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A MÖSSBAUER STUDY OF CONDUCTION IN MAGNETITE
J. Coey, A. Morrish, G. Sawatzky
To cite this version:
J. Coey, A. Morrish, G. Sawatzky. A MÖSSBAUER STUDY OF CONDUCTION IN MAGNETITE.
Journal de Physique Colloques, 1971, 32 (C1), pp.C1-271-C1-273. �10.1051/jphyscol:1971190�. �jpa- 00214517�
JOURNAL D E PHYSIQUE Colloque C 1, supplément au no 2-3, Tome 32, Février-Mars 1971, page C 1
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271J. M. D. COEY and A. H. MORRISH University of Manitoba, Winnipeg 19, Canada
and G. A. SAWATZKY
University of Groningen, The Netherlands
Résumé. - En étudiant les spectres Mossbauer obtenus en présence d'un champ appliqué de 47 kOe de quelques échantillons de la série Fe3Os-y Fe203 avec la structure spinelle on a observé un troisième spectre en plus des spectres normaux correspondant à Fe3+(A) et Fe2+/3+(B). Le champ hyperfin est de 503 kOe et le déplacement isomérique est de 0,54 mm.s-1 (relatif au chrome) pour ce spectre, qui est attribué au Fe3f ne participant pas à l'échange électronique dans les sites B. On peut expliquer le rapport d'intensité des trois spectres soit par des sauts d'électrons entre un couple d'ions de Fer, .soit par des lacunes du site B liées à cinq ions Fe3+.
Abstract. - A study of the Mossbauer spectra of several members of the spinel series Fe304-y Fe203 with and without an applied field of 47 koe has revealed a third pattern in non-stoichiometric magnetite in addition to the normal Fe3+(A) and Fe2+13+(B) patterns. The hyperfine field is 503 kOe and the isorner shift is 0.54 mm.s-1 relative to chromium. The pattern is ascribed to Fe3+ on B sites which is not involved in fast electron interchange. Its intensity may be accounted for either on the basis of pair-localiseci hopping or by assuming that each B site vacancy traps five Fe3f ions.
Introduction.
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Magnetite has the inverse spinel structure above the Verwey transition, TV. In a unit ce11 of the tetrahedral interstices in the f. c. c.oxygen lattice (A sites) are occupied by Fe3+, and equal numbers of FeZ+ and Fe3+ ions are randomly distributed in 16 of the octahedral interstices (B sites).
It is possible to prepare non-stoichiometric magnetite with a deficit of iron, Fe3-,O4, in which there are B site vacancies, the charge balance being preserved by a greater proportion of Fe3 + ions [l]. Such material may be represented by the formula
(Fe3+) [Fe:: ,,Fe? ,,u,I 0,
where the different brackets denote the difïerent sites.
I t is believed that the electrical properties of magnetite are intimately related to the B site cation distribution.
Verwey [2] first suggested that the high conductivity results from electron hopping between Fe3+ cores with an activation energy of the order 0.05 eV. More recently there have been descriptions of the electronic structure in terms of a band mode1 [3, 41. The eleven B site d electrons are distributed in spin up and spin down bands with an energy gap of several eV. Each is split by the trigonal field into three sub-bands, a singlet (lowest) and two doublets. The 3 d i band contains 10 electrons, and is full, while the eleventh electron half fills the 3 d ? singlet, and is responsible for the conductivity [SI.
The many experimental investigations of Fe30, include several using the Mossbauer effect. Two pat- terns are observed ; one is ascribed to Fe3 '(A) and the other to Fe2+f3+(B). Separate Fe2+(B) and Fe3+(B) spectra are not seen because the characte- ristic time for electron interchange z is less than the nuclear Larmour precession time z, ( N IO-* S) thus the nucleus experiences an average field of 3 d5 and 3 d6 electron configurations. The B site line width has been studied as the temperature is lowered and z approaches z, 16, 71, and we have found that the line broadening is constant between about 2500K and TV implying essentially zero activation energy for the hopping. Similar conclusions have been drawn from measurements of thermopower [8].
In order to elucidate the conduction mechanism we have investigated non-stoichiometric magnetite with a B site Fe2+ : Fe3+ ratio considerably different from 1 : 1.
Experiment. - Several members of the spinel series Fe30,-yFe203 were studied. The stoichio- metric magnetite absorber was made of crushed pure natural crystals. Materials with 6 = 0.03 and 0.08 were prepared by heating u Fe203 a t 1450 OC for 4 hours in nitrogen and air respectively, and then quenching in water. The latter contained 5 % of unconverted a Fe,O?, but when that spectrum was subtracted off, thespëctra for the 0.08 absorber were similar to those for a material with 6 = 0.09 which we purchased ("). No trace of y Fe,03 was detectable by X rays as a separate phase in any of these materials.
The y Fe203 itself (6 = 0.33) certainly contained less than 1 % of the a phase.
Typical room temperature Mossbauer spectra for the series are shown in figure 1. It is obvious that the intensity of the B site pattern in b) and c) is reduced quite out of proportion to the number of vacancies introduced. The area ratio of the first lines of the A site and B site patterns is 1 : 1.88 in the pure material.
(It is not exactly 1 : 2 because of the different recoilless fractions for the two sites [9].) However in Fe,.9, O, the ratio for the corresponding two lines is 1 : 1.06. This suggests that a part of the B site spectrum overlaps the A site pattern.
In figure 2, the A and B site spectra for the series are clearly resolved by applying a 47 kOe field. The width of the Fe2'I3
'
(B) line increases with 6. However the most striking feature of the data is the appearance of a third pattern which may be clearly seen in c).The values of the hyperfine fields and isomer shifts of al1 the patterns given in table 1 are extrapolated to zero applied field. Those of the new pattern are cha- racteristic of Fe3' on the B sites of an oxide spinel
(t) Research supported by the National Research Council of Canada.
(*) From the Fisher Scientific Company.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971190
C 1
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272 J. M. D. COEY AND A. H. MORRISH AND G. A. SAWATZKY-
g
,,, G-%~Y,. ; y*. .u,
.(c, p%? ?--S . P i l .
. . .. . . .
89
..
,- . Z . . . .
FIG. 1.
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Mossbauer spectra of the series Fes-a04 at 296 OKa) 6 = 0.00 b) 6 = 0.03 c) 6 = 0.09 d) 6 = 0.33.
FIG. 2. - Mossbauer spectra of the same series in a 47 kOe field applied paraIlel to the y-ray propagation direction.
sucb as y Fe203. The Verwey transition temperatures were obtained from magnetic measurements in the range 77-150 OK and from thermal scans of the Moss- bauer spectra. They are in agreement with the values expected on the basis of composition.
Discussion. - At first sight the separate Fe3'(B) pattern suggests pair-localised hopping as propo- sed by Daniels and Rosencwaig [IO, 1 11. This pattern is then due to ferric ions in excess of the number of F e 2 ' - ~ e ~ ' pairs, and the predicted Fe3+(A) : Fe3+(B) : Fe2+I3+(B) intensity ratio is 1 : 5 6 : (2- 6 6).
However it is possible to retain the band picture if we realise that the conduction electrons are not free to wander amongst al1 the Fe3+ cores in non- stoichiometric magnetite. Each vacancy is electrically equivalent to an additional charge of
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512 on an occupied B site which may be neutralised either by polarisation of the oxygen lattice or by an excess positive charge on the adjacent B sites. If we ignore the former mechanism we can simply suppose that 5 Fe3+ ions are trapped by the vacancy and cannot take part in the conduction process (i. e. the vacancy electrostatically repels conduction electrons from the cores). It turns out that the intensity ratio is just the same as that predicted on 'the hopping model, although 5 6 is now an upper limit on the intensity of the Fe3' (B) pattern on account of the polarisation effect.For 6 = 0.09 we observe acorrected ratio of 1 : 0.46 : 1.49 compared to the predicted 1 : 0.45 : 1.46. The agreement for 6 = 0.08 is equally good.
In the band picture the introduction of vacancies in Fe30, reduces the number of electrons, resulting in a slightly less than full band below TV and also a decrease in the correlation energy gap causing a reduction in T,. Although the electrons will be stron- gly scattered by the vacancies the net effect can still be an increase in the conductivity. On the other hand the band is slightly less than half full above TV and the scattering by the vacancies will decrease the conduc- tivity. The variation of the conductivity [8] as the temperature approaches Tc is likely a result of magne- tic scattering.
In conclusion we have shown that Our observations are consistent with a band picture of magnetite which can also explain quaiitatively the behaviour of the conductivity near TV and the variation of TV as a function of stoichiometry.
Hyperjîne Fields and Isorner Shifts for the Fe304-y Fe203 System
Hyperfine field (kOe) Isomer shift (+) (mm. s e l ) Material TV (OK) Fe3+(A) Fe3 '(B) Fe2+I3+ (B) Fe3'(A) F ~ ~ + ( B ) Fe2't3'(B)
- - -
-
- - --
Fe3.000, 118 490
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463 0.a3 - O. 83Fe2,9,04 108 490 (*> 463 0.41 ( *> 0.81
Fe2.9104 93 49 1 503 462 0.42 0.54 0.82
Fe,.,,o, - 499 505 - 0.41 0.52 -
(*) Pattern too weak to measure.
(+) Relative to Chromium.
References
[l] VERWEY (E. J. W.) and HAAYMAN (P. W.), Physica, [7] SAWATZKY (G. A.), COEY (J. M. D.) and MORRISH 1941, 8, 979. (A. H.), J. Appl. Phys., 1969, 40, 1402.
[2] VERWEY (E. J. W.), HAAYMAN (P. W.) and ROMEIJN [8] GRIFFITHS (B. A.), ELWELL (D.) and PARKER (R.), (F. C.), J. Chem. Phys., 1947, 15, 181. Phil. Mag., 1970, 22, 163.
[3] ADLER (D.), Solid State Physics, 1968, 21, 4. [9] SAWATZKY (G. A.), VAN DER WOUDE (F.) and MOR- [4] CULLEN (J. R.) and CALLEN (E.), J. Appl. Phys., RISH (A. H.), Phys. Rev., 1969, 183, 383.
1970, 41, 879. 1101 DANIELS (J. M.) and ROSENCWAIG (A.), J. Phys.
[ 5 ] YOSIDA (K.) and TACHIKI (M.), Prog. Theor. Phys., Chem. Solids, 1969, 30, 1561.
1957, 17, 331. [ I l ] ROSENCWAIG (A.), Can. J. Phys., 1969, 47, 2309.
[6] KÜNDIG (W.) and HARGROVE (R. S.), Solid State Comm., 1969, 7, 223.
