Luminescence of uranium-doped strontium tetraborate (SrB4O7)

Pergamon

PI1:S0022-3697(97)00002-4

J. Phys. Chera Solids Vol 58, No. 7, pp. 1143-1146, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-3697/97 $17.00 + 0.00

LUMINESCENCE OF URANIUM-DOPED STRONTIUM TETRABORATE (SrB407) P E T E R A. T A N N E R ' ~ , P E I Z H I - W U : ~ , L I N JUN~:, L I U Y U L O N G § a n d S U Q I A N G : ~ 1"Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong ~/Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, P.O. Box 1022, Changchun 130022, China §Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China

(Received4 September 1996; accepted 13 October 1996) A~tract--The luminescence and excitation spectra of uranium doped into strontium borate, SrB407:U, are reported. The emission spectrum is similar to the structureless green 'uranate' luminescence. The excitation spectrum is assigned to transitions from oxygen-derived orbitals to uranium 5f and 6d orbitals. © 1997 Elsevier Science Ltd. All rights reserved.

Keywords: A. optical materials, B. luminescence.

1. INTRODUCTION

2. EXPERIMENTAL

Strontium tetraborate, SrB407, is isostructural with EuB407 [1] and the structural framework consists of a three-dimensional (B407) network of BO4 tetrahedra, with Sr 2+ cations participating in SrO 9 polyhedra [2]. The tetrahedral borate group BO a is unusual because in most cases boron is triangularly coordinated by oxygen, forming the BO 3 group. The crystal structure of SrB407 gives rise to another unusual phenomenon. In this host lattice the lanthanide ions Eu 3+, Sm 3+, Yb 3+ and Tm 3+ can be reduced to their corresponding divalent states in air at high temperature without the introduction of reducing agents [3-5]. The actinide element U can also exist in different oxidation states, the most important of which are U(III), U(IV), U(V) and U(VI) [6]. Each oxidation state has a characteristic optical spectroscopic fingerprint [7-10]. Therefore, we have prepared the uranium-doped compound SrB407:U in a similar way to that described in ref. [3], and have investigated by optical spectroscopy whether reduction of uranium from its initial U(VI) state would occur. The nature of the site occupied by U in SrB407:U was also of interest. If it resided at vacancies of the major sites then UO9 or UO4 polyhedra would be expected. The ionic radii of U(IV) and U(VI) are clearly so much larger than the ionic radius of B 3+ at the tetrahedral coordination site, that it appears that uranium could not substitute at the boron site. Concerning substitution of the Sr2+ site, there have been no previous luminescence studies of uranium situated in a nine-fold coordination environment, a fact that provided further impetus to the present study.

Powder samples of SrB407 doped with U(VI) from U O 2 (NO3)2 • 6H20 at 1 and 0.1% by mole ratio U/Sr were prepared as described in ref. [3]. The sample was checked by X-ray diffraction after preparation and no other patterns than that of SrB407 were detected. Low resolution (1-2nm) excitation and emission spectra were recorded at room temperature (300 K) and 77 K using a xenon lamp and an SLM 4800C spectrofluorometer. Argon-ion laser excited spectra were recorded using a Spex 1403DM spectrometer equipped with an Oxford Instruments continuous flow cryostat with a base temperature ,-AOK. The peak positions marked on the observed broad features in Figs 1-3 are accurate to ± 20cm -1. 3. RESULTS Under 335-343 nm xenon lamp excitation, SrB407:U exhibits a green, structureless emission band at 300 K. The maximum of the band is near 534 nm and two shoulders are identified at ,,~20 nm on either side. Little change occurs on cooling to 77 K, but the shoulders become more sharply defined, and the maximum of the emission band shifts 100cm -I to the red (Fig. 1). Similar results are obtained for excitation wavelengths at 422 nm and 275 nm. The 300 K excitation spectrum (Fig. 2(A)) obtained by monitoring the emission at 535 :L 2 nm consists of (i) a series of sharp lines between 420 and 500 nm; (ii) a strong band peaking near 412 nm (ca. 24 100 cm- 1); (iii) a medium intensity band near 340nm (29 160cm-I); and (iv) a weaker band at 280nm (35 340cm-1). The

1143

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e t al.

wavelength (nm) 540

440

640

o

| I

22000

20000

18000

16000

wavenumber (cm "~) Fig. 1. The 343 n m ×enon lamp excited emission spectrum of SrB407:U (U/Sr = 0.01) at 77 K.

77 K excitation spectrum, obtained using the excitation energy marked by the arrow in Fig. 1, is shown in Fig. 2(B), and does not provide further information. The sharp lines were shown to be due to the xenon lamp excitation source. The 457.9nm laser-excited emission spectra of SrBaOT:U at 20K for both 0.1 and 1 mol% U/Sr are similar to spectra obtained using xenon lamp excitation, with two shoulders resolved on an intense, broad feature. However, the maximum shifts c a . 400 cm - l to low energy in the more concentrated material. Under longer wavelength (514.5 nm) excitation, the structure is more clearly resolved (Fig. 3), with the maximum emission intensity at lower energy than that with higher energy excitation. When the concentration of wavelength (rim) 300 400

200 •

,

,

l 4.)

i

uranium in the material is increased, the higher energy bands are weaker, and other features become broader and show a small shift to lower energy. 4. DISCUSSION The luminescence transitions of U(III), U(IV) and U(V) occur at lower energy than the emission bands observed in the present study [7-9]. The emission for the lowest energy zero-phonon line(s) ofuranium(VI), i.e. for the uranyl ion UO 2+, correspond(s) to the rig "-* ~-~g- (Dooh) transition, being associated with electron transfer from oxygen to a non-bonding uranium orbital. Under 6-, 5- or 4-equatorial coordination in crystalline materials, this transition is generally wavelength (nm) 560

~20 i

,

i

•

i

.

600 t

-

i

500

-

o

(h rl

II

o

°

i50000 40000

30000 wavenuzaber

20000 (q~a"1)

Fig. 2. Excitation spectrum of the 535 4-2nm emission of SrBa07 :U (U/Sr = 0.01) at (A) 300 K and (B) 77 K. The band peaking near 415 nm has been truncated in both spectra.

19400

17900 wavenu~be:"

16400 (cm "~)

Fig. 3. The 514.5nm laser excited emission spectrum of SrB407:U. (A) U/Sr = 0.001; (B) U/Sr = 0.01, both at 20K.

Luminescence of SrB4OT:U in the range from 471 nm (21 207 cm- t) [ 10] to 520 nm (19 239cm -l) [11]. In the present case, the observed bands are at low energy of 525 nm (19 040cm-1), so that the equatorial coordination number is more likely to be 4 (as in UO 6-) and/or lower than 7 (as in UO~2-). For uranyl crystals, many sharp bands are resolved in the low temperature emission spectra which correspond to progressions and other vibronic structure based on the zero-phonon line, with the wavenumber difference identifying the vibrational mode. In particular, strong progressions on the zero-phonon line are observed in the symmetric O - U - O stretching mode, even in the luminescence of uranyl-doped borosilicate glasses, which exhibit variable equatorial coordination [12]. At low temperature (Hg. 3), the separations of the three lowest energy bands (1823017515 = 715cm-1; 17515-16760 = 755cm -1) could be assigned, within the limits of experimental error, to the vs ( O - U - O ) progression. The intensities of these bands would indicate that the v = 0 member of the progression is strongest, as usual for uranyl compounds. The typical uranyl progressions are not clearly observed for several reasons. The transition intensity is from electronic origins, and not from vibronic origins as in centrosymmetric uranyl compounds, due to the substantial electric dipole change during electron excitation at the low symmetry site. First, a weakening of the formally triple-bonded uranyl moiety is envisaged, by electron delocalization to other oxygen neighbours. The weaker bond does not experience such a relatively large bond length increase on excitation. Second, the structure is inhomogeneously broadened and smeared out because the uranyl ions do not substitute at well-defined lattice sites, but experience a range of coordination geometries and environments. The higher energy strong features at 19040 and 18655cm -1 in Fig. 3 are assigned to inhomogeneously broadened zero-phonon lines at different sites. The formal charge on uranium is 6+, whereas that on Sr is +2, and the U 6+ ion is only about two-thirds as large as Sr2+. There is no evidence to show that uranium enters a well-defined (St 2+) lattice site. The low temperature emission spectra (Fig. 3) show that the inhomogeneous broadening increases with the concentration of uranium in the SrB407, since a wider range of different environments is experienced by uranium at higher concentrations. It is also clear that the maximum emission intensity shifts to lower energy when the concentration of uranium in the material is increased (Figs 2(A) and (B)), or when the temperature is decreased. Both of these phenomena arise from the energy transfer to lower energy uranium sites (traps). The transfer rate is higher when the

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number of neighbours increases, and the higher energy sites are depopulated at lower temperatures. The emission spectrum in Fig. 1 closely resembles that assigned to the 'uranate group', UO, [13]. In particular, the green emission ascribed to the octahedral UO66- ion in SrLaMgNbO6:U [14] is very similar to that of the present study. However, we have not assigned the emission to a single coordination geometry. The octahedral UO66- moiety would be expected to exhibit sharp-line, low-temperature luminescence with extensive vibronic structure. The excitation spectrum (Fig. 2) also closely resembles that of SrLaMgNbO6:U [14], which has been assigned by comparison with the UF 6 absorption spectrum [13]. We will re-examine the validity of this analogy elsewhere, but note that the lower energy bands in the excitation spectra are assigned to transitions from a closed shell configuration to uranium 5f orbitals in both cases. Similar to the excitation spectra of uranates [13], we find that the excitation spectra of uranyl compounds also exhibit one or two structureless bands at the high energy of 300 nm. These features, and two bands near 340nm (29 160cm -1) and 280nm (35340cm 1) in SrB407:U, correspond to transitions from orbitals derived primarily from oxygen (2p) to uranium 6d orbitals. The transitions are thus parity allowed, but not necessarily electric dipole allowed because the lowest 6d orbital transforms as Ag(Do~h). The featureless quality of the excitation spectrum of SrB4OT:U does not permit a more detailed explanation to be given. Finally, it is noted that the low-energy tail of the Xe lamp excited emission spectrum has a small 'dip' at 625nm (16 000 cm-1), with another smaller one near 670 nm (14 925 cm -l). One explanation of these features is that they are due to absorption of the emitted radiation by some U(IV) centres in the material. We are unable to obtain the absorption spectrum from our powder in order to verify this, but note that the presence of U(IV) in UO3-doped sodium tetraborate glasses has been confirmed at rather higher uranium concentrations than those employed in the present study [15]. 5. CONCLUSIONS The luminescence and excitation spectra of SrB407:U have been assigned to U(VI) ions situated in a disordered manner in the material. Features in the emission spectrum are inhomogeneously broadened, and are assigned to zero-phonon lines rather than vibratory structure. The luminescence and excitation spectra are similar to those observed from SrLaMgNbO6:U by Bleijenberg [13]. This author assigned green emission to the UO66- moiety, as

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P.A. TANNER et al.

opposed to red emission from UO42-. However, there appear to be several different coordination polyhedra of uranium doped into SrB40 7 because the emission spectra change with excitation wavelength and with uranium concentration. Whereas SrO 9 polyhedra are present in the neat borate host material, several different uranium polyhedra would be possible for a disordered arrangement, such as UO603 and UO207, where the first number denotes the nearest neighbours. The present interpretation of the optical spectra has involved the uranyl ion, although with rather long primary U - O bond distances. Acknowledgement--PAT thanks HK UGC RG#9040098for partial support of this work. REFERENCES 1. Machida, K., Adachi, G. and Shiokawa, J., J. Lumin., 1979, 21, 101.

2. Machida, K., Adachi, G. and Shiokawa, J., Acta Cryst., 1980, B36, 2008. 3. Pei, Z., Su, Q. and Zhang, J., J. Alloys Compds, 1993, 198, 51. 4. Peterson, J. R., Xu, W. and Dai, S., Chem. Mat., 1995,7, 1686. 5. Schipper,W. J., Meijerink, A. and Blasse, G., J. Lumin., 1994, 62, 55. 6. Katz, J. J. and Rahinowitch, E., The Chemistry of Uranium. Dover, New York, 1951. 7. Tanner, P. A., J. Mol. Struct., 1995, 355, 299. 8. Flint, C. D. and Tanner, P. A., Molec. Phys., 1984, 53, 801. 9. Shehelokov,R. N. and Bolotova, G. T., Soviet J. Coord. Chem., 1978, 4, 343. 10. Flint, C. D., Sharma, P. and Tanner, P. A., J. Phys. Chem. 1982, 86, 1921. 11. Tanner, P. A., Speetroehim. Aeta, 1990, 46A, 1259. 12. Flint, C. D., Tanner, P. A., Reisfeld,R. and Tzehoval,H., Chem. Phys. Lett., 1983, 102, 249. 13. Bleijenberg,K. C., Struct. Bonding, 1980,42, 97. 14. Azenha, M. E., van der Voort, D. and Blasse,G., J. Solid State Chem., 1992, 101, 190. 15. Culea, E., Milea, I. and Iliescu, T., J. Non-Cryst. Solids, 1994, 175, 98.