VALENCE STATE CHANGE OF Fe IMPURITIES IN LiNbO3 AFTER X-RAY IRRADIATION

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 Fe 3+ impurities can be converted to Fe. 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 impurities strongly influence the amount of local index changes, hence Mossbauer studies are expected to contribute to the understanding of the related basic mechanisms. Previous investigations by Mossbauer spectroscopy /2-4/ and optical or EPR methods /5,6/ indicate that the iron enters the lattice as Fe and/or Fe, their ratio depending on thermal treatment of the samples in reducing or oxidizing atmospheres, and some evidence is given that the photorefractive effect arises from a photoinduced charge exchange between the Fe and Fe irons /l/. Electrons from the Fe 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 local polarization and hence the birefringence via the electrooptic effect. In view of this model it is interesting to study the influence of ionizing radiation, 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 Fe. 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 atmosphere all Fe impurities in the sample have been converted to Fe . 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 distinguished, arising from slow electronic relaxation of the |± 5/2> and |± 3/2> electronic crystal field levels of the Sy2 state of the Fe 3 + . The sample has been irradiated using a Cu-X-ray source (60 kV, 40 mA,, Fe-filter) with a source-tosample 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 additional lines represent a quadrupole doublet originating from Fe 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 Fe to Fe has Fig. 1 : Mossbauer spectra of LiNb03 : Fe 3+ 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, supplement au n° 3, Tome 40, mars 1979, page C2-561 Resume.L'influence d'irradiation aux rayons X a 4.2 K sur l'etat de charge des impuretes de fer dans LiNb03 a ete etudiee par effet Mossbauer. Les impuretes Fe 3+ peuvent etre converties aux Fe par irradiation. Ensuite il est possible d'effectuer la transformation Fe en Fe + par chauffage a des temperatures >. 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 Msssbauer 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 behaviour. After 30 hours X-ray exposure time a steady 2+ state between formation of Fe and disintegration to Fe3+ during the irradiation is practically achieved.