ESR and Mössbauer studies of Bi2O3Fe2O3 glasses

Journal of Non-Crystalline Solids 109 (1989) 289-294 North-Holland, Amsterdam

289

ESR AND MOSSBAUER STUDIES OF Bi203-Fe203 GLASSES Katsuhisa TANAKA, Kanichi KAMIYA and Toshinobu YOKO

a

Department of Industrial Chemistry, Faculty of Engineering, Mie Unioersity, Tsu, Mie-ken 514, Japan

Setsuhisa TANABE, Kazuyuki HIRAO and Naohiro SOGA Department of Industrial Chemistry, Faculty of Engineerin~ Kyoto University, Sakyo-ku, Kyoto 606, Japan Received 17 November 1988 Revised manuscript received 2 March 1989

The local structure around iron ions in the Bi203-Fe203 glasses prepared by the twin-roller quenching method has been examined by means of ESR and Mrssbauer measurements. It has been observed that the isomer shift of iron ions increases monotonically with the content of Fe203. This behavior indicates that as the content of Fe203 increases, the covalency of Fe-O decreases. The effective g-value of the ESR spectra of Fe 3+ is slightly larger than 2.00 for all of the glasses. It is considered that this fact is attributable to the effect of the 6p orbital of Bi 3+ and the dipolar interactions between Fe 3+ ions. The distribution function of the electric field gradient around iron ions has been evaluated by fitting the calculated curve into the experimental MiSssbauer spectrum. The average value of the electric field gradient estimated from the distribution function increases with increasing the content of Fe203. Namely, the coordination sphere of 0 2 - around Fe 3÷ is more asymmetric in the glass with higher content of Fe203. From this result, it is considered that the magnitude of overlap between the 2p orbital of 0 2- and the 4s orbital of Fe 3+ decreases with increasing content of Fe203, which is consistent with the compositional dependence of the isomer shift.

I. Introduction Iron-based oxide glasses are interesting substances from a viewpoint of magnetism in amorphous solids. Most of the oxide glasses containing a large amount of iron oxide such as BaO-FeEOa-B203, LiEO-FeEO3-B203 and FeO-A1203-SIO2, show the behavior of spin glassor mictomagnetic transition [1-5]. The magnetic properties of these substances are inevitably related to both the local environment and the longrange arrangement of iron ions. For instance, it is known that the magnitude of the superexchange interaction among iron ions is determined by the degree of covalency of the Fe-O bond and the propagation of the spin correlation ultimately depends on the F e - O - F e bond angle. The valence a Present address: Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611, Japan. 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

state of iron ion also has an effect on the magnetic properties in a sense that the Fe E+ ion has a large magnetic anisotropy due to the strong spin-orbit interaction of the 3d orbital, while the magnetic anisotropy energy of the Fe 3+ ion is small because its orbital angular momentum is zero. From such a standpoint, the present authors have systematically studied the covalency of the Fe-O bond in several FeEOa-based binary oxide glasses recently [6]. Among the FeEO3-based glasses reported, the Bi203-Fe203 system is the most interesting because it has a wide glass-forming region and the two components of the basic system for the amorphous ZnO-Bi2Oa-Fe203 and CaO-Bi203Fe203which show ferromagnetic character [7,8]. In the present investigation, a further examination was carried out by means of the ESR and MiSssbauer measurements in order to obtain more detailed information about the local structure of iron ions in the binary BiEOa-Fe203 glasses. In

290

K. Tanaka et al. / E S R and M~)ssbauer studies of Bi203-Fe203 glasses

particular, the distribution of ligand field around iron ions and bonding characteristics of F e - O were discussed.

(a)

ul

LI

2. Experimental (b)

R e a g e n t - g r a d e B i 2 0 3 and F e 2 0 3 w e r e mixed in

nominal compositions of ( 1 0 0 - x)Bi203.-xFe203 (x = 5-70 mol%) and melted in a platinum crucible with an electric furnace or in an image furnace with a xenon lamp at 1000 to 1500 ° C in air. The melt was quenched by pouring onto a twin-roller made of stainless steel rotating at 3000 rpm. The specimens thus obtained were subjected to X-ray diffraction using Cu K a radiation in order to determine whether the specimen was amorphous or not. The glass samples were then subjected to ESR and Mtssbauer measurements• ESR measurements were carried out at room temperature using a Varian E-line spectrometer operating at the Xband frequency ( v = 9.5 G H z ) with a magnetic field modulation of 100 kHz. The magnetic modulation width and the time constant used were 8.0 G and 0.25 s, respectively. The Mtssbauer effect measurements were performed at room and liquid nitrogen temperatures using a 10 mCi 57Co in metallic rhodium as a ~,-ray source. The velocity calibration and the calculation of isomer shift w e r e - m a d e by using the spectrum of a-Fe foil obtained at room temperature.

20

3~0

4~0 5tO 60 2e (degree) Fig. 1. X-ray diffraction patterns of (a) 70Bi2Oy 30Fe203 and (b) 40Bi2Oy 60Fe203.

The absorption peak due to Fe 2+ was not observed. The spectra of the other glasses exhibited a similar appearance. The compositional dependence of the average value of the isomer shift obtained from the position of the doublet is shown in fig. 3. It is seen that the isomer shift tends to increase with increasing content of Fe203. This

•.

60Bi203.40Fe203

•

?

v

3. Results and discussion £

X-ray diffraction patterns obtained for 70Bi203 • 30Fe203 and 40Bi203- 60Fe203 are shown in fig. 1. Both patterns exhibit the characteristics of the amorphous state. The X-ray diffraction measurements indicated that the specimens with Fe203 content of 10 to 60 tool% were amorphous. This result is consistent with that reported by Ota et al. [7]. Figure 2 shows the Mtssbauer spectra of 60Bi203 • 40Fe20 ~ and 40Bi 203 • 60Fe203 glasses. The spectra consisted of one symmetric doublet which is attributable to the absorption by Fe 3 + at tetrahedral site, judging from its isomer shift [9].

tt~

....

~':~:'~;;~!).~!,~i~-~...~-~.,:..~. . . . . .

I,--

"

;.4

.-~.o.,~:,:~.~.:.;,:-

; 40Bi2C~. 60Fe203

.!. .i 7 I

-10

-5

I

I

0 5 10 Vetocit y (ram/s) Fig. 2. Mt~ssbauer spectra of 60Bi2Oa.40Fe203 and 40Bi2Oy 60Fe203 glasses at room temperature.

291

K. Tanaka et aL / ESR and MiJssbauer studies of Bi203-FezO 3 glasses

0.5 "~ 0.4 EEO. '

°

u')

0.2

lb

2'0 3'0 4'0

5b

e'o

Fe203 content (mol°/o) Fig. 3. The variation of the isomer shift with the content of Fe203.

means that the covalency of the Fe3+-O bond decreases as the content of iron ion increases [10]. ESR spectra of 90Bi203 • 10Fe203, 60Bi203 • 40Fe203 and 40Bi203- 60Fe203 glasses are shown in fig. 4. The intensive spectral line centered at about g = 2.0 is clearly seen for these three specimens. In addition, a weak signal at about g = 4.3 is seen for only the glass with 10 tool% Fe203. According to the previous investigations [11-14], for the oxide glass with the Fe203 content larger

E 25

than about 3 tool%, the spectral lines centered at g = 4.3 and g = 2.0 are ascribed to isolated Fe 3+ in the orthorhombic crystal field and Fe3+-O Fe 3÷ spin pair, respectively. Moon et al. [11], carried out ESR and magnetization measurements on BaO. 4B203 glasses containing Fe203 and revealed that the decrease of the relative intensity of the signal at g = 4.3 with increasing the content of Fe203 above 3 mol% corresponds to the formation of antiferromagnetic spin pairs between Fe 3+ ion. The fact that the clusters of Fe 3+ are present in the glass with low content (3-6 mol%) of Fe203 was also demonstrated for the L i 2 0 - F e 2 0 3 - B : O 3 glasses by means of MiSssbauer measurements at 4.2 K [3]. It is likely that the same interpretation on the ESR spectra of the present Bi203-Fe203 glasses is reasonable and it is concluded that for the present glasses almost all of the Fe 3+ ions form spin pairs or clusters where dipolar and superexchange interactions between Fe 3÷ ions occur. The compositional dependence of the effective g-value obtained from the ESR spectra is shown in table 1. The effective g-value is larger than 2.00 for all of the glasses and the deviation from 2.00 increases monotonically with increasing content of iron ion. The value at 50 tool% Fe203 departs somewhat from such a tendency. A similar departure is also observed for the case of the isomer shift as seen from fig. 3. These facts may indicate that any structural change occurs at 50 tool% Fe203, although decisive evidence has not been obtained. The deviation of the effective g-value from 2.00 is partly ascribable to the contribution of orbital angular momentum to the magnetic moment of

c r"

Table 1 Compositional dependence of the effective g-value Composition (mol%)

I

1

I

1

I

2 3 4 Magnetic field ( k G )

I

5

Fig. 4. ESR spectra of (a) 90Bi2Oyl0Fe203; (b) 60Bi2Oy 40Fe203; (c) 40Bi203-60Fe203 glasses at room temperature.

Bi 203

Fe203

90 80 70 60 50 40

10 20 30 40 50 60

Effective g-value

2.02 2.03 2.03 2.04 2.09 2.08

292

-:.-..~~-.~-_-~-~

K. Tanaka et aL / ESR and MOssbauer studies of Bi203-Fe203 glasses

Fe 3+. According to Kittel [15] and Van Vleck [16], the fraction of the magnetic moment due to the orbital angular momentum, Is(orb ) , to that due to spin angular momentum, Is(spin ) , is expressed as follows: I s (orb)//~ (spin) = ½g - 1,

+ s

G,

..

.,

•

..

~.~.o.-,.%,.

(1)

where g denotes the g-value obtained by the microwave magnetic resonance absorption experiments. The fact is explainable in terms of the effect of Bi 3+ ions: contribution of the orbital angular momentum to the magnetic moment of Fe 3+ is not absent in the present Bi203-Fe203 glasses in spite of the zero value of orbital angular momentum quantum number of a free Fe 3+. It is known that the spin-orbit interaction of the 6p orbital of Bi 3+ is large, and this interaction has an influence on the electronic state of Fe 3+ through the 2p orbital of O 2-. According to Shinagawa and Taniguchi [17], the spin-orbit coupling constant of the 2p orbital of O 2- bonded to Bi 3+, ~2~, is expressed as follows:

=

"

Vetocity ( rnm/s )

Fig. 5. MiJssbauer spectrum of 60BizO3.40Fe203 glass at room temperature. The solid curve is the spectrum simulated by considering the distribution of the electric field gradient (see text).

bution of the electric field gradient (EFG) was estimated from the Mrssbauer spectra. According to Eibschtitz et al. [19], the spectral line is expressed by using the E F G distribution function, p ( [ V 1), as follows:

(2)

where ~2p and ~6p are the spin-orbit coupling constants of the 2p orbital of a free 0 2- and that of the 6p orbital of Bi 3+, respectively, and S is the overlap integral between the 2p orbital of 0 2- and the 6p orbital of Bi 3+. This equation indicates that the Bi 3+ ion makes the spin-orbit interaction of the 2p orbital of 0 2- bonded to Bi 3+ ion larger. As a result, the 2p orbital of 0 2- with a large spin-orbit coupling constant interacts with the 3d orbital of Fe 3+ bonded to this 0 2- ion, leading to the appearance of the orbital angular momentum which contributes to the magnetic moment of Fe 3+. The reason for the experimental fact that the deviation from 2.00 increases with increasing the content of Fe203 is not d e a r at the present time. It may be due to an increase of strength of the dipolar interactions between Fe 3+ ions. It is considered that the strong dipolar interaction, which is more predominant in the glass with a higher content of Fe203, causes a localized magnetic field at the site of Fe 3+ and increases the effective g-value [18]. In order to obtain further information about the local environment around iron ions, the distil-

-

w ~ P ( ! V[-) dV, V) z

(3)

w 2 + 4(z-

where w is the natural linewidth of the MiSssbauer spectrum, p( I V [) is expressed as follows [19]:

(exp{--[(VM--V)/XlVM]y' } (0< v<

p(V)=lexp{_[(V_ VM)/X2VM]Y2}

(4)

(vM< v< oo), where V M is thc E F G giving the peak of the distribution and xl, x2, yl and Y2 are positive parameters. The result of the parameter fitting for the 60Bi203 •40Fe203 glass is shown in fig. 5, where the solid curve was calculated by using eq. (3) on the basis of thc distribution function shown in fig. 6. The agreement between the calculated curve and ,the experimental spectrum was very excellent. Similar results were also obtained for other glasses. The average value of the E F G defined as oo

I v I---fo

IVI'p(IVI)dlVI,

(5)

was evaluated from the distribution function for

K. Tanaka et al. / ESR and Mrssbauer studies of Bi 203-Fe 203 glasses

293

40 Bi203. 60 Fe203 77K

tO

0

1

2

IVl/IVMl J

...,'

Fig. 6. The distribution functmn of the electric field gradient for 60Bi 203 •40Fe203 glass. I

-10

each glass and plotted against the content of Fe203 in fig. 7. In this figure, the value of eQI V I where e is the elementary electric charge and Q is the quadrupole moment of the 57Fe nucleus is plotted as the ordinate. It is seen that the average E F G increases with increasing content of Fe203, indicating that the FeO 4 tetrahedra become more asymmetric with an increase of Fe203. In such a glass, most of the Fe 3+ ions take part in the constitution of the random network, which may lead to the asymmetry of FeO 4 tetrahedra. Since it is considered that the Fe 3+ forms the d3s or sp 3 hybrid orbital, this asymmetry leads to less overlap between the 4s orbital of Fe 3÷ and the 2p orbital of 0 2- . Therefore, in the glass with higher content of Fe203 where the degree of asymmetry of FeO 4 is larger, the 4s electron density of Fe 3÷ is smaller. This inference is compatible with the 2.2 2.1 ~ 2.0 0

J>_-- 1.9 ~3

~18 1.7 10

3 40 50 Fe203 content ( mo[*l, )

60

Fig. 7. The variation of the average value of the electric field gradient with the content of Fe203.

-5

I

I

0 Velocity ( turn I s )

5

10

Fig. 8. MiSssbauer spectrum of 40Bi203 • 60Fe203 glass at 77 K.

compositional dependence of the isomer shift, that is the 4s electron density, mentioned above. A slight decrease of the average E F G at the composition of 60 mol% Fe203 may be attributed to the precipitation of microcrystals in the 40Bi203 • 60Fe203 glass although they were not detected by X-ray diffraction analysis. It is inferred that the more symmetric configuration of 0 2- around Fe 3÷ in the microcrystal makes the average E F G for the 40Bi203 - 60Fe203 glass smaller. The precipitation of microcrystals is demonstrated in the M~Sssbauer spectrum at 77 K as shown in fig. 8. The weak peaks, split due to the internal field, are clearly seen at about - 7 . 0 , - 3 . 9 , 4.9 and 7.8 m m / s in addition to the doublet in the central position. These additional peaks are ascribable to the ferrior antiferromagnetic microcrystals having a magnetic transition temperature lower than room temperature or showing the superparamagnetism at room temperature, as in the case of BiFeO 3, ctFe203 and Bi 2FenOgThe MiSssbauer spectra presented here clearly indicate that the binary Bi203-Fe203 glasses do not show a ferromagnetic character at room temperature. The hyperfine structure observed for the 40BiaO 3 - 60Fe203 glass at 77 K arises from an inhomogeneity such as superparamagnetism due to the precipitation of microcrystals rather than the cooperative magnetic transition over the ensemble of spins in the glass network. Hence, the

294

K. Tanaka et al. / ESR and Mi~ssbauer studies of Bi 20s-Fe 203 glasses

ferromagnetic character of the ZnO-Bi203-Fe203 and the CaO-Bi203-Fe203 glasses with Curie temperature higher than room temperature must be ascribed to the effect of Zn2+ and Ca 2÷ ions on the glass structure, especially the local structure around iron.

4. Conclusion

M/3ssbauer measurements on the binary Bi203-Fe203 glasses with 10 to 60 mol% Fe203 revealed that the isomer shift of Fe 3÷ increases monotonically with increasing the concentration of Fe203. This fact indicates that the covalency of the Fe3+-O bond decreases as the concentration of iron ion increases. On the other hand, from the ESR measurements, it was found that the effective g-value is slightly larger than 2.00 for all of the glasses and the deviation from 2.00 increases with increasing Fe203 content. The deviation from 2.00 is attributed to the large spin-orbit coupling constant of the 6p orbital of Bi 3÷ and the dipolar interactions between Fe 3+ ions. The distribution function of the electric field gradient around Fe 3÷ in the present glasses was estimated by analyzing the M/Sssbauer spectra. The average value of the electric field gradient calculated using the distribution function increases with the content of Fe203. In other words, the FeO4 tetrahedra are more asymmetric in the glass containing large amounts of Fe203. This compositional dependence of the average electric field gradient is compatible with that of the isomer shift. The present authors would like to thank Professor K. Sugiyama of the Electrical Engineering

Department of Mie University for the ESR measurements. They would also like to thank Dr. Y. Isozumi of the Radioisotope Research Center, Kyoto University, for the M~ssbauer measurements.

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