MÖSSBAUER SPECTROSCOPY OF Fe IMPURITIES IN LiNbO3

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MÖSSBAUER SPECTROSCOPY OF Fe IMPURITIES

IN LiNbO3

S. Date, W. Keune, H. Engelmann, U. Gonser, I. Dezsi

To cite this version:

JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 12, Tome 37, Décembre 1976, page C6-117

MOSSBAUER SPECTROSCOPY OF Fe IMPURITIES IN LiNb0

3 S. K. DATE (*), W. KEUNE (**), H. ENGELMANN, U. GONSER and I. DEZSI (***)

Fachbereich Angewandte Physik, Universitat des Saarlandes, 6600 Saarbrucken, Germany

Résumé. — Nous avons étudié l'effet Môssbauer d'ions Fe3+ et Fe2+ dilués dans des

monocris-taux de LiNbOî. Pour Fe3+ nous observons à basse température des spectres possédant une

struc-ture hyperfine paramagnétique caractéristique d'une relaxation électronique lente. Il est possible de modifier réversiblement le rapport Fe2+/Fe3+ dans un cristal par un traitement thermique oxydant

ou réducteur. La relaxation électronique de Fe3+ à basse température reste lente même en présence

d'une large proportion d'ions Fe2+.

Abstract. — The Mossbauer effect of dilute Fe3+ and Fe2+ ions in LiNbo3 single crystals is

reported. For Fe3+ typical paramagnetic hyperfine spectra were observed at low temperature which

are characteristic of slow electronic relaxation processes. TheFe2+/Fe3+ ratio in a crystal can be

changed reversibly by oxidizing or reducing heat treatment. The spin relaxation for Fe3+ at low

temperature remains slow even in the presence of a large fraction of Fe2+ ions.

Iron doped LiNb03 is an axial ferroelectric material

which is of technological importance as a potential high-resolution holographic storage medium [1]. The rhombohedral crystal structure of L i N b 03 is closely

related to that of corundum (<x-Al203); it is obtained by

replacing two A l3 + ions by a Li1 + and a N b5 + ion

alternately in an ordered array along the crystallo-graphic c-axis [2]. Li1 + and N b5 + ions lie in slightly

distorted oxygen octahedra with trigonal site sym-metry C3. EPR studies by Herrington et al. [3]

indicat-ed that the Fe impurity can enter the lattice substitu-tionally as high-spin F e3 + on an axially symmetrical,

unique lattice site with the symmetry axis parallel to the c-axis. Previous Mossbauer results [4] obtained with L i N b 03 single crystals Fe57-doped by diffusion

indi-cated the presence of high-spin F e3 + and F e2 + valence

states their ratio depending on the thermal treatment of the samples. The F e3 + and F e2 + ion each was found to

be on a unique axial lattice site with the symmetry axis parallel to the c-axis.

In this communication we present further Mossbauer effect results for dilute Fe5 7 impurities in L i N b 03

single crystals. The samples were doped with 0.22 W % Fe2Os (91 % enriched in Fe57) by growing from the

melt (Czochvalsky technique), and thus have a more homogeneous impurity distribution than diffusion doped crystals used previously.

Quite similar to a-Al203 (corundum) LiNb03 is a

suitable host lattice for dilute Fe3 + impurities to study

the effect of electronic spin relaxation on the Fe5 7 (*) Tata Institute of Fundamental Research, Bombay, India. (**) Laboratorium fur Angewandte Physik, Gesamthoch-schule, 4100 Duisburg, Germany.

(***) Central Research Institute for Physics, Budapest, Hungary.

nuclear hyperfine structure. This is demonstrated in figure 1, where typical spectra of F e3 + in L i N b 03

(oxidized sample) are shown at different temperatures. At room temperature and above the spectrum exhibits a central non-Lorentzian absorption with wings extend-ing to about + 10 mm/s. At 77 K and 4.2 K the spectra clearly show the magnetic hyperfine patterns which arise because of an increase of the electronic spin-relaxation time at low temperatures. Our spectra are similar to those reported for F e3 + in <x-Al203 by

several authors [5,6]. The spectrum in figure \d (4.2 K) was analyzed following the criteria suggested by Wertheim and Remeika [6]. The crystalline field splits the 6S5 / 2 state of the F e3 + ion into three Kramers'

doublets | ± 5/2 > , | ± 3/2 > and | + 1/2 > . Each of them produces an effective hyperfine field at the Fe5 7 nucleus if the respective electronic relaxation

times are large compared to the nuclear Larmor pre-cession time. Neglecting off-diagonal elements for | + 5/2 > and | ± 3/2 > states in the hyperfine interaction tensor one expects, in the limit of long rela-xation times, one six-line pattern from each state. As off-diagonal terms for the | ± 1/2 > state are signifi-cant, a complicated pattern of eleven lines is expect-ed [6].

In the low-temperature spectra in figure 1 (top) clearly a superposition of two six-line patterns, one due to the | + 5/2 > state and one arising from the

| + 3/2 > state, is observed (indicated by the bar diagrams). Since the crystallographic c-axis was per-pendicular to the ^-propagation direction in this experi-ment, the relative intensities of the hyperfine lines in each six-line pattern which are expected if the c-axis and the principal axis of the EFG tensor (quantization axis) coincide, are 3 : 4 : 1 : 1 : 4 : 3 . The pronounced

C6-118 S. K. DATE, W. KEUNE, H. ENGELMANN, U. GONSER AND I. DEZSI

V e l o c i t y I m m I s ]

FIG. 1.

-

Fe3+ Mossbauer spectra of single crystal LiNb03 - 0.22 W % Fe57O3, oxidized in air at 800 C, (a) at 970 K, c-axis parallel to y-ray, (b) at 295 K, c-axis parallel to y-ray, (c) at 77 K, c-axis perpendicular to y-ray, ( d ) at 4.2 K,

c-axis perpendicular to y-ray (source : C027 in Rh).

(Am = 0)-lines observed in each six-line pattern demonstrate that the principle EFG axis is approxima- tely colinear with the c-axis, as for Fe3+ in a-A1203. The resulting hyperfine fields are 529 kOe and 317 kOe for the

I

f

512

>

and

I

f 312

>

levels, respectively, i. e. a value of 212 kOe/spin is obtained which is somewhat smaller than 220 kOe/spin obtained for Fe3+ in a-A1203 [6, 73. The quadrupole interaction constant e2 qQ = eVzz Q for Fe3

*

was determined from the measured difference of the two outer lines to the right (S,) and the difference of the two outer lines to the left (S,) of the

I

f 512

>

pattern :

assuming axial site symmetry

(r

x

0) and C Sz

>

and V,, parallel to the c-axis (8 = 0). We obtained a value e2 qQ =

+

1.04 f 0.08 mm/s, which is equal in sign and magnitude to that of Fe3* in corundum [6].

Thus, the local environment of Fe3+ in LiNbO, is similar to that of Fe3+ in a-A1203, i. e. an oxygen

octahedron, and a possible charge-compensating defect, e. g. an oxygen vacancy, is remote. The center shift at 4.2 K with respect to a-Fe at room temperature was found from the

I

+

512

>

spec- trum to be

+

0.47 f 0.05 mm/s.

As for Fe3+ in a-A1203 [6] the theoretically predicted eleven absorption lines caused by the

1

+

112

>

level also could not be observed down to 4.2 K for Fe3+ in LiNbO,. The effect of the

I

f 112

>

state in the mea- sured low-temperature spectra apparently is an unresolved background smear extending up to higher velocities and the appearance of two most central absorption peaks (Fig. Id) which cannot be explained by the I $- 512

>

and

I

f 312

>

patterns. The influence of various reducing thermal treatments on the iron valence state in LiNbO, is demonstrated in figure 2.

V e l o c i t y [ m m l s l

RG. 2. - Mossbauer spectra of a LiNb03 - 0.22 W % Fe2703 absorber measured at 77 K after various annealing procedures at 1 000 C : (a) as grown, (b) 760 tom argon atmosphere, 50 h, (c) 760 torr Ar, additional 50 h, ( d ) 50 torr Ar, additional 10 h, (e) 0.2 torr Ar, additional 20 h (c-axis always perpendicular

to y-ray).

MOSSBAUER SPECTROSCOPY O F Fe IMPURITIES IN LiNb03 C6-119

reported earlier [4], and which is caused by a small fraction of high-spin Fez+ ions. Starting from the as-grown state of the sample continued annealing at 1 000 C in Ar atmosphere results in an increasing Fe2+/Fe3+ ratio, since in figure 2a-e the line intensity of the Fez+-doublet is seen to increase relative to the Fe3+ intensity, indicating the enhanced conversion of Fe3+ to Fez'. For the different reduction' stages (described in detail in the caption of figure 2) the rela- tive amount of Fez+ increases from about 15 % (Fig. 2a) to m 90

% (Fig. 2e).

As can be seen from figure 2 the Fe3 + electronic rela-

xation time remains sufficiently long even in the pre- sence of Fez+ ions to produce the paramagnetic Fe3+ hyperfine pattern. This observation is in contrast to results for Fe3+ in u-A1203 where the Fe3 + relaxation

time is decreased by the presence of Fez+ impurities [6], presumably by cross-relaxation.

Annealing the samples at 800 C in 1 atm. of Ar atmosphere for 24 hours results in 100 % reduction to Fez+ impurities [4], i. e. no trace of Fe3+ ions has been detected in Mossbauer spectra of such samples. The line intensity ratio of the Fez+ doublet indicates axial symmetry at the Fez+ site along the c-axis, and a negative quadrupole coupling constant 141.

Further experimental evidence for this result is given in another contribution at this conference [8]. In figure 3 measured values of AEp = 112 eVZz Q for

Fez' are plotted versus temperature. A strong tempe- rature dependence of AEQ is evident. The negative sign

Temperature IKI

FIG. 3. - Temperature dependence of AEQ = 112 ez qQ for Fez+ in LiNbO3.

of YZZ indicates that the electronic ground state is an orbital singlet, which appears together with an energe- tically higher-lying doublet because of removal of degeneracy of the T,, level by the trigonal crystal field. Acknowledgments. - This work was supported by the Deutsche Forschungsgemeinschaft under SFB 130. One of the authors (SKD) wishes to thank the Alexander von Humboldt Foundation for the award

of postdoctoral fellowship.

References

[I] VON DER LINDE, D., GLASS, A. M., AppI. P h y ~ . 8 (1975) 85. [5] JOHNSON, C. E., CRANSHAW, T. E., RIDOUT, M. S., in : [2] ABRAHAMS, S. C., REDDY, J. M., BERNSTEIN, J. L., J. Phys. Proceed. Intern. Conf. on Magnetism, Nottingham

Chem. Solids 27 (1966) 997 ; (The Institute of Physics and the Physical Society, ABRAHAMS, S. C., HAMILTON, W. C., REDDY, J. M., ibid. 27 London, 1964) p. 147.

(1966) 1013 ; [6] WERTHEIM, G. K., REMEIKA, J. P., Phys. Lett. 10 (1964) 14 ; ABRAHAMS, S. C., BERNSTEIN, J. L., ibid. 28 (1967) 1685. WERTHEIM, G. K., REMEIKA, J. P., in : Nuclear Magnetic

Resonance and Relaxation in Solids, Proc. Coll. Ampere

[3] HERRINGTON, J. B., DISCHLER, B., SCHNEIDER, J., Solid State _{XIII, edited by L. van Gerven (North-Holland, Amster-}

Commun. 10 (1972) 509. _{dam) 1965, p. 147.}

141 KeuNE, W., DATE, S. K., DEZSI, I., GoNsER, U., J . APP~. Phys. [7] WICKMAN, H. H., WERTHEIM, G. K., Phys. Rev. 148 (1966)

46 (1975) 3914 ; 211.

KEUNE, W., DATE, S. K., GONSER, U., BUNZEL, H., Ferro- [8] GONSER, U., SAKAI, H., KEUNE, W., J. Physique Colloq. 37