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JAHN-TELLER COHERENT TRANSITION AND
OTHER PROPERTIES OF FeBO3
H. Pollak, R. Quartier, W. Bruyneel, P. Walter
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, Dkcembre 1976, page C6-589
JAHN-TELLER COHERENT TRANSITION AND
OTHER PROPERTIES OF FeBO,
H. POLLAK (*), R. QUARTIER, W. BRUYNEEL and P. WALTER Dkpartement de Physique, Groupe de 1'Etat Solide
Universitk Nationale du ZaTre, Campus de Kinshasa, Zai're
R6sum6.
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Des microcristaux de borate ferrique ont kt6 etudiks par NGR entre la temperature ordinaire et celle oh il rkagit avec le carbone de remplissage dans le creuset pour former un carbure. L'evolution de la largeur des raies quadrupolaires qui prksente une brusque variation B 551 Kest interpr6tke cornme le rksultat d'un effet tunnel coherent entre deux distorsions Jahn-Teller. Abstract.
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Polymicrocrystals of ferric borate were studied by NGR as a function of temperature up to the point where they react with the carbon packing material to form an iron carbide.The variation of the line width shows a sudden decrease at 551 K which is interpreted as arising from a coherent tunnel effect between two Jahn-Teller distorsions.
1. Structure and properties.
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Iron borate FeBO, crystallizes in the calcite rhombohedra1 structure of space group R3c( ~ 3 6 ~ ) .
The colour of this birefringent compound, transparent to visible light, is dark green. Ferric borate is a typical weak ferromagnet (canted antiferromagnet) having a transition temperatureTN
at 348 K to the antiferromagnetic state. Both magnetic and magneto-optical propreties have intensively been studied [I, 21. FeBO, posesses some special features typical of compounds where electron charge transfer is known to occur (e. g. the dark green colour, the dichroi'sm and an IR absorption band at about 11 000 cm-l). We therefore decided to look with NGR for evidence of such behavior.
From its crystallographic structure, we conclude that the iron is octahedrally coordinated by six oxygen ions in FeBO,. Each octahedron shares a common face of 3 oxygen ions with the neighbouring ones. This confi- guration is different to that in the silicates where the octahedra only share one common edge. In these compounds, the charge transfer IR absorption band is located at about 14 000 cm- l.
It is well known from the work by Weyl [3], that the electron transfer between ferrous and ferric ions is facilitated by 02- ions. This transfer occurs more easily, the smaller the polarisation of the oxygen ions. Here the boron-oxygen bonds cause a very small polarisation effect, as determinated by optical absorp- tion measurements [4].
2. Experimental results.
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Polymicrocrystals of ferric borate were studied as function of temperature in the range between 300 and 920 K.(*) Centre Regional &Etudes Nucleaires de Kinshasa.
From 348 to 901 K we observe only two absorption peaks. Owing to the crystallographical structure, where an unique iron is present, we have performed a systematic study of the Mossbauer parameters taking account of a quadrupole doublet.
In the above temperature range, the quadrupole splitting remains almost constant, which is uncommon, with a value of 0.19 & 0.005 mmls, whilst the isomeric shift, which has a value of 0.23
+
0.005 mm/s relative to metallic iron at 348 K, slowly varies according to the second order Doppler shift. Both parameters are typical of high-spin ferric iron.However, the behavior of the line width shows an abrupt discontinuity at about 551 K. Since the recoilless fraction and the Q . S. and I. S. show no discontinuity at this point, it seems difficult to imagine a phase transition or an order-disorder transformation taking place at this temperature. Above this temperature we observe a decrease of the line width (see Fig. 1).
FIG. 1. -Line width of FeB03 versus the temperature. The
activation energy is deduced from the slope of the high tempe- rature curve.
C6-590 H. POLLAK, R. QUARTIER, W. BRUYNEEL AND P. WALTER
3. Interpretation.
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For a compound like the lographic parameters with temperature, which affects studied one, the only explanation of this change is a most drastically the Jahn-Teller effect. This explains relaxation process. Since the 6 (except for the second the sudden line width decrease.order Doppler shift) and the AE show no explicit
temperature dependence, we cannot attribute- this relaxation process to an electron hopping process. We suggest that the discontinuity arises by a coherent transition from one Jahn-Teller distorsion to another one by tunnelling effects. In that case, for a tunnel pulsation less than AE, only two lines appear in the
spectrum. If it is of the same order of magnitude than the tunnel effect, each line becomes a doublet whose intensity depends on the tunnel frequency 151. This is to be considered as a fine structure where both lines have the same width as their daugther. In the temperature of 551 to 900 K we are in the first regime, whilst between 348 and 551 K we are in the second one. The fine structure is too weak to cause a full separa- tion of the resonance lines. Therefore one only sees a broad peak whose width is markedly larger compared
4. Magnetic behaviour. - Surprinsingly our specimen did not reveal, at room temperature, any magnetically ordered structure allthough it is well known that ferric borate is weakly ferromagnetic at this temperature 111. However, after cooling down, the sample shows a magnetic structure, which appears at the transition temperature of TN = 348 K.
The hyperfine field follows the classical Brillouin curve with
p
= 0.35. The thermal treatment has there- fore enhanced the magnetic orientation to a large extend. Optical parameters of the sample did not change at all during this treatment done under vacuum better than torr. When heated again, the magnetic structure disappears atTN
and the quadrupole doublet is found as expected. The process is reversible.to the one observed in the higher temperature range
where the tunnel effect pulsation is lower. The change Acknowledgements.
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The sample has kindly in the fluctuating rate of the tunnel effect at 551 K can been provided by Dr R. Diehl, Forschungsinstitut fur be assumed to arise from a small change of the crystal- Angewendte Festkorpertechnik, Freiburg.References
[I] EIBSCHUTZ, M., PFEIFFER, L. and NIELSEN, J. W., J. Appl. [3] WEYL, W. A., J. Phys. Chem. 55 (1951) 507.
Phys. 41 (1970) 1276. [4] ALLEN, G. C. and HUSH, N. S., Progr. in Inorg. Chem. 8
[2] WOLFE, R., KURTZIG, A. J. and LECRAW, R. C., J. Appl. (1967) 357.
