MOSSBAUER EFFECT OF 57Fe IN Ti2O3

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MOSSBAUER EFFECT OF 57Fe IN Ti2O3

C. Blaauw, F. Leenhouts, F. van der Woude

To cite this version:

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JOURNAL DE PHYSIQUE ColIoque C6, supplkment au no 12, Tome 37, Fkcembre 1976, page C6-607

MOSSBAUER

EFFECT

OF

57Pe

IN Ti203

C. BLAAUW

Department of Physics Dalhousie University Halifax, N. S., Canada and

F. LEENHOUTS and F. VAN DER WOUDE

Solid State Physics Laboratory Materials Science Centre University of Groningen Groningen, The Netherlands

R6sum6.

-

Le composB Ti203 a etk BtudiB par spectroscopie Mossbauer de 57Fe. On trouve que le fer existe comme Fez+ aussi bien comme Fe3+. La proportion Fe3+/Fe2+ de l'intensite s'agrandit avec la tempbrature. On donne une explanation utilisant le concept d'un Btat lie associB avec le Fez+. La dependence de la tempkrature de l'interaction quadrupolaire est en accord avec les rksultats d'un calcul du champ crystalline. Les rksultats de l'investigation ne donnent pas d'indication d'ordre magnktique, ce qui est en accord avec les rksultats d'une investigation aux diffractions des neutrons.

Abstract. - The Ti203 system has been investigated with the Mossbauer effect technique using 57Fe. It is found that iron exists in divalent as well as in trivalent oxidation states, the Fe'+/Fez+ intensity ratio increasing with temperature. An explanation for this is given in terms of bound states associated with the Fez+ ions. The temperature dependence of the Fez+ quadrupole splitting is in agreement with the results of a crystal field calculation. No evidence for magnetic ordering in Ti203 is found, in agreement with neutron diffraction results.

1. Introduction.

-

Titanium sesquioxide Ti203 has been of interest recently because of the anomalous temperature dependence of its physical properties. Resistivity measurements indicate that a semiconduc- tor-metal transition occurs within the temperature region 500-800 K, which is probably of electronic origin and may be explained by a band overlap model [I-51.

Ti203 has the structure of corundum, spacegroup R ~ c - D ~ ~ , which may be described with a rhombohe- drall cell containing two formula units of Ti203, or a hexagonal cell. Only one kind of cation and one kind of anion position are present. Cations are approximately anti-prismatically surrounded by six anions. Ti203 retains the corundum structure at all temperatures but the lattice parameters vary anomalously. In the temperature region corresponding to that of the resistivity anomaly, a large change is found in the lattice parameters, consisting of an expansion of the hexagonal c-axis and a contraction of the hexagonal a-axis. Neutron diffraction experiments [6] have shown that no antiferromagnetism exists in Ti203. Our Mossbauer experiments will be seen to support this conclusion.

2. Experimental. - Starting materials for sample preparation were Ti metal and rutile (TiO,), both in a powdered form and of 99.9

%

purity. Iron, enriched

up to 90

%

in 57Fe, was available in the form of Fe20, powder. Ti203 was prepared by arcmelting pellets of the appropriate mixture of Ti and TiO, in an Ar atmosphere, homogenizing the resulting material for a few days in evacuated, sealed, quartz tubes at z 1 000 K and then quenching in water to room temperature. The introduction of Fe into the lattice was accomplished by arcmelting pellets of Ti203 mixed with Fe203 in an Ar atmosphere. In this way most of the iron and sometimes all of it was incorporated in the lattice, the remaining Fe being in the metallic state. This metallic iron could sometimes be removed from the sample as small iron particles with a magnet. In most cases we started with a 1

%

Fe203/Ti203 ratio so that the Fe/Ti ratio actually present in the lattice varied from a few pro mille to one percent. The structure of all samples was determined by means of x-ray diffraction analysis.

Samples were prepared of nominal composition Tio.99Feo.010,, with x = 1.40, 1.47, 1.50(2 x), 1.53, 1.60 and 1.67. By correlating the Mossbauer spectra with the structural composition of those samples it was possible to determine the Mossbauer spectra characteristic of Fe in the phases Ti0 and Ti3O,, which are adjacent to Ti203 in the Ti-0 phase diagram. The spectra for x = 1.50, containing Fe3+ as well as Fez+, were found to be characteristic of iron in Ti203, and will be discussed in more detail in the next section.

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C6-608 C . BLAAUW, F. LEENHOUTS AND F. VAN DER WOUDE

Mossbauer spectra of one of these samples at different temperatures are shown in figure 1. Isomer shifts here, as elsewhere in this paper, are relative to metallic iron at room temperature. Both Fez+ and Fe3+ are pre-

-

Veloc~ly lmm/s)

-

Velac~ty lmm/s)

FIG. 1.

-

Mossbauer spectra of 57Fe in Ti203 at different temperatures. The smooth lines show the result of a computer fit in terms of two doublets of Lorentzian lines at T 2 295 K

and in terms of two Lorentzian lines at T = 89 K .

sent in the samples, the spectra of each showing a quadrupole split doublet. However, not only is the Fe3+/Fe2+ intensity ratio different for the two samples, this ratio is also temperature dependent.

3. Discussion.

-

3.1 THE Fez+ SPECTRUM.

-

The Fez+ subspectrum was fitted with two Mossbauer lines. Because at low temperatures the high energy line was well separated from the other components of the spectrum, at low temperatures no constraints were imposed except the condition that both lines of the doublet should be equal. At high temperatures the linewidth was constrained to 0.28 mm/s typical of our equipment. This was necessary in order to retain acceptable values of the linewidths in that region. The isomer shift (I. S.) at room temperature is 1.0 mm/s, a value seen more often in metal oxides for high spin Fez+. The high temperature limit of the shift was found to be

-

0.69 x mm/s K. This is a value which is normal for a second order Doppler shift and suggests that the electronic configuration of Fez +, in terms of effective numbers of s and d electrons, is not temperature dependent.

The temperature dependence of the quadrupole splitting (Q. S.) of the lines is shown in figure 2. We have based our analysis of this temperature dependence on the results of a crystal field calculation. Different orbital 3d states correspond to different spatial charge

0.01 I

0 200 LOO 600 800 1000

T( Kl-

FIG. 2. - The temperature dependence of the 57Fe quadrupole splitting of Fez+ in TizO3. The smooth line is a least squares fit

of the data to a function discussed in the text.

densities and therefore contribute differently to the electric field gradient at the Fez+ nucleus. The temperature dependence of the occupation of these states will result in a temperature dependence of the quadrupole interaction. The splitting of the 3d energy levels was determined from a crystal field calculation, assuming that Hund's rules apply, and calculating the effect of the six 0'- ions, octahedrally surrounding the ~ e ' + ion, on the five orbital 3d states available for the sixth 3d electron of this ion. Spin-orbit interaction was neglected.

The calculation was carried out for the low temperature as well as for the high temperature lattice parameters of pure Ti203 as given by Rao et al. [7] For

<

r 2

>

and

<

r4

>

of the 3d electron we used the Fez+ free ion values given by Watson 181. The result of the calculations is given in figure 3. It is interesting to not that one level, 6f approximately 3 z2 - r 2

FIG. 3.

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The crystal field splitting of the Fe 3d electron energy levels in Ti203 as determined from a point charge approximation

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MOSSBAUER EFFECT OF 57Fe IN Ti203 C6-609 symmetry in our coordinate system, is appreciably

lower than the other four. Though the results of such a calculation must be considered to be crude, they do suggest that a reasonable method of analysis might be to start with a low lying 3d singlet separated by a gap Eg from the remaining four states which we have assumed to be degenerate. This approximation is reasonable if the splitting between the four upper levels is small compared to E,. In this approximation the temperature dependence of the quadrupole splitting AEQ of the Mossbauer lines is

Here AEQ(0) corresponds to the splitting associated with the occupation of the lowest 3d level at T = 0. This approximation did not give a reasonable quali- tative fit to the experimental data. To obtain a better agreement with the data we introduced refinement, respectively

where D expresses a contribution resulting from the lattice, which is taken to be independent of T, and

where a temperature dependent energy gap Eg(l - A T ) has been assumed. Introducing the refinements sepa- rately yielded in each case an improved agreement with the experimental data. The solid curve in figure 2 corresponds to the first refinement but the second one gave an almost identical curve. In table I the results are given for both approximations.

The results of a least squares jit of the Fe2 + quadrupole splitting data to the three functions

discussed in the text

E,(O) E, D A

Function (mmls) (eV) (mmls) (K-l)

Because each approximation alone gave a good result, it is not possible to say a priori which one is best. The first approximation yields a temperature independent quadrupole interaction term of

-

0.21 mmls, a value which does not seem unreaso- nable for a lattice contribution. The second approxi- mation gives a reduction of the energy gap Eg from a value of 0.055 eV at T = 0 K to 60

%

of this value at 1 000 K. Because of the change in the lattice para-

meters with temperature, and the resulting changes in the positions of the coordinating 0'- ions which largely determine the crystal field splitting of the 3d levels, one does expect the gap t o be temperature dependent. Comparison of the results of the calculation at T = 300 K and T = 600 K (Fig. 3) also suggests the possibility of a decrease of the gap with increasing temperature.

Without more information it is not possible to say which of these hypotheses, both of which appear to be physically reasonable, is correct, or whether the physical reality is perhaps a mixture of both. The experimentally found value of Eg is only about 25

%

of the one found in the crystal field calculation. However, this calculation is only expected to give an order of magnitude for the gap.

The experimental point taken at liquid helium temperature was not included in the data used to obtain the solid curve. It is interesting to not that the quadrupole interaction at this temperature is smaller than that at liquid nitrogen temperature. Though this result could possibly be explained by assuming the lattice contribution to the Q. S. to increase strongly at low temperatures, a different explanation is suggested by Ingalls' work [9]. He showed that in those cases where the spin-orbit coupling constant

L

is of the order of 20-30

%

of E,, a small decrease of the quadrupole splitting at low temperatures might be observed. This could be the case in our results if we take 1 to be of the order of the free ion value

A,

= 130 cm-l, as our data gives E, % 450 cm-I. The absolute value of the Q. S. at low temperatures seems somewhat low for a single state. However, this may also be explained by the influence of spin orbit interaction through the .reduction of the low tempetature quadrupole splitting associated with the relatively small ratio of E,/A [9].

3.2 THE Fe3 + SPECTRUM. -Below room temperature

the intensity of the Fe3+ spectrum was generally so small that no meaningful analysis could be carried out. At higher temperatures we tried fitting the spectra in several ways, none of which was completely satis- factory, but acceptable fits could be obtained by accept- ing large values of the linewidths. The I. S. at room temperature was found to be % 0.4 mm/s, a value normal for high spin ~ e ~ + . The average value of the Q. S. decreased approximately linearly with increasing temperature from w 0.5 mm/s at T = 300 K to % 0.2 mm/s at T = 900 K, again suggesting a signifi- cant temperature dependence of the positions of the 0'- ions coordinating the Fe3+ in this temperature region.

3.3 THE Fe3+/FeZf INTENSITY RATIO.

-

The

Fe3+/Fe2+ intensity ratio in the Mossbauer spectra was found to depend on temperature and also on the sample considered. In figure 4 the idealized temperature dependence as estimated from the Mossbauer results for the two Ti203 samples is shown. Two

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C6-610 C. BLAAUW, F. LEENHOUTS AND F. VAN DER WOUDE

3; " J

200 LOO 600 800

FIG. 4. - The idealized temperature dependence of the Fe3+/FeZC intensity ratio of 57Fe in Ti203 for sample 1 (lower

curve) and sample 2 (upper curve).

temperature regions may be distinguished, a low temperature region, in which a, possibly exponential, increase of the ratio is observed, and at high temperature a saturation region.

For a possible explanation of these results we first consider a stoichiometric, Fe doped, Tiz03 sample, i. e., one of composition Ti, +,Fe,O,. The fact that at low temperatures Fe2' is found in the spectra indi- cates that relative to a ~ i q - , ~ e P + 0 ; - description one electron has been transferred from the Ti 3d band to each Fe3+ ion. In this way a hole has been created which will be attracted to the extra negative charge associated with an Fez+ ion and may form a bound state with it. At elevated temperatures the extra electron on the Fez+ ion will recombine with the bound hole, thereby forming Fe3+. From the temperature dependence of the Fe3+/Fe2+ intensity ratio in the low temperature region it is found that the activation energy associated with this recombination is of the order of 0.02 eV. Here we have assumed that the recoilless Mossbauer fractions for ~e~ + and Fe3+ are similar. At higher temperatures the density of unoccupied electronic states in the vicinity of the Fermi level becomes important in determining the Fe3+/Fe2+ ratio. In stoichiometric material the number of bound holes will be equal to the number of Fe impurity atoms. If we disregard the population of bands at higher energies this means that as T -, co the Fe3+/Fe2+ intensity ratio will approach unity.

MORIN, F. J., Phys. Rev. Lett. 3 (1959) 34.

HONIG, J. M. and REED, T. B., Phys. Rev. 174 (1968) 1020. VAN ZANDT, L. L., HONIG, J. M. and GOODENOUGH, J. B.,

J. Appl. Phys. 39 (1968) 594.

[4] ZEIGER, H. J., KAPLAN, T. A. and RACCAH, P. M., Phys.

Rev. Lett. 26 (1971) 1328.

[S] VAN ZANDT, L. L. and EKLUND, P. C., Phys. Rev. B 7 (1973)

1454.

This ratio is affected if the sample composition deviates from stoichiometry. For instance, it was observed during Mossbauer experiments that when our samples were heated to temperatures T

2

900 K, metallic iron was formed but simultaneously the Fe3+/Fe2+ intensity ratio increased strongly, e. g. in sample 2, from 0.6 at T = 870 K to 1.1 at T = 950 K. This phenomenon can be explained if one considers what happens when deviations from stoichiometry occur. Increasing the oxygen content, for instance, will create more holes in the Ti 3d band .This increases the number of states available for Fez+ donor electrons and will lead to an increase in the Fe3 + /Fez+ intensity ratio at high temperatures. Similarly a decrease of oxygen content will reduce the number of empty states available for the Fez+ donor electrons and decrease this ratio.

4. ConcIusions.

-

An important result of these experiments is that no evidence is found for antiferro- magnetic ordering in Ti203. This conclusion is in agreement with recent views on Ti203, where it is now considered that the temperature dependent metal- nonmetal transition in this material is related to the gradual disappearance of the gap between two bands of mostly 3d character instead of a picture in which the transition is related to a doubling of the unit cell upon the appearance of antiferromagnetism below the transition temperature.

Our experiments do not give information about the transition directly. The experimental results are explained reasonably we11 using a localized description of the Fe impurity ions. The temperature dependence of the Fe3'/Fe2+ intensity ratio is explained, ,assuming that bound states are formed by ~ e ions and ~ + holes in the Ti 3d band.

Acknowledgement.

-

This work was performed at the University of Groningen as part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (Foundation for Fundamental Research on Matter - F. 0. M.) and was made possible by financial support from the Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek (Netherlands Organization for the Advancement of Pure Research -

z.

w.

0.).

aces

[6] SHIRANE, G., PICKART, S. J. and NEWNHAM, R., J. Phys.

Chem. Solids 13 (1960) 166.

171 RAO, C. N. R., LOEHMAN, R. E. and HONIG, J. M., Phys.

Lett. 27A (1968) 271.

[8] WATSON, R. E., Techn. Rep. No. 12, Solid State and Mol. Theory Group, MIT (1959).