Determination of the lattice site of Fe in photorefractive LiNbO3

Solid State Communications, Vol. 83, No. 10, pp. 819-821, 1992. Printed in Great Britain.

0038-1098/92 $5.00 4- .00 Pergamon Press Ltd

DETERMINATION OF THE LATTICE SITE OF Fe IN PHOTOREFRACTIVE LiNbO3 C. Prieto and C. Zaldo Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas, Campus Universitario de Cantoblanco, C-IV, 28049 Madrid, Spain

(Received 8 April 1992 by M. Balkanski) The coordination of Fe ions in LiNbOa single crystals has been investigated by Extended X-ray Absorption Fine Structure technique. From the analysis of the data it is found that Fe a+ sits in the Li-site and that the displacement of Fe along the c-axis is very similar to that of Li+. 1. INTRODUCTION

2. EXPERIMENTAL METHODS

LiNbO3 SINGLE CRYSTAL doped with Fe (hereafter LiNbO3:Fe) has been one of the pioneer materials to study the photorefractive effect [1]. LiNbO3 :Fe shows a unidirectional contribution to the photocurrent (the photovoltaic effect) even in the absence of an applied electric field [2]. It has been generally assumed that this contribution arises from an asymmetric interaction between Fe and its neighbors, however the actual lattice position of Fe and its displacement along the c-axis of the lattice have been a matter of controversy. Previous works have determined the (73 symmetry of the site occupied by iron using EPR [3, 4] and M6ssbauer [5] spectroscopies, however, a conclusion with regard to the Li+ or Nb 5+ substitution was not obtained. The main obstacle in the determination of the lattice site is that the three possible sites in the lattice (Li, Nb or vacancy positions) have C3 symmetry, furthermore the mean distance from the impurity to the first oxygen shell is very similar in the three cases. Recently, by Proton Induced X-ray Emission/ Channelling (hereafter PIXE) technique [6], it has been determined that Fe substitutes Li+ lattice cations. Though from PIXE measurements it is concluded that the Fe position is very close to that of Li+, a direct measurement of the displacement of Fe along the c-axis :is not provided. Extended X-ray Absorption Fine Structure (hereafter EXAFS) spectroscopy has been applied previously to determine the lattice site of other impurities in LiNbO3 Single crystals [7, 8]. In this work we analyze the EXAFS spectrum of Fe in LiNbO3. The coordination numbers and neighboring distances obtained in the analysis show that Fe 3+ substitutes Li+ ions and that the displacement along the c-axis is 6 = -0.5/~.

Congruent LiNbOa : Fe single crystals have been grown by the Czochralski method. The molar Fe concentration in the melt was 1%. The K-edge, X-ray absorption spectra of Fe were acquired at room temperature (RT) in the fluorescence mode. We used synchrotron radiation emitted by the LURE (Orsay) DCI storage ring. Details on the experimental set-up and measurement methods have been reported previously [8]. 3. EXPERIMENTAL RESULTS Figure 1 shows the X-ray absorption spectrum, #(E), of LiNbO3 : Fe taken from the K-edge of Fe. It is well known that the Fe3+/Fe 2+ ratio in LiNbO3 : Fe samples can be modified by thermal treatments [5]. In order to investigate the oxidation state of the sample used to perform the EXAFS experiments, a twin sample was reduced in vacuum at 800°C. Figure l(a) shows a comparison between the K-edge of Fe in our oxidized and reduced samples, the spectrum corresponding to Fe20 3 is also shown for reference. In Fig. l(a) it is observed that the X-ray absorption spectrum of oxidized LiNbO3: Fe (long dashed line) is very close to that of Fe203, however, there is some minor contribution of Fe 2+. From the comparison it has been estimated that the ratio [Fe3+]/[Fe] in our oxidized sample is 0.85. The EXAFS measurements reported below have been performed using an oxidized LiNbO3 sample, thus the EXAFS information derived is mainly related to Fe 3+. Figure 1(b) shows the X-ray absorption spectrum of oxidized LiNbO3 : Fe. After removing the background, the EXAFS signal, x(k), has been analyzed with a standard procedure described previously [8]. Figure 2 shows the Fourier transform (hereafter

819

820

Vol. 83, No. 10

D E T E R M I N A T I O N OF THE LATTICE SITE OF Fe 0.050 1.0

~

0.0z5

~ f ~

~, o.0oo

0.5

~.-o.o25F.

0.0090 "

1.0

7110

~

'

0.025 ~-'/(~

7150

~ * ~

'

r

.

(b)

- _ = : I v , , , ,6 ,

0.5

10

k , A -I

t,,~J,

I

i

;0

'

I

,

0"O000 7200 74 0 7600 PHOTON ENERGY , eV

7800

Fig. 1. (a) Room temperature X-ray absorption spectra (taken in fluorescence mode) of Fe in thermally reduced LiNbO3 (short dashed line), oxidized LiNbO3 (long dashed line) and powdered Fe203 (solid line). (b) Room temperature X-ray absorption spectrum (taken in fluorescence mode) of oxidized LiNbO3 : Fe. The spectra have been recorded at the K-edge of Fe. we refer to this function as pRDF) of the k3x(k) signal. In Fig. 2 a first peak can be observed at about 1.7 A and some structure in the range 2.2-4.5 A. In previous EXAFS works [7, 8] it has been shown that the peak at shorter distance is associated with the first shell of oxygens and the second structure at larger distance is related to the niobiums of the lattice. To obtain the "true" distance of the neighbors the position of the peaks observed in Fig. 2 has to be corrected because of the phase Shift, q~(k), of the EXAFS signal. To perform this correction we have separated each peak filtering the distance range of interest. The filtered pRDF peaks have been fitted

Fig. 3. Fit of the x(k) spectrum contribution of the significant peaks observed in the p R D F function. The points stand for the experimental data and the solid lines for the theoretical fit. (a) Fit of the x(k) contribution ascribed to F e - O coordination. (b) Fit of the x(k) contribution ascribed to F e - N b coordination.

6.0

5.0

.3A(3) u~ N

0.02

~0.01 z o

1

I

I

"1

I

I

1

2

3

4

5

8

DISTANCE .

Fig. 2. Fourier transform function (pRDF) of k3x(k) from k = 4-10/~ -~ of the EXAFS signal of oxidized LiNbO 3 : Fe.

)

]

4.0

3.0

2.0

I

SHIFT

T

~

¢,3

1"-00.75

0

I

8

4

0.00

~...~o---"

o.ooo

(b)

.i

..

:t

7130

(a)

i

- 0 25

I

I

0.25

I

0.75

FROM THE CENTER ,

Fig. 4. Radial distances of different coordination shells of atoms around Li site in LiNbOa. The distances have been calculated from the mean point between the upper and bottom oxygen planes. The displacement 6 along the c-axis is considered positive in the positive direction of the c-axis. Dashed lines refer to oxygen ions and solid lines to niobium ones. Li ions are not considered because their low backscattering amplitude. The first digit in the notation indicates the order of the shell. A and B labels mean ions above and below of the center. While c indicates ions along the c-axis. Finally the number in brackets stands for the number of equivalent atoms, The points indicate the distances found for the F e - O ( 0 ) and F e - N b (11) coordinations.

Vol. 83, No. 10

D E T E R M I N A T I O N OF T H E L A T T I C E SITE OF Fe

821

Table 1. Summary of the results obtained from the fits of the EXAFS spectrum of oxidized LiNb03 : Fe sample, e is the figure of merit of the fit Pair

Nj

Rj (A)

try (A)

Fj (A -2)

Fe-O

3 3

2.041 2.298

0.086 0.105

1.0 1.0

Fe-Nb

4 3

3.20 3.40

0.060 0.049

1.0 1.0

using the phase and amplitude values reported by McKale [9]. Figure 3 shows the fits performed for the oxygen-related p R D F peak [Fig. 3(a)] and for the niobium-related one [Fig. 3(b)]. The numerical results obtained are summarized in Table 1. In the fittings, we have selected the coordination numbers, Nj, equal to those expected from the LiNbO3 lattice. Other parameters: the Fe-neighbor distance, Ry; the Debye-Waller, crj and the free electron path-related one, Fj, have been adjusted (in k and R spaces) to minimize the error between experimental data and theoretical calculations. In Table 1 it may be observed that the peak at about 1.7 A in Fig. 2 corresponds to the presence of 3 + 3 oxygens at distances 2.041 and 2.298A respectively. With regard to the other p R D F structure, 2.2-4.5 tk in Fig. 2, the best fit is obtained assuming a coordination of 4 + 3 niobiums at 3.20 and 3.40 A, respectively. The distances reported in Table 1 do not agree with the assumption of Fe in the niobium octahedron, because in that case the closer (6) niobiums are at a mean distance of about 3.8 A. Therefore the only assumptions that would agree with the coordination and distances reported in Table I are that Fe ions site Li + or vacancy lattice sites. The latter possibility is not favored by the theoretical energy calculation [10]. Moreover it has been mentioned above that from PIXE experiments it has been concluded that Fe replaces the Li + ions of the lattice [6]. Figure 4 shows a comparison of the coordination of Fe summarized in Table 1 with the distances expected in the LiNbO 3 lattice if Fe 3+ replaces Li +. A very good fit is found if a displacement in the negative sense of the c axis, 6 = -0.5 ik, is considered for the Fe 3+ impurity. This displacement is very close to the displacement of Li + (6 = -0.442 A).

AEo (eV) -8.9 -8.9 -25 -25

e 0.00031 0.00005

4. CONCLUSIONS In summary we have concluded that Fe 3+ impurities in LiNbO3 single crystals sit in a welldefined lattice position: the Li + position of the LiNbO3 lattice. Moreover the displacement 6 of Fe 3+ and Li + ions are very similar. Because of the low sensitivity of EXAFS to diluted concentrations of atoms, from our results other lattice positions of Fe ions present in minor concentrations cannot be excluded.

Acknowledgements - The authors acknowledge the experimental facilities provided by LURE, Orsay, France. This work has been partially supported by DGICyT, under project number MAT91-0216. REFERENCES 1.

G.E. Peterson, A.M. Glass & T.J. Negran, Appl. Phys. Lett. 19, 130 (1971). 2. A.M. Glass, D. von der Linde & T.J. Negran, Appl. Phys. Lett. 25, 233 (1974). 3. H.H. Towner, Y.M. Kim & H.S. Story, J. Chem. Phys. 56, 3676 (1972). 4. G.I. Malovichko & V.G. Grachev, Soy. Phys. Solid State 27, 1678 (1985). 5. W. Keune, S.K. Date, I. Dezsi & U. Gonser, J. Appl. Phys. 46, 3914 (1975). 6. L. Rebouta, M.F. da Silva, J.C. Soares, M. Hage-Ali, J.P. Stoquert, P. Siffert, J.A. SanzGarcia, E. Di+guez & F. Agull6-Ldpez, Europhys. Left. 14, 557 (1991). 7. C. Zaldo, F. Agull6-L6pez, J. Garcia, A. Marcelli & S. Mobilio, Solid State Commun. 71, 243 (1989). 8. C. Zaldo, C. Prieto, H. Dexpert & P. Fessler, J. Phys.: Condens. Matter 3, 4135 (1991). 9. A.G. McKale, B.W. Veal, A.P. Paulikas, S.K. Chan & G.S. Knapp, J. Am. Chem. Soc. 110, 3763 (1988). 10. H. Donnerberg, S.M. Tomlinson, C.R.A. Catlow & O.F. Schirmer, Phys. Rev. B44, 4877 (1991).