HAL Id: jpa-00226038
https://hal.archives-ouvertes.fr/jpa-00226038
Submitted on 1 Jan 1986
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
INVESTIGATION OF THE DOUBLE-PHASE TRANSITION OF Tm3Fe5O12, BY EXAFS
SPECTROSCOPY
H. Beuthien, D.V. Ahlften, W. Girnus, A. Torkler, W. Gunsser, W. Niemann
To cite this version:
H. Beuthien, D.V. Ahlften, W. Girnus, A. Torkler, W. Gunsser, et al.. INVESTIGATION OF THE DOUBLE-PHASE TRANSITION OF Tm3Fe5O12, BY EXAFS SPECTROSCOPY. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-733-C8-735. �10.1051/jphyscol:19868138�. �jpa-00226038�
JOURNAL DE PHYSIQUE
Colloque C8, supplbment au n o 12, Tome 47, dbcembre 1986
INVESTIGATION OF THE DOUBLE-PHASE TRANSITION OF TmgFe501, BY EXAFS SPECTROSCOPY
H. BEUTHIEN, D.v. AHLFTEN, W. GIRNUS, A. TORKLER, W. GUNSSER and W. NIEMANN*
Institute of Physical Chemistry. University of Hamburg, Bundestrasse 45,,D-2000 Hamburg 13, F.R.G.
" ~ a l d o r Topsoe Research Laboratories, DK-2800 Lyngby
Introduction In previous studies amorphous Erbium-iron garnets have been investigated by EXAFS measurements /1,2/ and Massbauer spectros-
copy / 3 / . An immediate increase of bond length and coordination num- ber of Er-0 in the region of the transition from non-crystalline to crystalline garnet was found. In this work EXAFS experiments at the narrow transition region of a-TmIG to c-TmIG at the LIII edge of Tm confirms the double-phase transition observed by DTA measurements.
Abstract Coprecipitated Thulium-iron hydroxides with garnet stoichio- metry (TmIG) have been investigated at several annealing temperatures
(from 333 K to 1453 K) using differential thermal analysis (DTA), x-ray diffraction (XRD) and EXAFS at the L edge of Tm. Two phase transitions at 1 0 4 3 K and 1 0 9 1 K are obser*6& by DTA. XRD proves that the crystallization of TmIG is completed at 1 0 9 1 K. The analysis of the nearest neighbour shell of the EXAFS data shows the presence of three temperature regions separated by the two phase transitions. The different oxide geometries in these regions are elucidated by
EXAFS fits.
Experimental Amorphous Thulium-iron oxides with the garnet stoichio- metry were prepared by coprecipitation of hydroxides from aqueous solutions of their nitrates /4/ in ultrasonic fields ( 2 3 kHz). The mixed hydroxides have been dried at 333 K and annealed in a DTA apparatus at various maximal temperatures and a constant heating and cooling rate. Only in the temperature region of the two phase tran- sitions the maximal temperature has been constant about one hour be- fore cooling down.
XRD experiments have been performed on the samples at various annea- ling temperatures.
EXAFS measurements have been done at the EXAFS I1 beamline / 5 / of the Hamburger Synchrotronstrahlungslabor (HASYLAB) at liquid Helium tem- perature.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868138
C8-734 JOURNAL DE PHYSIQUE
EXAFS experiments at 4 K have been performed at samples which were annealed at various temperatures in order to investigate the Tm-0 co- ordination. A series of fine structures is shown in Fig. 2. The cor- responding Fourier transform of k-X(k) is displayed in Fig. 3.
Results and discussion Desorption of water is observed for
l i l . , , , . temperatures up to - 6 7 3 K. A similar
Fig. 2 Fine structures of Tm-0 Fig. 3 Fourier transform of the in a-TmIG and c-TmIG spectra shown in Fig. 2
~ m - F ~ - G ~ c ~ ~ ~
--
.I
exothcrmtc > , , , , , , 11 72 , endolhermlcThe backtransform of the corresponding nearest neighbour peaks are fitted by a two shell fit using standard data evaluation technique /6/ and experimental backscattering amplitudes and phases taken from the polycrystalline sample the structure data of which were taken from reference / 7 / . Results of the fitting procedure are shown in Figs. 4 and 5 where the bond length and the coordination number of Tm-0 are displayed as a function of the annealing temperature.
Three different regions which coincide with those found by DTA are clearly visible. For T>T only one shell is present wheras for TtTl the data have been Zitfed by a two-shell model with Rl%0.23 nm
behaviour has been reported for HoIG /4/. Two exothermic phase transitions are found at T = 1043 ? 6 K and at T = 1091 + 6 (Fig.1). In reference
/ i /
a double-phase transition has alsobeen claimed. However, the preparation of the samples was different to the method used in this investigation.
Below TI the samples do not show distinct diffraction patterns but only
'00 600 800 1000 an amorphous halo. Within the phase
T I K I
transition region a gradual increase Fig.1 DTA diagramm of intensity of the [4201 reflex of
garnet with increasing temperature is observed indicating the formation of garnet structures. The crystallization of TmIG is completed at T2.
Additional reflexes of a second phasenot yet identified have been found in the transition region. The broad patterns indicate that this phase is not well crystallized.
and R2%0.26 nm. A contraction of the Tm-0 distance for both shells is observed for T<Tl,wheras in the narrow region T1<T<T2 a drastic increase of bond length towards the crystalline value occurs.
The shell with the larger radius shows a coordination number of N% 2.5. The EXAFS contribution of this shell vanishes for T>T ,
wheras the coordination number of the shell with the lower x a h u s in- creases by approximately the same value of AN % 2.5.
300 500 700 1000 1300 300 500 700 1000 1304
annealing temperature I K l annealing fempemture T K I
Fig.4 Tm-0 bond lengths Fig. 5 Tm-0 coordination numbers
References
/I/ R. Priess, Thesis, University of Hamburg, 1985
/2/ A. Seyler, Diploma-Thesis, University of Hamburg, 1985 /3/ W. Girnus, H. Beuthien, R. Priess and W. Gunsser,
Journ. d. Phys. Colloqu. C8, suppl. n012, tome 46 (1985), C8-137 /4/ V.P. Chalyi, Z.Y. Makarova, K.P. Danil'chenko and L.G. Gavrilova,
Inorg. Mat. 16 (1980), 1536
/5/ W. ~alzfeldt,~. Niemann, R. Haensel, P. Rabe,
~ u c l . Instr. Meth. 208 (1983), 359-360
/6/ P.A. Lee, P.H. Citrin, P. Eisenberger, B.M. Kincaid, Rev. Mod. Phys. 53 (1981), 769-806
/7/ F. Theou, Solid =ate Comm. 8 (1970), 1745 and 1751
