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VALENCE STATE CHANGE OF Fe IMPURITIES IN LiNbO3 AFTER X-RAY IRRADIATION
J. Lauer, H. Pfannes, W. Keune
To cite this version:
J. Lauer, H. Pfannes, W. Keune. VALENCE STATE CHANGE OF Fe IMPURITIES IN LiNbO3 AFTER X-RAY IRRADIATION. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-561-C2-563.
�10.1051/jphyscol:19792195�. �jpa-00218573�
VALENCE STATE CHANGE OF Fe IMPURITIES IN LiNb0
3AFTER X-RAY IRRADIATION
J. Lauer, H.D. Pfarmes and W. Keune
Labovatorivm fur Angewandte Physik, Geeamthoehsehule Duisburg, D-4100 Duisburg, Germany
Abstract.- The influence of X-rays irradiation at 4.2 K on the charge state of iron impurities in LiNb03 has been studied by Mossbauer spectroscopy. By the irradiation F e3 + impurities can be con- verted to Fe2 +. A reconversion is possible by annealing at temperatures >. 160 K for a sufficiently long time.
Iron doped LiNbCh is an axial ferroelectric compound being of considerable interest as storage material for optical volume phase holograms which result from light-induced changes of the refractive index (photorefractive effect) /l/. The iron impu- rities strongly influence the amount of local index changes, hence Mossbauer studies are expected to contribute to the understanding of the related ba- sic mechanisms. Previous investigations by Mossbauer spectroscopy /2-4/ and optical or EPR methods /5,6/
indicate that the iron enters the lattice as F e3 +
and/or F e2 +, their ratio depending on thermal treat- ment of the samples in reducing or oxidizing atmos- pheres, and some evidence is given that the photo- refractive effect arises from a photoinduced charge exchange between the F e2 + and Fe3 + irons /l/. Elec- trons from the Fe2 + levels may be excited to the conduction band and subsequently trapped by Fe ions in the dark regions of the hologram pattern.
The resulting change in space charge effects the lo- cal polarization and hence the birefringence via the electrooptic effect. In view of this model it is interesting to study the influence of ionizing ra- diation, particularly of X-rays, on the impurity charge state.
Our sample was a single crystal platelet grown from a LiNb03 : 0.22 wt% Fe203 melt, which was 91 % enriched in 5 7Fe. The physical thickness was 0.25 mm corresponding to a single-line effective thickness of T =2.3.
A
By annealing in a controlled oxidizing atmos- phere all Fe impurities in the sample have been converted to Fe3 . The corresponding Mossbauer absorption spectrum at 4.2 K is shown in figure 1 (top). As has been pointed out more in detail /2-4,6/
mainly two magnetic hyperfine subspectra can be dis- tinguished, arising from slow electronic relaxation of the |± 5/2> and |± 3/2> electronic crystal field levels of the iSy2 state of the Fe3 + .
The sample has been irradiated using a Cu-X-ray source (60 kV, 40 mA,, Fe-filter) with a source-to- sample distance of about 12.5 cm. Care was taken that no light reached the sample during irradiation and recording of the Mossbauer spectra. After the irradiation two additional lines appeared in the absorption spectrum as indicated in figure 1 (bottom) for an irradiation time of 30 hours . As can be seen by comparison of this spectrum with that of LiNb03 : Fe after reducing heat treatment /2,3,6/ the addi- tional lines represent a quadrupole doublet origi- nating from Fe2 ions (isomer shift + 1.19 mm/s vs.
a-Fe, quadrupole splitting 2.42 mm/s). Thus by X-ray irradiation a partial conversion of F e3 + to F e2 + has Fig. 1 : Mossbauer spectra of LiNb03 : F e3 + at 4.2 K unirradiated (top) and after 30 h. X-ray irradiation at 4.2 K (bottom). The y-direction was perpendicular tc the crystallographic c-axis.
JOURNAL DE PHYSIQUE
Colloque C2, supplément au n° 3, Tome 40, mars 1979, page C2-561
Résumé.- L'influence d'irradiation aux rayons X à 4.2 K sur l'état de charge des impuretés de fer dans LiNb03 a été étudiée par effet Mossbauer. Les impuretés F e3 + peuvent être converties aux F e2 +
par irradiation. Ensuite il est possible d'effectuer la transformation F e2 + en Fe + par chauffage à des températures >. 160 K pendant un temps suffisant.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792195
C2-562 JOURNAL DE PHYSIQUE
been achieved. We studied this behaviour versus the X-ray dose. In figure 2 the inner parts of the Msss- bauer spectra after several irradiation times are shown. With increasing irradiation dose the Fe2+
absorption is enhanced. A total conversion however seems not to be possible, i.e. the portion of Fe 2+
versus the irradiation time shows a saturation beha- viour. After 30 hours X-ray exposure time a steady state between formation of Fe 2+ and disintegration to Fe3+ during the irradiation is practically achie- ved.
VELOCITY m m / s
Fig. 2 : ~sssbauer spectra of LiNbOs : Fe3+ at 4.2 K after X-irradiation at 4.2 K for 0 to 30 hours irra- diation time.
The amount of Fe2+ once created after irradia- tion does not change with time up to temperatures of about 77 K. Annealing the sample at room temperature for about one day however reconverts the Fe2+ com- pletely to Fe3+. We examined the reconversion more in detail in the following way. Starting from the 30 hours X-ray irradiated sample we heated it to the temperatures T indicated on the abcissa of figure 3 and annealed for one hour (isochronal annealing).
After each annealing period the sample was cooled again to 4 . 2 K and a Mijssbauer spectrum was recor- ded. The heating rate was about 7 degrees/min., the cooling rate (until 77 K) amounted to about 6 de- greeslmin. In figure 3 the absorption of the ye2+
doublet after annealing at the temperature T rela- tive to the ~ e ~ + - absorption at 4.2 K before annea- ling is shown. As can be seen the transition I'e2+
to Fe3+ occurs mainly in a temperature range around about 160 K but the curve in figure 3 does not tend to zero at room temperature i.e. not all Fez+ will be converted to Fe3+ since the annealing time was only one hour in this case. We performed also iso- thermal annealing by heating the sample to 160 K
for times t from t = 1 h. to t 2 4 0 0 h. and the Fe2+ /Fez+,
.
2K-ratio has been obtained from the160K,t
corresponding spectra.
ANNEALING TEMPERATURE T/K
Fjg. 3 : Absorption of Fe$+ after isochronal annea- lrng (At = Ih.) at temperatures from 40 K to 240 K relative to the Fez+ absorption of the unannealed sample. The measuring temperature was always 4.2 K.
The same procedure has been done for an annealing temperature of 200 K. Also for these last two measu- rements we started from the 30 h.-X-ray irradiated sample. In the case of the 200 K annealing, e.g.
Fe2+2 o OK, t / ~ e ~ + , . , ~ was found to be 0 . 4 6 for t = 1 h.
and 0 . 3 8 for t = 4 h. The time dependence of our pre- liminary data cannot be explained by only one expo- nential process for the thermal reconversion of Fe2+
to ~ e ~ + .
In the framework of the model used for the pho- torefraction effect we can explain our experimental results as follows. Due to the X-ray irradiation an electron is transferred from the valence band to the conduction band and subsequently trapped by an Fe3+.
In the valence band a hole is remaining. The process may be reversed at higher temperatures by two diffe- rent processes or perhaps one process with different kinetics, where the lower mobility of the hole in
the valence band compared to that of the electron in the conduction band could play some role. Additional- ly we observed that the Fe2+ to Fe3+ reconversion also can be achieved by irradiation of the sample with He-Ne-Laser light at 4.2 K, in agreement with results obtained by other experimental techniques /7/. This is also compatible with the model.
Acknowledgement.- This work was partially supported by the Deutsche Forschungsgemeinschaft (SFB 130,
~aarbriicken)
.
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