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Submitted on 1 Jan 1980
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INFLUENCE OF X-RAYS, LIGHT AND ANNEALING ON THE Fe-CHARGE-STATES IN LiNbO3 : Fe
H. Pfannes, J. Lauer, W. Keune, Y. Maeda, H. Sakai
To cite this version:
H. Pfannes, J. Lauer, W. Keune, Y. Maeda, H. Sakai. INFLUENCE OF X-RAYS, LIGHT AND ANNEALING ON THE Fe-CHARGE-STATES IN LiNbO3 : Fe. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-453-C1-454. �10.1051/jphyscol:19801177�. �jpa-00219665�
JOURNAL DE PHYSIQUE Colloque Cl , suppic!ment au n O 1, Tome 41, janvier 1980, page C1-453
INFLUENCE OF X-RAYS, LIGHT AND ANNEALING ON M E Fe-CHARGE-STATES I N tiNb03 : Fe
H.D. Pfannes, J. Lauer, W. Keune, Y. ~aedt? and H. ~ a k a i "
I;aburatoriwn fUr Angewandte Physik, GesamthochschuZe Duisburg, D-4100 Duisburg, Gemany.
x Research Reactor I n s t i t u t e , Kyoto University, 590-04, Japan.
The charge state of iron impurities in LiNbO crystals is influenced by crystal preparation (atmospheres, thermal treat- 3 ment /I-3/), irradiation by X-rays
/4,5/
and by absorption of W-light/6/. By light irradiation in the range of 350-
650
n m photocurrents with and with-out applied electric field (photovoltaic effect) and refractive index changes (photorefractive effect, PE) are generatEd
/2,7/.
Based on the PE storage of volume holograms in LiNb03:Be is achieved/a/.
Photoinduced charge transfer between Fe2+
and pe3+ions has been suggested as under- lying the PE/7/.
We investiihted by Miossbauer spectroscopy the change of the Fe-charge state after
Mitssbauer spectrum at 77K after irradia- tion with co60- $-rays (dose ~ 5 . 5 ~ 1 0 7 rad) at 77K is shown in fig.?. The indi- cated ~ e ~ + - ~ u a d r u ~ o l e doublet ( A E
Q
%2.36 mms", I . S . W + 1 . 2 mms" vs. d - F e ) appears after irradiation and saturates in intensity with increasing irradiation dose. The maximum absorption ratio
2+ sat /(pe2++pe3') of roughly 3% was reached with an incoming dose of ~ 1 . 4 - I 0
7
rad in the case of ~ o ~ O - ~ - r a ~ s . The con- version, of I?e3+ to I?e2+ is possible a l s o by X-ray-irradiation /5/, which yields the same pe2+-doublet as with F-irradia- tion. However, Fe2+ sat/(~e2*+~e3+) x,
7%
as determined from the spectrum at 4.2K after 30h exposure at 4,2K to X-rays from a Cu tube (60kV, 40mA, Fe-filter, 12.5cn distance to sample) yielding an incoming dose of roughly 10 rad. 6
Fig. 1 : Mossbauer absorption of LiNb03 :
pe3+ at 77K after 16h ~ o ~ z ~ - i r r a d i a t i o n O K
(3.4-lo6 rad/h) at 77K. 0 80 160 2LO
annealing temperature/K-
i
X- and a*-irradiation. The sample (0.26
mm thick, 0.22 wt% Fe20j (91% enriched in Fig. 2: I?@*+-absorption after annealing pe5?), single crystal with c-axis at various temperatures relative to the
r-direction) contained only pe3+ prior absorption of the unannealed sample, to irradiation
/5/.
The inner part of the Fe sat, 2+ VS. the annealing temperature,I annraling~ time: x 1 h
2 h
0 4 h + 8 h
- - x - x - ~ 16 h
0 20 h
\' \
3'.
...+'.
,' \ x-
.,*4=\\ +... -0.' X\
+..,
0
I I I
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801177
cl-454 JOURNAL DE PHYSIQUE
The ~e'+ charge state once formed by irradiation remains stable up to tempera- tures T
s 7713..
In order to study the thermally activated reconversion of pe2+to l?e3+ we performed isochronal and iso- thermal annealing of the 30h X-irradiated sample.
In fig.2 the ratios ~ e ~ + / as ob- ~ e ~ + ~ ~ ~ tained from the spectra recorded at 4,2K
for various annealing temperatures and annealing times are shown. A large frac- tion of the pe2+ ions is converted to Fe3+
within a temperature range between 120 to 240K. It is possible to obtain only Fe3+
by annealing for about
,<
8 days at room temperature.The reconversion kinetics can be seen more clearly in fig.3 for annealing tem- peratures of 160, 180 and 200K. The curves
X- or-bcirradiation excites electrons from the valence band into the conduction band leaving holes in the valence band.
At low temperatures these holes are cap- tured at acceptor type defects forming small polarons ("self -trapped 0-"), The electrons may be trapped either as small polarons at Nb sites yielding I?b4+ or
-
more likely in our case of LiNb03:Fe
-
captured at pe3+ impurities forming Fe 2+
.
Our low value for the activation energy of about O.leV which is comparable to that of irradiated pure LiNb03,
/9/,
suggests that the self trapped holes are thermally released, thus mobile in the valence band, and recombine with the trapped electrons of ~e~~ leaving I?e3+. The, observed recon- version of ~ e to ~ e ~ ' ~ + by irradiation of the sample with He-Ne-laser light,/5,9/,
as well as the conversion of Fe3+ to Fe2+
by W-irradiation, /6/, harmonize with the proposed model and the optical ab- sorption data.
Fig. 3 : Fe2+-absorption af t er isothermal annealing relative to Pe 2+ sat VS. the annealing time
.
may be analyzed below 180K by assuming one single thermally activated reconver- sion process. We estimate an activation energy of about O.1eV. Our data indicate that a process governed by one single activation energy from 120K to room tem- perature seems unlikely.
Following the suggestion of Schirmer and von der Linde for pure LiNb03, /9/, we interpret our results by the following model :
Acknow1edaement.- One of us (W.K.) great- fully acknowledges a research fellowship of Kyoto University.
Ref erences
/I/ Phillips,W,, Amodei,J.J., Staebler, D.L., RCA Rev. (1972) 94.
/2/ Kurz,H,, Xratzig,E., Keune,W., Engel- mann, H.
,
Gonser,
U.,
Dischler,
B.,
Ra'u-ber,A., Appl. Phys.
12
(1977) 335 /3/ Bollnann,W., phys. stat. sol.(a)40 (1977) 83
-
/ 4 / Bernhardt ,Hj., phys, stat. sol. (a)
(1976) 217
/5/
Lauer,J., Pfannes,H,-D,, Keune,W,, J, Physique Colloq,3
(1979) (22-561/6/ Engelmann,H., Gonser,U., Ferroelec- trics (to be published)
/7/
von der Linde,D., Glass,A.M., Appl.P~Ys-
8
(7975)85
/8/ Kurz,H., Optica Acfa
&
(1977) 463/9/
Schirmer,O.F., von der Linde,D.,Appl. Phys. Letters (1978)
