Mössbauer spectroscopic and direct current electrical conductivity study of some iron (II) dicarboxylates

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Journal of Magnetism and Magnetic Materials 131 (1994) 189-198

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M6ssbauer spectroscopic and direct current electrical conductivity study of some iron (II) dicarboxylates A . K . N i k u m b h *, M . M . P h a d k e , A . A . L a t k a r Department of Chemistry, Universityof Poona, Ganeshkhind, Pune 411 007, India (Received 29 June 1993)

Abstract

The thermal decomposition of ferrous succinate tetrahydrate, FeC4H404.4H20 and ferrous malate dihydrate FeC4H405 • 2H20 have been studied by two probe direct current electrical conductivity measurements under the atmospheres of static air and dynamic dry nitrogen. The products at each decomposition stage were determined by M6ssbauer spectroscopy, infrared spectroscopy, X-ray powder diffraction and gas chromatography. The change in the oxidation state of the iron ion is dearly followed through the course of decomposition via the changes in the isomer shift and quadrupole splittings. The formation of intermediates such as FeO, Fe304, ~/-Fe203 and ot-Fe203 has also been discussed and the results of the M6ssbauer measurements have been correlated with the thermal analysis of these dicarboxylates.

1. Introduction

The thermal decomposition of metal carboxylate has become a subject of recent interest due to their wide use as medicinal agents and antiseptics. Thermal decomposition studies of iron (II) carboxylates have become important research projects in the preparation of gamma ferric oxide ('~-Fe203) due to its application in magnetic recording tapes [1,2] and ferrite components. The usual method of preparation of v-Fe203 is by reduction of tx-Fe203 (obtained by dehydration of ct-FeOOH) to Fe304 and reoxidation to ~/Fe203 [3]. There are reports on the preparation of v-Fe203 from iron (II) carboxylate by using an iron (II) oxalate dihydrate [4-7]. Another method

* Corresponding author.

for the synthesis of ~/-Fe203 from thermal decomposition of ferrous oxalate dihydrate using direct current electrical conductivity for characterization has been reported [8,9]. Recently the synthesis of ~/-Fe203 by thermal decomposition of iron (II) dicarboxylates have been reported [10-141. There is a great lack o f studies of iron (II) carboxylates except for iron (II) oxalate dihydrate [6,15-18]. It was indicated that the decomposition process of iron (II) oxalate is very complicated, mainly due to the variable valence of iron. Interest has recently been shown in the use of M6ssbauer spectroscopic techniques in the study of solid-state decomposition of iron (II) formate, malonate, fumarate and tartarate [19-23]. The present work was undertaken (a) to investigate the formation of iron-oxides during the thermal decomposition of ferrous succinate tetrahydrate

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190

A.K. Nikumbh et al. /Journal of Magnetism and Magnetic Materials 131 (1994) 189-198

(EeC4H404.4H20) and ferrous malate dihydrate ( F e C 4 H 4 0 5 • 2H20) and (b) to obtain some

information about the structural properties of iron-oxides formed. The phase analysis and structural characterization were performed using STFe M6ssbauer spectroscopy, dc electrical conductivity measurements and X-ray diffraction studies•

2. Exl~Hmentai Ferrous succinate tetrahydrate ( F e C 4 H 4 0 4 • 4H20) and ferrous malate dihydrate ( F e C 4 n 4 0 5 • 2H20) were prepared using the methods described previously [11,14]. Elemental analyses were made in wt% for F e C 4 H 4 0 4 . 4 H 2 0 (C, 19.55 (19.68); H, 1.58 (1.64); Fe, 23.1 (22.89)) and for FeC4H405 • 2 H 2 0 (C, 21.5 (21.4); H, 3.60 (3.57); Fe, 25.04 (24.94)), where the values in parentheses are calculated ones. The IR spectra showed frequencies corresponding to the carboxylate group, hydroxyl group, metal oxygen etc.

-

The bidentate linkage of the succinate or malate group with the metal was confirmed on the basis of the difference between the antisymmetric and symmetric stretching frequencies [24]. The X-ray diffraction pattern showed that the sample was polycrystalline in nature and comparable with data reported for succinate [11] and malate [14]. The presence of water of crystallization was confirmed on the basis of the thermal analyses curves. The compounds F e C 4 H 4 0 4 " 4 H 2 0 and FeC 4 H a O s . 2 H 2 0 have magnetic moments 4.90 and 5.04 /z B respectively, which indicates that the compounds have free spins with sp3d 2 hybridization. The procedures used for measurements of direct current electrical conductivity, infra-red spectra, X-ray powder diffraction and gas liquid chromatography were similar to those reported earlier [8,11,14]. For the Mfssbauer study, the samples were heated isothermally in a silica boat at different temperatures for one hour in a controlled tern-

Temperature,T ('C)

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A.K. Nikumbh et al. /Journal of Magnetism and Magnetic Materials 131 (1994) 189-198

perature furnace under an atmosphere of static air and dynamic dry nitrogen. These isothermal samples were kept in a vacuum desiccator and removed at the time of measurement. The constant acceleration M6ssbauer spectrometer (Austin Science S-600) was used to record the M6ssbauer data in conjunction with 10 mCi 57Co in a rhodium source. The spectrometer was calibrated using a natural iron foil. All the spectra were recorded at 298 + 2 K. A sample containing approximately 10 mg cm -2 of natural iron was taken for each measurement. The values of isomer shift have been reported with respect to natural iron. The uncertainties in isomer shift, quadrupole splitting and internal magnetic field values are + 0.02 mms- 1, + 0.03 mms- 1 and + 5 kOe respectively.

3. Results and discussion 3.1. Thermal decomposition processes o f ferrous succinate tetrahydrate

The plot of log o- vs. T- 1 in Fig. l(a) exhibited two peaks (region B' and B") for the dehydration step. The isothermally heated sample of FeC 4 H 4 0 4 " 4H20 under static air at 180°C showed no H - O H bands in the IR spectrum, the X-ray diffraction pattern showed polycrystallinity of the sample with a decrease in interplanar spacing. The C and H analysis of this isothermally heated sample agreed well with the formula FeC4H40 4 (anhydrous ferrous succinate). After the dehydration step the cr value steadily increased from 215 to 2820C (region C). The IR spectrum of the isothermally heated sample of FeC4H40 4 • 4H20 at 260°C showed a decrease in the intensities of the coordinated carboxylate bands; in addition bands at 290 cm-1 (s) and 360 cm-1 (m) occurred for metal-oxygen stretching frequencies due to the presence of iron oxide [25]. The X-ray diffraction pattern of this isothermally heated sample was generally broad. Nevertheless the peaks corresponding to both FeO and FeCaH40 4 were observed. Although a sharp increase in o, was observed at 285°C (region D) the characteristic high value

191

of Fe304 (3.0 ll-1 cm-1) could not be obtained under dynamic conditions. However, the X-ray diffraction studies confirmed that mainly Fe304 was formed at this temperature. The intermediate obtained in region E (350-430°C) was mainly ~-Fe203 with traces of Fe304. The X-ray powder pattern was generally broad showing a peak corresponding to ~/-Fe203 and traces of Fe304. The IR spectrum of FeC4H404.4H20 heated at 400°C, showed no bands corresponding to coordinated carboxylate bands, but strong, broad bands of F e - O stretching frequencies were observed. This part of the curve is followed by region F (i.e., above 445°C) corresponding to the complete reversible phase transformation of ~/-Fe203 to ~-Fe203. A comparison of conventional thermal analysis [11] in static air with conductivity analysis in static air of FeC4H404 • 4H20 shows that the conductivity analysis gives a much more detailed view of the decomposition process. TGA showed two regions of weight loss, one upto 110°C corresponding to loss of two water molecules, and the other upto 215°C corresponding to further loss of two water molecules. DTA and DTG gives two endothermic peaks, one at 94°C and other at 180°C corresponding to complete dehydration, a broad exothermic peak at 293°C corresponding to oxidative decomposition of FeC4H404 to the crystalline a-Fe20 3. The M6ssbauer spectra of the original sample and product at each decomposition stage are shown in Fig. 2 and the spectral parameters are summarized in Table 1. The M6ssbauer spectrum of FeC4H404 • 4H20 ((a) in Fig. 2) gave two absorptions with equal width and intensity, indicating that Fe 2+ is in one ion site, probably in an octahedral environment. The isomer shift and quadrupole splitting at room temperature are respectively 1.28 and 2.42 mms- 1. The values of the isomer shift and quadrupole splitting were in good agreement with those in the literature [16]. FeC4H404" 4H20 is therefore, a spin-free compound with some interaction between the Fe 2+ ions and the asymmetrically arranged succinate groups. The M6ssbauer spectrum of anhydrous succinate ((c) in Fig. 2) showed two absorptions, but they were broad and had different widths.

192

A.K. Nikumbh et al. ~Journal of Magnetism and Magnetic Materials 131 (1994) 189-198

The broadening reflects an unstable environment around Fe 2+, which is consistent with the fact that the anhydrous suecinate was in the amorphous state. It should be noted here that a sample of FeCaH404 stored in a desiccator for more than seven days shows a similar M6ssbauer spectrum. These observations indicate that a slow oxidation of the anhydrous material could not be detected. The spectrum of the dihydrate ((b) in Fig. 2) was almost the superpositions of those of the original and anhydrous succinate. This may be an indication that the dihydrate was a mixture of both compounds. The Mfssbauer spectrum of a sample of FeC4H404.4H20 heated isothermally at 260°C ((d) in Fig. 2) showed two asymmetric peaks sepa' rated by a distance of 0.73 mms-1 and the more acute spectrum indicated an apparent Fe 2+ quadrupole doublet. In addition to this a M6ssbauer peak at 2.55 mms -1 indicates the presence of small amount of FeC4H404. The values of isomer shift and quadrupole splitting were near the published data of FeO [26]. It has been observed in the literature [26] that the appearance of an asymmetric peak height led to the identification of the major peak as arising from quadrupole split Fe 2+ and the distortion zero velocity as arising from a small amount of Fe 3÷.

The detail shape and temperature dependence of the M6ssbauer spectra of FeO have been described [26]. However, the present data may not be very accurate because of the instrumental difficulties. We therefore conclude that, although decomposition occurred, the thermal energy at this temperature was much lower than the activation energy required for migration of the active material into the lattice sites of Fe20 3 (even at 275°C). The appearance of a central peak at 0.00 mms-1 is due to oxidation of some of the atoms on the surface of the solid to the tervalent state, so that a spin free F e 3+ M6ssbauer spectra appears. Here we tentatively assume that the sample obtained at 2600C, is a mixture of FeO, FeC4H404 and a small amount of Fe 3+ is also formed. The X-ray diffraction pattern and IR spectrum showed the formation of FeO with traces of FeC4H404. For the FeC4H404"4H20 sample heated isothermally at 320°C, the M6ssbauer spectrum consisted of several peaks ((e) in Fig. 2), in that, in addition to the doublet due to low-spin Fe 2+, the emergence of a sextet has been observed. This behaviour marks the beginning of the formation of an oxide of iron. The isomer-shift, quadrupole splitting and an internal magnetic field value of 0.50 mms-1, 0.95 mms -1 and 483

Table 1 M6ssbauer parameter " of FeC4H404 • 4H20 and its product at each decomposition stage. The M6ssbauer measurement were carried out at room temperature (25*(2) Atmosphere

Temperature ranges (*C)

Region

Predicted products

Isomer shift [ + 0.02 rams- i]

Quadrupole splitting

Internal magnetic field [ 5:5.0 kOe]

[+0.03 rams-~] Static air

Dynamic dry nitrogen

25 50-110 110-215 215-282 282-350 350-430 > 450

A B' B" C D E F

FeC4H404 • 4H20 FeC4H404 - 2H20 FeC4H404 FeO d- FeC4H404 Fe304 ~-Fe20 3 a-Fe20 3

1.28 1.20 1.35 0.65 0.50 0.28 0.35

2.42 2.49 2.91 0.51 0.95 0.24

483 495 515

25 50-110 110-210 210-390 390-450

A B' B" C D

FeC4H404 • 4H20 F c C 4 H 4 0 4 • 2I-I20 FeC4H404 FeO + F e C 4 H 4 0 4 Fe304

1.25 1.19 1.39 0.68 0.47

2.30 2.41 2.88 0.64 0.89

485

a With respect to natural iron foil.

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kOe respectively have been calculated (Table 1). These values are in close agreement with the formation of Fe304 [27]. Samples which were heated at 4000C showed a simpler M6ssbauer spectrum of only six lines ((f) in Fig. 2) which were narrower than those of Fe304. The isomer shift and internal magnetic field were similar to those for ~/-Fe203 [28]. No quadrupole splitting was observed, which is consistent with the cubic

193

symmetry. Gamma ferric oxide (-y-Fe203) is a ferrimagnetic compound with a spinel structure, in which iron (III) cations occupy both the tetrahedral (A) and octahedral (B) sites, and is usually assumed to have a collinear magnetic structure consisting of two sublattices. In the case of the sample, FeC4H404 • 4H20, heated isothermally at 450°C, the M6ssbauer spectrum showed a six-line pattern due to magnetic hyperfine splitting ((g) in Fig. 2) with the isomer shift, quadrupole splitting and internal magnetic field values of 0.35 rams-l, 0.24 mms-1 and 515 kOe, respectively. These values are in good agreement with the formation of a-Fe20 3 [29,30], The crystal structure of ct-Fe20 3 has been reported to be a corundum type (~t-A1203). For ~t-Fe203, there is a closed packed oxygen lattice where in Fe 3+ cations occupy the octahedral sites [31]. The compound ot-Fe20 3 is a magnetically unusual complex, being antiferromagnetic at low temperature, then undergoing a transition above the so called Morrin temperature to a weak ferrimagnetic state, due to spin canting, before becoming paramagnetic at high temperatures [32]. When the reaction is carried out in an atmosphere of static air, the gaseous product acted as a gas buffer for the solid state reaction and some of the reaction is poorly defined. For example the role of water molecules in FeC4H404 • 4H20 and the role of atmospheric oxygen in the solid state reaction in static air could be clarified by comparing the different physical properties for the reaction carried out in a dynamic dry nitrogen atmosphere. Regions B' and B" in the plot of log tr vs. T -1 in Fig. l(b) correspond to the dehydration steps. The sample obtained by isothermally heating FeC4H404"4H20 in region C (330°C) showed that the IR bands corresponding to F e - O stretching frequencies become more intense, and that due to the coordinated carboxylate bonds decreased in intensity. The X-ray diffraction was very similar to that of the isothermally heated sample under static air at 2600C, indicating that the sample was a mixture of FeO and FeC4H404. A steep increase in tr has been observed in the region D corresponding to Fe304 formed as the final product. The sample thus obtained at 450°C

194

A.IC Nikumbh et aL ~Journal of Magnetism and Magnetic Materials 131 (1994) 189-198

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obtained by heating FeC4H404-4H20 at 450°C in dry nitrogen atmosphere. shows a negligible variation in tr with variation of temperature. X-ray diffraction analysis has confirmed the formation of this phase. No line which

can be assigned to metallic iron could be detected. The DTA, D T G and T G A of F e C 4 H 4 0 4 • 4 H 2 0 under dry nitrogen atmosphere have been reported [11]. The decomposition steps could be seen on D T A and D T G curve at 360°C. The T G A curve showed a continuous weight loss from 242°C until it crystallized to F e 3 0 4. The M6ssbauer spectrum of the samples F e C 4 H 4 0 4 • 4 H 2 0 heated isothermally at 90, 180 and 330°C under dry nitrogen atmosphere were quite similar to that of Fig. 2((a) to (d)) and the spectral parameters are summarized in Table 1. At 450°C the M6ssbauer spectra (Fig. 3) consists of two superimposed six line patterns due to Fe 3+ (A) and to Fe 2+'3+ (B). The relative line width of both of the pattern as well as the somewhat greater broadening of the outer lines are ascribed to impurity and vacancy effects• The excess broadening of the outermost lines of the B-site pattern may also be due to the fact that the electronic interchange of B sites is not extremely fast compared to the 57Fe Larmor frequency. Isomer shift, quadrupole splitting and hyperfine

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A.K. Nikumbh et al. /Journal of Magnetism and Magnetic Materials 131 (1994) 189-198

fields are in good agreement with the values reported by Kiindig [33].

3.2. Thermal decomposition processes of ferrous malate dihydrate Region B in the plot of log cr vs. T-1 in Fig. 4(a) corresponds to dehydration of F e C 4 H 4 0 5 • 2 H 2 0 under static atmosphere. The o- value steadily increased from 175 to 230°C (region C), and the IR spectrum of the isothermally heated FeC4H405 • 2 H 2 0 sample at 210°C showed a decrease in intensity of coordinated carboxylate bands, in addition, bands at 395 cm-1 (s) and 350 cm -1 (m) occurred for metal-oxygen stretching frequencies due to the presence of iron oxide [25]. The X-ray powder diffraction pattern of this isothermally heated sample showed the structure to be polycrystalline in nature, the peaks corresponding to both FeC4H405 and FeO were observed. Even though a tendency for a sharp increase in cr was observed at 235°C (region D), the characteristic high cr value of Fe304 could not be obtained under dynamic conditions and a change in slope was observed at 300°C (region E), probably due to the formation of semiconducting -yFe20 3. The X-ray powder diffraction pattern was generally broad, having peaks corresponding mainly to 7-Fe20 3 and traces of Fe304. This part

195

of the graph is followed by region F, i.e. above 420°C, corresponding to the complete transformation of ~/-Fe203 to ct-Fe20 3. Simultaneous TGA, DTA and D T G of F e C 4 H 4 0 5 - 2 H 2 0 under the atmospheres of static air and dynamic dry nitrogen have been reported earlier [14]. TGA shows a continuous mass loss between 60-180°C. DTA and DTG gives an endothermic peak at 140°C corresponding to the dehydration of the two water molecules, a broad exothermic peak at 208°C (hump at 255°C) corresponding to the oxidative decomposition of FeCaH405 to the crystalline (x-Fe203. A continuous weight loss was shown on the TGA curve in this region. The MSssbauer spectra of ferrous malate dihydrate and anhydrous FeC4H405 are shown in Fig. 5 and the spectral parameters are summarized in Table 2. The spectrum of F e C 4 H 4 0 5 • 2 H 2 0 ((a) in Fig. 5) gave two peaks with almost equivalent intensity, indicating that Fe 2+ is in one ion site, probably in an octahedral environment. The values of isomer shift and quadrupole splitting were in good agreement with those in the literature [19]. The M6ssbauer spectra of anhydrous FeC4H405 ((b) in Fig. 5) showed the larger quadrupole doublet, mainly attributing to the charge distribution in the Fe 2÷ (i.e. more symmetrical). It is well established that the con-

Table 2 M6ssbauer parameter a of FeC4H405 • 2H20 and its products at each decomposition stage. The M6ssbauer measurements were carried out at room temperature (25°(2) Atmosphere

Temperature ranges (°C)

Region

Predicted products

Isomer shift [ + 0.02 rams- 1]

Quadrupole splitting (+0.03 nuns -1)

Static air

25 40-175 175-230 230-295 295-350 350-425

A B C D E F

FcC4H405 • 2H20 FeC4H405 FeO + FeC4H40 5 Fe304 ~/-Fe20 3 ot-Fe20 3

1.19 1.32 0.71 0.45 0.26 0.31

2.37 2.95 0.60 0.86 0.20

486 493 518

25 40-160 160-250 250-440

A B C D

FeC4H405 • 2H20 FcC4H405 FeO + FeC4H405 Fe30 4

1.22 1.35 0.75 0.48

2.35 2.90 0.64 0.84

484

Dynamic dry nitrogen

a With respect to natural iron foil.

Internal magnetic field [ + 5.0 kOe]

196

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tribution to the electric field gradient around high-spin Fe(II) from the valence electrons and from surrounding charges are of opposite sign [34]. The Mfssbauer spectrum of the product at 210°C ((c) in Fig. 5) could be assigned to FeO on the basis of the conclusion for the ferrous succihate. The Mfssbauer spectrum of the product at 265°C ((d) in Fig. 5) is somewhat broad and split, suggesting that the product is a mixture of several compounds. The values of isomer shift, quad-

rupole splitting and internal magnetic field were near the published data of Fe304 [27]. The Mfssbauer spectrum of FeC4H405 • 2 H 2 0 heated at 325°C represents a six-line pattern ((e) in Fig. 5) with isomer shift and internal magnetic field, indicating the formation of 7Fe20 3 [28]. In the case of sample FeC4H405 • 2 H 2 0 , heated isothermally at 425"C, the M6ssbauer spectrum showed a six-line pattern due to magnetic hyperfine splitting ((f) in Fig. 5) with the isomer shift, quadrupole splitting and the internal magnetic field. These values are in good agreement with the formation of a-Fe20 3 [291. The plot of log cr vs. T- 1 in Fig. 4(b) showed a clear peak at 143°C (region B) corresponding to the dehydration of FeC4H405 • 2H20 under dry nitrogen atmosphere. This curve clearly showed the different intermediate phases which occurred during decomposition. Here the cooling curve was also recorded, to test the purity of Fe304 formed. The IR spectrum and X-ray diffraction pattern for sample heated isothermally at 440°C showed for pure Fe304. No line which could be assigned to metallic iron could be detected. The M6ssbauer spectra of the same FeC4H4Os.2H2 O heated isothermally at 140, 210 and 440°C under dry nitrogen atmosphere were quite similar to that Fig. 5(b) and (c) and Fig. 3 respectively. The isomer shift, quadrupole splitting and internal magnetic field values are given in Table 2 and are in good agreement with the reported value for Fe304 [33]. The gaseous products obtained by thermal decomposition of FeC4H404 -4HzO under a dynamic pure and dry nitrogen atmosphere was indicated by the gas chromatogram shown in Fig. 6. These chromatogram showed the presence of both types of gases, i.e. polar (viz. H 2, CO 2, etc.) and non-polar gases (viz. C2Hz, C2H4,etc.). The gases were collected at around 350°C. A similar gas chromatogram was obtained by thermal decomposition of F e C 4 H 4 O s . 2 H 2 0 under dry nitrogen atmosphere. The different paths followed by the decomposition of F e C 4 H 4 0 4 . 4 H 2 0 in different atmospheres showed complete dehydration, as was seen from conductivity measurement, Mfissbauer spec-

A.I£ Nikumbh et al. /Journal of Magnetism and Magnetic Materials 131 (1994) 189-198

t

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>Fe304

+ 2 C 2 H 4 1" + 4CO1` + [O];

tra and IR spectrum. A transformation of FeC4H404 to FeO was also detected in static and dry nitrogen atmosphere. A separate phase of FeO could be obtained; this compound always occured with FeC4H404. Thus the transformation of FeO and FeC4H404 seems to be an equilibrium reaction. This mixture of FeO and FeC4H404 is then transformed to Fe304, which is the final product obtained in dry nitrogen atmosphere. F e C 4 H 4 0 4 • 4 H 2 0 50-110°C > F e C 4 H 4 0 4 2 H 2 0

+ 2 H 2 0 1`;

(1) 110-210*C)FeC4H404

+ 2H201" ;

(2)

F e C 4 H 4 0 4 21°-39°*C~FeO + C 2 H 41' -~- C O 1' + C O 2 1' ;

(3)

2 F e O + F e C 4 H 4 0 4 39°-45°°c ~Fe304 + C E H 4 1" + 2 C O $.

(4)

The decomposition paths of FeC4H404 • 4 H 2 0 in static air was found to be similar except the step for formation of the intermediate v-Fe20 a. F e C 4 H 4 0 4 • 4 H 2 0 50-110°C ) FeC 4H 4042H20 + 2H2OT ;

(5)

F e C 4 H 4 0 4 • 2 H 2 0 110-215°C ) F e C 4 H 404 + 2H2OT ;

282-3500C

(8)

Time ----*,-

Fig. 6. Gas-liquid chromatograms for gases obtained during the thermal decomposition of FeC4H404'4H20 or FeC4H4Os.2H20 under dynamic nitrogen atmosphere.

FeC4H404" 2H20

197

(6)

2Fe304 + [O]

350-430"C

~3'v-Fe203;

~/.Fe203 > 450°c ~ot.Fe203.

(9) (10)

A similar reaction path of F e C 4 H 4 0 5 " 2 H 2 0 were observed in static air and dry nitrogen atmosphere.

4. Conclusions

The results of the present study allow us to make the following important observations regarding the solid state decomposition of FeC 4 H 4 0 4 • 4 H 2 0 and F e C 4 H 4 0 5 • 2H20. (a) The dehydration of FeC4H404 • 4 H 2 0 and F e C 4 H 4 O s - 2 H 2 0 , yielding anhydrous FeC4H 4 04 or FeC4H405, took place in static air and dynamic dry nitrogen atmosphere considered. (b) The oxidative decomposition behaviour of F e C 4 H 4 0 4 " 4 H 2 0 or F e C 4 H 4 0 5 - 2 H 2 0 was better understood from the study of dc electrical conductivity measurements, which showed different regions of conductivity for the intermediates formed, whereas the oxidative decomposition behaviour could not be dearly understood from the thermal curves. (c) M6ssbauer spectrum of FeC4H404 • 4 H 2 0 or F e C 4 H 4 0 5 . 2 H 2 0 gave two absorptions with equal width and intensity, indicating that Fe E+ is in one ion site, probably in an octahedral environment. (d) The M6ssbauer spectrum of anhydrous succinate and malate showed the larger quadrupole doublet, mainly attributable to the charge distribution in the Fe 2+. (e) M6ssbauer spectra of isothermally heated samples of F e C 4 H 4 0 4 . 4 H 2 0 and FeC4H405. 2 H 2 0 under static air and dynamic dry nitrogen

198

A.I£ Nikumbh et al. /Journal of Magnetism and Magnetic Materials 131 (1994) 189-198

were comparable, except the final product obtained in nitrogen atmosphere w a s F e 3 0 4. (f) The gas chromatograms showed that both polar and nonpolar gases were obtained during the thermal decomposition.

[12]

[13] [14]

5. Acknowledgements

The authors are grateful to Head, Department of Chemistry, University of Poona, Pune 411 007, for his interest and encouragement. They also thank Dr. S.K. Date and Dr. P.P. Bakare, Physical Chemistry Division, National Chemical Laboratory, Pune, India, for the facilities given for part of the work.

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