Pr3+-β″-Al2O3

Journal of

~YS A~D COMPOUND5 ELSEVIER

J o u r n a l of Alloys a n d C o m p o u n d s 225 (1995) 152-155

Optical and magnetic investigations of Na +/Pr 3+ - /o~ -" A 12 0

3

F. Tietz a, E. Zanghellini a, G. Mariotto a, T. D e d e c k e b W. Urland b a Universitgl di Trento, Dipartimento di Fisica, l,qta Sommarive 14, 1-38050 Povo, Trento, Italy b Universitiit Hannover, Institut fiir Anorganisehe Chemie und SFB 173, Callinstr. 9, D-30167 Hannover, Germany

Abstract The Na ÷ ions in the solid electrolyte if'alumina (Na÷-if'-Al203) have been exchanged with Pr3÷ ions. The Pr3+ ion has been used as an optical and magnetic probe to investigate the local structure within the conduction planes of this oxide. With this aim, optical spectra and susceptibility measurements have been carried out. Both fluorescence and excitation spectra, related to the 3Po~ 3F2,3H4 transitions, have been recorded on single crystals. The magnetic susceptibility shows temperatureindependent paramagnetism at low temperatures. The physical properties have been interpreted with the help of ligand-field calculations by applying the angular overlap model. It has been deduced that the Pr3÷ ions occupy mostly the eightfoldcoordinated mid-oxygen (mO) sites. Keywords: Optical properties; Magnetic properties; Solid electrolyte; Ligand-field calculations; Angular overlap model

1. Introduction Since the discovery of fast ion exchange of trivalent rare-earth ions for sodium ions in/3"-alumina [1], these doped materials have been studied in some detail. Especially the neodymium and europium isomorphs have been the target of intensive investigations: the work on the neodymium compound prompted interest for its possible laser application [2,3] and that on the europium isomorph, with the well-known transitions SDo ~ 5Fj (J= 0-4) of the Eu 3+ ion, was mainly aimed at studying the sites of the luminescent ions [4-7]. These investigations, together with X-ray diffraction measurements on highly ion-exchanged crystals [8], have shown that within the conduction planes of ff'-alumina the majority of the lanthanide ions occupy eight-coordinate mid-oxygen (mO) sites and only a few percent of them seven-coordinate Beevers-Ross (BR) sites with C3v symmetry. The symmetry of the mO sites, originally C2h, can easily be reduced (a) by a different second-shell coordination with residual Na + ions [4], (b) by distortions of the coordinated in-plane oxygens [9], (c) by minor shifts of the lanthanide ion within the ab-plane [9] or along the crystallographic c-axis [10]. These dislocations are in principle also valid for the BR sites, and the * Dedicated to Professor Haus-Uwe Schuster on his 65th birthday. 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSD! 0 9 2 5 - 8 3 8 8 ( 9 4 ) 0 7 0 2 4 - 5

resulting atomic distribution in the conduction region is highly disordered. This is well reflected by the inhomogeneously broadened optical spectra of trivalent fl"-aluminas [2-7]. Our interest in the transport properties of Na ÷/Ln 3+/3"-A1203 (Ln =Pr-Lu) [11,12] is closely connected with the knowledge of site occupancy and local structure. Therefore we have chosen praseodymium as a representative for a light and large trivalent lanthanide ion to probe its coordination and ionic conductivity in ff'-alumina crystals. With regard to the structural aspects of this activity we have carried out X-ray diffraction investigations on crystals with different degrees of Pte ÷ exchange [10], as well as spectroscopic and magnetic measurements. Here we present results of the latter two techniques obtained on highly exchanged samples. 2. Experimental details Single crystals of sodium/T-alumina have been grown by a flux evaporation technique [13] and have been ion-exchanged using PrC13 under argon atmosphere [11,14]. The amount of the exchange was determined by electron-probe microanalysis [13]. For the optical measurements the crystals were mounted into an optical flux-cryostat and the sample temperature was maintained below 10 K. Broad-band excitation and emission spectra of Pr 3+ were obtained

F. Tietz et al. / Journal o f Alloys and Compounds 225 (1995) 152-155

using a Xe arc-lamp source of 250 W, filtered by a 20-cm Jobin Yv0n monochromator with a spectral bandpass of 4 A. For narrow-band excitation, different lines of an Ar+-ion laser were used. The fluorescence was analyzed at 90° by a double monochromator of 1 m focal length (Jobin-Yvon, Ramanor HG2-S) and detected by a standard photon counting system. Further experimental details are given elsewhere [15]. Powder samples of highly exchanged (up to 98%) Na +/Pr 3+-/3"-A1203 were measured with a SQUID magnetometer (Quantum Design, MPMS5) in the 1.7 to 300 K temperature range at various field strengths. The magnetic susceptibility data were corrected by a specially developed method [16] for diamagnetism.

153

C

a

i

[

i6200

i

i

i

15800

15400

03 4-9

3. Results and discussion

-r--I

c

(a)

3.1. Optical investigations Broad-band fluorescence spectra, tr- and ~--polarized, of a 90% Pr 3 +-exchanged Na +-/3"-alumina crystal are shown in Fig. 1. The spectra were recorded at 10 K under excitation within the 3p1(116) band at 459 nm. The emissions from the 3Po level show strong inhomogeneous broadenings. An assignment of the different transitions observed, based on the energy-level scheme of Pr 3+ [17], is also reported. Some excitation spectra in ~--polarization, showing the spectral region from 430 to 490 nm with the excitation bands of the 3pj, q6 0 r = 0 , 1, 2) states, are plotted in Fig. 2. The spectra were detected by monitoring the intense it-polarized line peaked around 15 500 cm -~ of the 3Po--->3 F 2 transition shown in the inset. As can be seen clearly from spectrum (c), recorded at 15 510 cm- ~, the 3P o level consists of at least three components with intensity maxima at 477.2, 478.3 and 480.0 nm.

i

3

O0 - 3 H41I '~! ~

i

'-k

3Po-3Fa

r I

f.__

u~ z klA

Co) z

I (d) I

I

I

I

I

440

450

460

470

480

WAVELENGTH [nm] Fig. 2. ~-ipolarized broad-band excitation spectra of the 3po---~3F2 emission line (inset), detected at: curve (a), 15 570 c m - t ; curve (b), 15 550 cm-Z; curve (c), 15 510 c m - l ; curve (d), 15 465 c m - t ; curve (e), 15 450 c m - t . T h e spectra were recorded at 10 K with k~;'cll~.

20000

JBO00

16000

WAVENUMBER [cm-1] Fig. 1. Fluorescence spectra obtained at 10 K from a Pr~+-exchanged (90%) /3"-alumina crystal under broad-band excitation at 459 nm, in o-polarization (continuous line) and in m-polarization (dashed line).

It is also well shown by the sequence of these spectra that the sites with higher 3Po energy are detected at the low-energy part of the fluorescence band and vice versa (spectra (a) and (e)). When the detection wavenumber decreases, the intensities of the three 3P o components change continuously without a clear separation of even one of them, neither in or- nor in ~--

F. Tietz et al. / Journal of Alloys and Compounds 225 (1995) 152-155

154

polarization. From these observations we conclude that the sites occupied by the P r 3+ ions are very similar and show a continously varying surrounding. The energy levels of 3P1 and 3p2, peaked at about 455--463 nm and 441-445 nm, respectively, have a similar site dependence as observed for the 3P 0 ~ 3H 4 transition. In contrast, the two bands between the 3P o and 3P 1 levels, located at 468 and 474 nm for spectra recorded at low energy (spectrum (e)), show a remarkably different behaviour. One of these bands shows a siteindependent character and stays fixed at 468 nm. In contrast, the other band is continously shifting from 474 nm to 469 nm, in the opposite direction with respect to the 3pj levels. The similarity of the P r 3 + sites is also confirmed by fluorescence spectra. The emission lines of the 3P o "q' 3 H 4 transition as well as the 3P o ~ 3Fz transition, excited at various wavelengths within the 3 p 2 - 3 p a ( l I 6 ) - 3 P 0 region by a c.w. Ar+-ion laser, show strong inhomogeneous broadenings and an overall splitting of the ground state of up to 1000 cm -a (Fig. 3). At all excitation wavelengths the spectra look similar, but show slightly different Stark splittings.

3.2. Ligand-field calculations The luminescence spectra of the 3P 0 ~ 3 H 4 transition have been compared with results of ligand-field calculations to get information about the local structure of the Pr 3+ ions within the conduction planes of/3"alumina. The calculations were carried out on the basis of the angular overlap model (AOM) for f electrons [18,19]. The AOM parameters e,~ and e~ were derived from the fitting of the crystal-field splitting of P r 3 + in garnets [20,21]. For the simulation of the first coor-

dination sphere of the trivalent ions in BR sites as well as in mO sites, the crystallographic data of N d 3+/3"-A1203 were used [8,22]. In addition, the bridging oxygen atoms within the conduction planes, O(5), were moved towards the cations up to a distance of 2.3 to model their possible displacements and to create non-equivalent Pr-O(5) bonds. The results of these calculations clearly indicate an occupancy of the mO site by Pr 3÷. The internal splitting of the 3 H 4 level is in satisfactory agreement with the fluorescence spectra. The varying ground-state splittings can be understood in terms of different Pr-O(5) bond lengths. As an example, the ground-state splitting of the sites excited at 472.7 nm, shown in the left part of Fig. 3, have been calculated with the following AOM parameters (in cm-1): e,~.o(3)= 155, e = . o ( 3 ) = 3 8 , e,,o(4)=163, e~.o(4)=48 for the spinel block oxygens, and e,,o(sa) = 419, e~.o(sa)= 310, e~.o(sb)= 449, e,~,O(5b) = 340 for the conduction-plane oxygens. The values for the 0(5) ions correspond to bond lengths of 2.43 /~ and 2.41/~, respectively, which are in excellent agreement with X-ray diffraction work [8,22]. The 3H 4 splitting for the BR site is much smaller and cannot explain the broad emission lines above 700 cm- 1 far from the excitation line. The maximum splitting for the BR site, with one long (3.25-2.8 /~) and two very short (2.3/~) Pr-O(5) distances, gave only values around 630 cm-1. It is evident that BR site occupation cannot explain the main features of the observed spectra. But we also cannot exclude the possibility that the peaks between 20 800 and 20 550 cm-i and at around 19 800 cm -1 are due to a low P r 3+ content in BR site, although these extra peaks may be explained also in the framework of our model, e.g. by mO sites with very short Pr-O(5) bonds.

3.3. Magnetic investigations 3Po - 3 F 2

4J

3P o - a l l 4 ~,~

>b-

3 Po - ~ H6 ii

OO Z W Z

21000

20000 WAVENUMBER

16500

15500

[cm -a ]

Fig. 3. 3po--~3H4, 3H6, 3F2 emission spectra in o--polarization (continuous line) and in ,n-polarization (dashed line) of a 90% ionexchanged Na+/pr3+-/3"-alumina crystal at 10 K excited at 472.7 nm. T h e vertical bars are the result of the ligand-field calculation (for details, see text).

The observed reciprocal magnetic susceptibility of a 98% pr3+-exchanged Na+-/3"-AlzO3 powder sample is shown in Fig. 4. The low-temperature (T< 15 K) susceptibility indicates temperature-independent paramagnetism. In order to understand the observed data, we again carried out ligand-field calculations applying the AOM for Pr 3+ located in the mO as well as in the BR site. The calculated data are also displayed in Fig. 4, showing the good agreement with our observed data for the model in which Pr 3+ ions occupy the mO sites. In this case we found the following average values for the AOM parameters (in cm-1): e,~.oo.4)=126, e~,o(3,4) = 37, e,,o(sa,b)= 383, e m o ( S a , b ) = 77. The details of the calculations are given elsewhere [16]. Thus from our measurements we can infer that the Pr 3÷ ions are preferentially located in mO sites. Observed deviations of the AOM parameters used for the curves in Fig. 4 with respect to the values given above for the spec-

F. Tietz et al. / Journal of Alloys and Compounds 225 (1995) 152-155 200 X "1 c n l tool

3

i .

180

160

'

i

7--

i

'

~

,

i

'

Acknowledgments

i

PrJ+-~"'Al~Or (obs. d a t a )

~ , ~- -

. _ ca/c. data for P r ~+ in t h e B R site

140

- - t a l c , d a t a f o r P r ~+

120

in t h e m O site

~ . ~ .

80

/

'

:"

The authors gratefully acknowledge the Deutsche Forschungsgemeinschaft for financial support. F.T. thanks the Alexander von Humboldt foundation for a Feodor Lynen grant.

"" .'.'

~ . •"

25

,

i

,

'

5

' ~

i

,

10

'

15

./

60

1

References

40

20

155

~,: [_ ,

0 I 50

J 100

,

, 150

I 200

,

I 250

,

I 300

15

/

T/K Fig. 4. Reciprocal magnetic susceptibility for Pr 3÷ in Pr 3+-/3"-alumina between 1.7 and 300 K. The experimental data ( • ) were obtained with a field strength of H = 20 000 Oe. The solid and dashed lines are the result of the ligand-field calculations for Pr 3÷ in mO site and BR site, respectively. The low-temperature range (hatched region) is shown as an enlarged inset.

troscopic measurements are due to the large temperature range for our magnetic measurements.

4. Conclusions Our optical and magnetic results indicate a single site occupancy of Pr3 ÷ ions in Na +/Pr 3+-ff'-A120 3. This site is assigned as the mO site, which shows a continuum of distortions probed by the optical spectra. Crystalfield calculations confirm such an interpretation of the experimental data. In a recently published optical study of Na+/Pr 3+~n-A1203 crystals [23], it was claimed that the emissions observed arise from Pr3+ ions in BR sites. As we mentioned above, we cannot confirm this interpretation; neither with our experimental results in X-ray diffraction, optical spectroscopy or susceptibility measurements nor with the theoretical calculations.

[1] B. Dunn and G.C. Farrington, Solid State lonics, 9/10 (1983) 223. [2] M. Jansen, A. Alfrey, O.M. Stafsudd, B. Dunn, D.L. Yang and G.C. Farrington, Opt. Lett., 10 (1984) 119. [3] H. Dai, O.M. Stafsudd and B. Dunn, Appl. Opt., 30 (1991) 4330. [4] A.P. Brown and D.J. Simkin, J. Chem. Phys., 89 (1988) 5377. [5] M.A. Saltzberg and G.C. Farrington, J. Solid State Chem., 83 (1989) 272. [6] A.P. Brown, J. Solid State Chem., 100 (1992) 49. [7] M. Laberge, D.J. Simkin, G. Boulon and A. Monteil, J. Lumin., 55 (1993) 159. [8] W. Carrillo-Cabrera, J.O. Thomas and G.C. Farrington, Solid State lonics, 28/30 (1988) 317. [9] A. Alfrey, O.M. Stafsudd, B. Dunn and D.L. Yang, J. Chem. Phys., 88 (1988) 707. [t0] J. KOhler and W. Urland, Z. Kn'st., in press. [11] F. Tietz and W. Urland, J. Alloys Comp., 192 (1993) 78. [12] F. Tietz and W. Urland, Solid State lonics, in press. [13] F. Tietz and W. Urland, J. Cryst. Growth, 118 (1992) 314. [14] F. Tietz and W. Urland, Solid State Ionics, 46 (1991) 331. [15] G. Mariotto, F. Tietz, E. Zanghellini and M. Ferrari, J. Lumin., 60~61 (1994) 216. [16] T. Dedecke and W. Urland, J. Magn. Magn. Mat., in press. [17] S. Hiifner, in Optical Spectra of Transparent Rare Earth Compounds, Academic Press, New York, 1978, p. 34. [18] W. Urland, Chem. Phys., 14 (1976) 393. [19] W. Urland, Chem. Phys. Lett., 46 (1977) 457. [20] J.B. Gruber, M . . Hills, R.M. Macfarlane, C.A. Morrison and G.A. Turner, Chem. Phys., 134 (1989) 241. [21] E. Antic-Fidancev, J. H61s~i, J.-C. Krupa, M. Lemaitre-Blaise and P. Porcher, J. Phys.: Condens. Matter, 4 (1992) 8321. [22] M. Wolf and J.O. Thomas, Acta Crystallogr. B, 49 (1993) 491. [23] P.R. Nelson, D.J. Vickers and O.M. Stafsudd, J. Phys. (Paris), 4 (1994) C4~19.