192 Journal of Alloys and Compounds, 193 (1993) 192-193 JALCOM 2172

Valency states and spectral characteristics of samarium in MeBeF4 (Me = Ca, Sr, Ba)

Shi Chunshan and Cao Zhicheng Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Academy of Sciences, Changchun 130022 (China)

Abstract

A simple technique for the preparation of powder compounds MeBeF4 (Me-= Ca, Sr, Ba) doped with divalent samarium ions is described. Using this method, samarium ions have been stabilized in the divalent state in SrBeF4 and BaBeF4, but no obvious emission of Sm 2+ is observed in CaBeF4. The emission spectra and valency stability of samarium ions are briefly discussed. Our experiments indicate that the valency state of samarium ions depends strongly on the properties of the matrix.

1. Introduction

Under some conditions the valency of samarium ions is variable. Usually the trivalent state is more stable than the divalent state. The electronic configuration of Sm 2+ is [Xe]4f 6, the ground spectral term is 7F o and the lowest excited state of 4f 6 is 5Do. Generally speaking, the energy level of the 4f55d excited state is higher than the 5D 0 energy level, so only the f-f transition is observed in Sm2÷-activated phosphors. Interest in the luminescence behaviour of Sm 2÷ ions in solid com- pounds has increased greatly in recent years [1, 2], but it is difficult to synthesize Sm2÷-activated phosphors, especially binary fluoride phosphors [3]. Therefore, from the viewpoints of basic research and finding new ma- terials, it is important to study the stability of divalent samarium ions and to look for a simple method to synthesize Sm2÷-activated phosphors.

2. Experimental details

The MeF3 (Me-= Be, Ca, Sr, Ba) compounds are of analytical purity. SmF3 is prepared from highly pure Sm203. SmF2 is synthesized by reducing SmF8 several times with H2 at high temperature. The prepared SmF2 is a black powder, whose X-ray diffraction data tally with those of the JCPDS 27-1399 standard card.

To synthesize samarium-ion-activated phosphors, ap- propriate amounts of each component are mixed, ground and heated to 800-900 °C for 6-10 h in H 2 o r air. All the samples obtained are white powders.

A Rigakud/Max-II B X-ray diffraction instrument is used to analyse the crystal structure of the samples. The emission spectra are recorded using an MPF-4 speetrofluorimeter made in Japan.

3. Results and discussion

All the phosphors synthesized in our experiments are analysed by X-ray diffraction; the results are in accord with the data of ref. 4. No obvious change is observed in the X-ray diffraction patterns when the matrixes are doped with 1 mol% SmF2.

The emission spectra are shown in Figs. 1 and 2. The emission wavelengths and designations are given in Table 1. Only Sm 3÷ emission peaks are observed when the phosphors are synthesized in air. When the phosphors are synthesized in H2, Sm 2÷ emission peaks are observed in SrBeF4 and BaBeF4, but no Sm 2÷ emission is observed in CaBeF4.

By investigating the emission spectra, we notice that even under the same synthesis conditions the valency state of the samarium ion is not the same in different matrixes. It seems that the trivalent state of the samarium ion is more stable than the divalent state in the CaBeF4 matrix, because only Sm 3+ emission is observed in the CaBeF4 matrix. In the SrBeF4 and BaBeF4 matrixes the situation is quite different. Sm 2÷ emission is observed in both SrBeF4 and BaBeF4 when the phosphors are synthesized in H2, i .e. divalent samarium ions can exist in the SrBeF4 and BaBeF4 matrixes.

To explain the phenomenon that the valency state of the samarium ion is affected by the composition of

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C. Sh~ Z, Cao Valency states o f Sm in MeBeF4 193

__L #

It II II I t I t

I

It

(a)

,, II . . . . . . . . . . _ . . . . . . . . . . . . . . . . . . . . . (b)

i i i i i 500 600 700

~. (rm)

Fig. 1. Emission spectra of Sm 3+ in (a) CaBeF4, (b) SrBeF4 and (c) BaBeF4 synthesized in air; A(EX)=400 nm.

j_ (b)

TABLE 1. Emission wavelengths (nanometres) and designations of samarium ions in MeBeF4 (Me---Ca, Sr, Ba)

Synthesis Matrix Designation condition

CaBeF4 SrBeF4 BaBeF 4

Synthesized 560 559 562 4Gst2---, 6H5/2 in air 597 597 598 4Gsa--* 6HTr2 (Sm 3 + ) 642 639 643 4Gsr 2 --* 6H9/2

691 - - 4Gsf 2 ~ 6 H l l / 2

Synthesized - 678 677 5D o-, 7F o in H 2 - 687.5 685 5D1 " + 7 F 3

(Sm 2 ÷) - 693 692.5 59 o ~ 7F 1 - 718 716 5D o ~ 7 F 2

of samar ium ions (both divalent and trivalent), so samar ium ions can be stabilized on the sites of Sr 2+ and Ba 2+ ions in the valency form whose radius is larger, i.e. the divalent state is more stable than the trivalent state. We think that with increasing radius of the substi tuted cations it will become easier for sa- mar ium ions to en ter the matrix in the divalent form. This can lower the crystal disorder because o f the difference in radius be tween samar ium ions and sub- st i tuted cations. Thus we predict that the order o f the stabilizing ability of divalent samar ium ions should be

Ca 2+ < S r 2+ < B a 2+

670 0 '690 700 7 O 720 73U

x (nm)

Fig. 2. Emission spectra of S m 2+ in (a) SrBeF4 and (b) BaBeF4 synthesized in H2; A(EX)=320 nm.

the matrix, we compa re the radius o f the samar ium ion with the radius o f the cat ion for which the samar ium ion is substituted. We think it reasonable to assume that samar ium ions occupy the Me 2+ sites in MeBeF4 ( M e - - C a , St, Ba). Divalent eu rop ium ions are found to substi tute for Me 2+ cat ions in these systems [5]. The radius o f the S m 2+ ion is larger than that o f the C a 2+ ion, but the radius o f the Sm 3+ ion is smaller than that o f the Ca 2+ ion. Therefore , when samar ium ions enter the sites o f Ca 2+ ions, the trivalent state will be stable but the divalent state will not. The radii o f bo th Sr 2+ and Ba 2+ ions are larger than the radius

4. C o n c l u s i o n s

Samar ium ions exist only in the trivalent state when the phosphors are synthesized in air. W h e n the phos- phors are synthesized in HE, samar ium ions can exist in the divalent state in some matrixes. I f the radius of the substi tuted cat ion is larger than that o f the divalent samar ium ion, samar ium ions can exist stably in the divalent state. Wi th increasing radius o f the subst i tuted cation, the divalent state o f the samar ium ion becomes more stable. The composi t ion of the matrix and the synthesis condit ions are very impor tant factors affecting the valency state of samar ium ions.

R e f e r e n c e s

1 E. Banks, S. Nakajima and M. Shone, J. Electrochem. Soc., Solid-state ScL Technol., 10 (1980) 127.

2 A. Gros, F. Gaume and J. C. Gacon, J. Solid State Chem., 36 (1981) 324.

3 L. L. Chase, S. A. Payne and G. D. Wilke, J. Phys. C: Solid State Phys., 20 (1987) 953.

4 B. Labourrette, J. Grannec, C. Fouassier and P. Hagenmuller, Mater. Res. Bull., 11 (1976) 135.

5 Shi Chunshan and Ye Zeren, Sinica Rare Earth Acta, 1 (1983) 1.