSiO 3 : P b 2+ AND SrS iO 3 : P b 2+ 453

emission and excitation spectra for CaSiO3:pb2+ and SrSiO3:pb2+ polymorphs, and these luminescence properties are presented in Table 2. Among the atmospheric phases of CaSiO3:pb2+ and

300 400 500

/ \ / \

\

\ -

3O0 400 5~)0 ~. (rim)

300 400 500

~ Excitation / ~ /

/ . . . . ,form

b) ~ form E

O" 50

r,-

3OO 400 500 ?, (nm)

FIG. i. FIG. 2.

Relative emission and exci- tation spectra for the poly- morphs of CaSiO3:pb2+(5 at%).

Relative emission and exci- tation spectra for the poly- morphs of SrSiO3:pb2+(5 at%).

SrSiO3:Pb 2+, the 8 form shows a violet-blue emission and is an efficient material giving the bright luminescence under 253.7 nm excitation because its excitation spectrum peaks at about 257 nm. For ~-CaSiO3:pb2+ and ~-SrSiO3:pb2+, however, the emission spectra (light-blue) are broad and weak, and furthermore, the excitation spectra peak around 230 nm. Therefore, their luminescence is not sensitive to 253.7 nm excitation. These observations agree with the results reported by Froelich and Fonda (3).

By high-pressure treatment of e-CaSiO3:pb2+ and e-SrSiO3:Pb 2+, the resulting materials were found to give-luminescences sensitive to the 253.7 nm excitation. A high-pressure phase, 6-CaSiO3:pb2+ , shows a strong violet-blue emission (ca. 341 nm) with a half-width of 37 nm. The excitation spectrum coEsists of a band peaking at about 244 nm, and hence 6-CaSiO3:pb2+ also may be useful for an ultraviolet-exciting phosphor. Similarly, 8- and 8~SrSiO3:pb2+ give violet-blue emissions peaking at about 337 and 331 nm. The peak positions of the excitation spectra are around 250 and 255 nm. It was found that the emission intensity of SrSiO3:pb2+ drastically increased when the host lattice (~ form) transformed into the 6 and 8' forms. In order to discuss the differences among the emission intensities of CaSiO3:Pb 2+ or SrSiO3:Pb 2+ polymorphs, we have

454 K. MACHIDA, et al. Vol. 17, No. 4

TABLE 2.

Luminescence properties for the polymorphs of CaSiO3:pb2+ (5 at%) and SrSiO3:pb2+(5 at%)

Phase Imax (nm) a I/2 (nm) b I ( % ) c T50 (K) d

~-CaSiO 3 light-blue broad 6

8-CaSiO 3 346 45 32 430

6-CaSiO 3 341 37 26 440

e-SrSiO 3 light-blue broad weak

~-SrSiO 3 337 37 ii 330

6~SrSiO 3 331 37 6 320

almax= Peak position of the emission band at 300 K.

bl/2= Half-width of emission band.

cI= Relative emission intensity estimated by integrating the corresponding area below the emission curve obtained under 254 nm excitation at 300 K, where the intensity of CaWO4:Pb 2+ (NBS 1026) is defined as 100 %.

dT50= Quenching temperature at which the intensity of lumi- nescence is half of that at 77 K.

considered their structural properties and the temperature de- pendences of their luminescence.

As the main quenching effects on the luminescence of Pb 2+ ions, one can point out the interactions between neighboring Pb 2+ ions and the lattice vibrations of matrix. The ionic radii of Ca 2+, Sr 2+, and Pb 2+ are summarized in Table 3. The ionic radius of Pb 2+ is considerably larger than that of Ca 2+. In 8- or d-CaSiO 3, the Ca 2+ ions occupy two kinds of sites, especially a third of £hem occupy seven- or eight-coordinated sites. Consequently, the large Pb 2+ ions in the 8 and 6 forms are expected to be selectively located at those large sites.

The luminescence properties of 8-Cal_xPbxSiO 3 (0.01~x~0.20) are given in Table 4. The emission band of sample shifted to longer wavelength and broadened with increasing the content of Pb 2+ ions. For the samples with xZ0.07, the emission spectra were found to consist of two bands and a concentration quenching effect on lumi- nescence was observed. Possibly the Pb 2+ ions at seven-coordinated sites give the emission band at shorter wavelength, while another band may be responsible fQr the Pb 2+ ions at six-coordinated sites. This suggests that the Pb 2+ ions in the 8 or 6 form which must be selectively located at the large sites are effectively dispersed in the matrix, and hence the interactions between the neighboring Pb 2+ ions must be relatively small. Therefore, 8- and ~-CaSiO3:pb2+

Vol. 17, No. 4 CaSiO3:Pb 2+ AND S r S i O 3 : P b 2+ 455

give strong emissions compared with e-CaSiO3:pb2+ , in which the Pb 2+ ions arbitrarily occupy only the eight-coordi- nated sites in the matrix. In a similar manner as CaSiO3:pb2+, ~-SrSiO3 which is isostructural with ~-CaSiO 3 has two kinds of sites for Pb 2+ ions, and thus, the Pb 2+ ions may be effectively dispersed compared with those occupying the eight-coordinated sites in e- and d-SrSiO3:pb2+. However, thi~ dispersion effect of Pb2+ions in 6-

TABLE 3.

Ionic radii (~)a for some divalent-metal cations

Ion VI b VIII b

Ca 2+ 1.00 1.12

Sr 2+ 1.18 1.26

Pb 2+ 1.19 1.29

aRef. 14.

bThese numbers represent the coordination numbers to oxygen.

TABLE 4.

Luminescence properties for 8-Cal_xPbxSiO 3

pb 2+ content a Imax (nm) I/2 (nm) I (%) (x)

0.01 336 33 21

0.03 337 34 31

0.05 346 45 32

0.07 348,355 54 29

0.10 349,356 54 24

0.20 350,359 55 16

aExcitation at 254 nm and 300 K.

SrSiO3:pb2+ is small because the difference between the ionic radii of SrZ+ and Pb 2+ is small (see Table 3).

The temperature dependences for the luminescence of CaSiO3:pb2+ and SrSiO3:pb2+ are shown in Fig. 3. The light output at 77 K was adopted as a standard for the temperature dependence of the sample. The quenching temperatures of ~-CaSiO3:pb2+ and e-SrSiO3:pb2+ could not reproducibly be measured because their luminescences were weak. From the quenching curves, the temperatures required to quench the luminescences of 8- and 6-CaSiO~:Pb 2+ can be seen to be high com-

' ' - " 2 + ' pared wlth those of 6- and 6-SrSlO3:Pb . Thelr T50 values are 320-440 K (see Table 2). This result suggests tha£-the quenching effects caused by the lattice vibrations in 8- and 6-CaSiO3:pb2+ are smaller than the corresponding effect in 8- and ~SrSiO3:pb2+ , and that the emission intensities of the former materials are strong compared with those of the latter materials. The phase, ~ SrSiO3:pb2+, which has the same T50 value as 6-SrSiO3:pb2+, gives relatively strong emission as well as that of the 6 form although the dispersion effect of the 8' form cannot be expected as previously

456 K. MACHIDA, eta]. Vol. 17, No. 4

100 • .-.. ~ " ~ . \

""'-.~. \ " . , , • H o s t l a t t i c e s : ""°~..k..%..k\ ~ , "*'.%' \ ~ , B-CaSi03; '.~.

"-..\ "\ - - - - - , ~ - C a S i O 3 ; ".\\ ,

"-.~ X . . . . . 6- Sr SiOs;

0 I I I 78 250 5O0 75O

=- T(K) FIG. 3.

Temperature dependences for the light outputs of CaSiO3:pb2+(5 at%) and SrSiO3:pb2+(5 at%) polymorphs.

described.

Conclusion

The high-pressure phases, ~-CaSiO3:pb2+ , ~-SrSiO3:pb2+ and ~ SrSiO3:pb2+, are the violet-blue emitting phosphors sensitive to 253.7 nm excitation. Particularly, 6-CaSiO3:pb2+ gives strong emission and is one of several efficient phosphors. This is thought to result from the effective dispersion of Pb 2+ ions in the matrix and the small lattice vibration.

Acknowledgement

This work was supported by a Grant from The Asahi Glass Foundation for Industrial Technology for one of the authors (G. A.) The authors wish to thank Mr. N. Ogawa for the preparation of samples at high-pressure.

References

i. T. A. Edison, U. S. Pat., 865367 (1907).

2. F. J. Studer and G. R. Fonda, J. Opt. Soc. Am., 3_~9, 655 (1949).

3. H. C. Froelich, J. Electrochem. Soc., 9_~3, 101 (1948); G. R. Fonda and H. C. Froelich, ibid., 9_~3, 114 (1948); G. R. Fonda and F. J. Studer, J. Opt. Soc. Am., 42, 360 (1952).

Vol. 17, No. 4 CaSiO 3:Pb 2+ AND SrSiO 3:Pb 2+ 457

4. N. N. Toropov, V. P. Barzakovskii, V. V. Lapin, and N. N. Kurtseva, " Handbook of Phase Diagrams of Silicate Systems, Vol. I. -- Binary Systems," pp. 20-44, 2nd ed., Israel Program for Scientific Transitions, 1972 Jerusalem.

5. A. E. Ringwood and A. Major, Earth Planet. Sci. Lett., 2, 106 (1967).

6. Y. Shimizu, Y. Syono, and S. Akimoto, High Temp.-High Pressures, ~, 113 (1970).

7.

8.

9.

i0.

ii.

12.

13.

M. Shimada, N. Ogawa, M. Koizumi, F. Dachille, and R. Roy, Am. Ceram. Soc. Bull., 5_88, 519 (1979).

K. Machida, G. Adachi, and J. Shiokawa, J. Lumin., 21, 101 (1979); K. Machida, G. Adachi, J. Shiokawa, M. Shimada, and M. Koizumi, Inorg. Chem., 19, 983 (1980).

J. W. Jeffery and L. Heller, Acta Cryst., 6, 807 (1953).

D. R. Peacor and C. T. Prewitt, Am. Mineral, 48, 588 (1963); F. J. Trojer, Z. Krist., 127, 291 (1968).

F. J. Trojer, Z. Krist., 130, 185 (1969).

K. Machida, G. Adachi, J. Shiokawa, M. Shimada, and M. Koizumi, Acta Cryst., in press.

K. Machida, G. Adachi, J. Shiokawa, M. Shimada, M. Koizumi, K. Suito, and A. Onodera, Inorg. Chem., in press.

14. R. D. Shannon, Acta Cryst., A32, 751 (1976).