The characterization of Co xZn7-xSb2O12 spinel obtained by the pechini method

The characterization of Co_xZn_7-xSb₂O₁₂ spinel obtained by the pechini method

Deiby Santos Gouveia^I,*; Raul Rosenhaim^II; Severino Jackson Guedes Lima^III; Elson Longo^IV; Antonio Gouveia de Souza^II; Iêda Maria Garcia dos Santos^II

^IIPEN, Instituto de Pesquisas Energéticas e Nucleares, IPEN/CNEN-SP, Brazil

^IILTM, Departamento de Química, CCEN, Campus I, Universidade Federal da Paraíba, 58059-900 João Pessoa - PB, Brazil

^IIILSR, Departamento de Tecnologia Mecânica, CT, Universidade Federal da Paraíba, 58059-900 João Pessoa - PB, Brazil

^IVCMDMC, Departamento de Química, Universidade Federal de São Carlos, Rod. Washington Luís, Km 235, 13565-905 São Carlos - SP, Brazil

ABSTRACT

Oxides with spinel structures have been studied for many decades as they have been used in a variety of applications like pigments and refractories. They have also been used as a model structure to evaluate the relative stability of ions in octahedral and tetrahedral sites. Zinc- antimony spinels (Zn₇Sb₂O₁₂) were synthesized by the Pechini method and the cation Zn²⁺ was substituted by Co²⁺, taking into consideration the stoichiometry of Co_xZn_7-xSb₂O₁₂ (x = 0 - 7). Characterization of the thermal properties of pyrolyzed powders was carried out with a TG/DTA analyzer, and mass losses were determined as a function of the cobalt content in the resin. The powders were calcined at temperatures in the range 600 to 1000 °C and characterized by XRD, BET and IR spectroscopy. Maximum cristalinity was obtained in powders calcined at 1000 °C. Materials with x = 5 - 7 revealed the presence of a secondary phase (Co, Zn)Sb₂O₆. Infrared analysis facilitated in the determination of the possible sites of Zn²⁺, Co²⁺ and Sb²⁺ ions in the lattice.

Keywords: spinel, Zn₇Sb₂O₁₂, pechini, cobalt

1. Introduction

Increase in the number of compounds with spinel structures and their technological importance has lead to increasing interest in the crystallographic and chemical properties of these binary compounds¹. Cation distribution in these compounds has been successfully explained, considering its preference for tetrahedral or octahedral sites. In inverse spinels (B)[AB]O₄, the divalent cations and half the trivalent cations are in octahedral sites, while the other half of the trivalent cations are in tetrahedral sites. These structures have some properties that has permitted their use in specific areas such as ceramics. A good example is MgAl₂O₄, which has high thermal stability, and is used as a refractory material.

In this investigation, the inverse spinel Co_xZn_7-xSb₂O₁₂ was studied. Substitution of Zn²⁺ by Co²⁺ is possible, leading to the formation of a substitutional solid solution between the two elements. This occurs as both the cations have the same oxidation state and the difference between their ionic radius is ~ 2.7%. This spinel has been used in important applications in the electronic industry, and is a secondary phase in ZnO varistors.

Conventional methods to obtain ceramic materials are not appropriate for obtaining many advanced ceramics, due to reduced reactivity, limited reproducibility, heterogeneities and poor control of stoichiometry. Better results can be obtained by using chemical synthesis routes, which render good stoichiometric precision, powder morphology control and phase purity².

A chemical synthesis method based on metal chelation was developed by Pechini³. In this method a polymeric chain is formed from a reaction between polyalcohol and a polycarboxylic acid with metallic cations distributed homogeneously along the chain. Heating of the polymeric resin above 300 °C leads to polymer breakage and an further heat treatments to formation of a fine oxide powder.

In this investigation, the spinel phase Zn₇Sb₂O₁₂ was synthesized, substituting Zn²⁺ with Co²⁺, taking into consideration the stoichiometry of Co_xZn_7-xSb₂O₁₂ (x = 0 to 7). The Pechini method of synthesis was used.

2. Experimental Procedure

The precursors used were citric acid (Vetec), ethylene glycol (Reagen), antimony oxide (Vetec), cobalt acetate (Vetec) and zinc acetate (Vetec).

An aqueous solution of citric acid was diluted in ethylene glycol (in the proportion, 60/40 wt/wt)^2,5 under agitation and heated at 50 - 60 °C. After complete dissolution, antimony oxide was added along with some drops of nitric acid to aid dissolution. Then zinc acetate and/or cobalt nitrate was added to the solution in stoichiometric amounts. After dissolution, the temperature was increased to 90 - 110 °C, leading to.

Primary calcination of the resin was done at 350 °C for one hour. After de-agglomeration (200 mesh), the material was analyzed using simultaneous thermogravimetric and differential thermal analysis (TA-Instruments – SDT 2960), with a heating rate of 10 °C.min^-1, in a current of air.

The material was then calcined at temperatures in the range 600 to 1000 °C, for one hour, at heating rate of 10 °C.min^-1. Powders were characterized using X-ray diffraction analysis (Siemens – D 5000), infrared spectroscopy (Bomem – MB – 102, for the range 800 to 400 cm^-1, using KBr pellets; and Bomem model FT – DA8, for the range 400 to 200 cm^-1, using) and BET surface area analysis (Micromeritics – ASAP 2000).

To determine crystallite size and lattice parameter, X-ray diffraction peaks were fitted using a pseudo-Voigt function. Lattice parameters were measured using the Win-Index and Win-Metric programs provided by SIEMENS.

Relative crystallinity was calculated using diffraction peak intensities, according to Equation 1.

where: I₁₀₀ is the intensity of the most intense peak of the most crystalline sample; I₀ is the intensity of the most intense peak of the least crystalline sample and l is the intensity of the most intense peak of other samples.

Crystallite size (t) was measured using the most intense diffraction peak of the plane (2 2 2), according to Scherrer equation (Equation 2).

where: t is the crystallite size, l is the wavelength (Cu Ka₁), q is the diffraction angle and b is equal to:

where: B_obs is the full width to half maximum of the diffraction peak of the sample and b is the full width to half maximum of the diffraction peak of the standard (quartz).

3. Results and Discussion

Thermogravimetric curves of the spinel powders (Co_xZn_7-XSb₂O₁₂, x = 0 - 7) are shown in Figure 1. Three thermal decomposition steps can be observed. The first step is due to loss of water and adsorbed gases. The second step is due to decomposition of organic material and the third step, above 800 °C for x > 3, is due to decomposition of carbonates or oxide. Two decomposition peaks were observed in the second step, with the smaller one at lower temperatures. This second decomposition peak was only observed in DTG curves (Table 1), probably due to the presence of organic chains with different sizes.

Upon substitution of zinc with cobalt, two different mass losses are observed in the second decomposition step. For x = 0 - 2, a high mass loss is obtained during pyrolisis and for x = 3 - 7, the mass loss is much smaller (Table 1).

A variation of 18% in the mass loss is observed between x = 2 and x = 3. This variation may be related to the amount of cobalt in the polymeric structure. Both cobalt and zinc present tetrahedral and octahedral coordination. Preference for tetrahedral sites by Zn²⁺ and for octahedral sites⁶ by Co²⁺ was observed⁶. A change in cation coordination may be altering the polymeric structure of the resin.

The variation in DTG peak temperature with cobalt content is similar to mass loss variation with cobalt content: in the absence of cobalt, this temperature is 497 °C and when no zinc is present, the temperature decreases to 343 °C. This suggests that the addition of cobalt causes a less stable polymeric structure to form, requiring less energy for pyrolisis. A discontinuity in the linearity of the curve is observed between x = 2 and x = 3 (Figure 2).

All samples show exothermic transitions in the DTA curves. The sample without cobalt (x = 0) presents a peak at 484 °C and the sample without zinc (x = 7) presents a peak at 339 °C (Table 1), indicating a decrease in temperature as cobalt is added to the resin. This result is in agreement with the thermogravimetric data (Figure 3).

In the presence of carbon, an additional peak is observed during thermal decomposition with increasing sample temperature. This leads to faster crystallization and to an increase in particle as well as crystallite size. As samples with different amounts of cobalt present different mass losses, an isothermal study was necessary to determine the ideal temperature and time for carbon reduction (Table 2). These samples were calcined prior to crystallization, at temperatures below 360 °C, to have the same amount of carbon as the sample with x = 7 (16%).

Formation of crystalline phases (Figures 4 and 5) was studied by X-ray diffraction using JCPDS index-cards 15-517 and 15-687.

All the samples containing zinc and cobalt revealed the presence of a solid solution. According to Kroger Vink7 notation, the substitution reaction can be represented by Equation 4.

Samples calcined at 600 °C were amorphous. Crystallization occurred at 700 °C, with the appearance of the spinel phase [(Co, Zn)_2.33Sb_0.67O₄], along with a small amount of ilmenite [(Co, Zn)Sb₂O₆], in samples with x = 5 and 7. At 800 °C, the samples were crystalline. The sample Zn₇Sb₂O₁₂ presented the highest degree of crystallinity at this temperature. At 900 and 1000 °C, a decrease in the degree of crystallinity was observed, due probably to decomposition of the Zn_2.33Sb_0.67O₄ phase to ZnO and Sb₂O₅ (Figure 4).

For samples with x = 1, 2, 3 and 4, only the (Co, Zn)_2.33Sb_0.67O₄ phase was observed at 900 and 1000 °C. The other samples (x = 5, 6 and 7) also indicated the presence of the secondary phase (Co, Zn)Sb₂O₆, identified by the JCPDS index-card 18-0403 (Figure 5). This phase was also observed when Nobre⁸ synthesized the spinel Zn₆CoSb₂O₁₂.

Using the lattice parameter results, graphs of unit cell volume as a function of calcination temperature and cobalt content were plotted (Figure 6a and 6b). According to the JCPDS index-card, the unit cell volumes of the phases Zn_2.33Sb_0.67O₄ and Co_2.33Sb_0.67O₄ are 632.73 and 622.84 ų, respectively.

When the amount of cobalt in the structure increased, the unit cell volume decreased linearly, as shown in Figure 6a. This behavior can be explained by considering the ionic radius of cobalt and zinc. The ionic radius of zinc (Zn2+: rIV= 0.074 e rVI = 0.089 nm) is higher than that of cobalt (Co2+: rIV = 0.071 e rVI = 0.088 nm), in both the octahedral and tetrahedral positions, favoring thus a decrease in unit cell volume, as the cobalt concentration increases. Therefore, the lattice contracts when cobalt substitutes zinc. Figure 6a also shows a small discontinuity in the curve when 3 < x < 4. This can be related to the entrance of cobalt in the tetrahedral site, as observed by Poleti⁹, Gama¹⁰ and Hie¹¹.

The unit cell volume as a function of calcination temperature was also determined (Figure 6b). Nobre synthesized Zn₇Sb₂O₁₂ and observed an increase in unit cell volume with increase in temperature. The unit cell volumes changed from 636.28 to 636.94 ų at 920 and 1020 °C for 1 hour, respectively. In this investigation, no change was observed in the samples without cobalt. Samples with x = 3, 5 and 7, showed an increase in unit cell volume above 900 °C – higher the amount of cobalt, greater the volume. This increase can be attributed to the entrance of cobalt in the tetrahedral site, leading to an increase in lattice parameter.

The relative crystallinity was calculated as a function of calcination temperature and amount of cobalt (Figure 7a and 7b). Substitution of cobalt by zinc (Figure 7a) lead to reduction in crystallinity in samples with x = 1 to 6. Samples with x = 0 and 7, revealed very low crystallinity. While zinc has a preference for tetrahedral sites¹², cobalt has a preference for octahedral sites. In samples with only zinc or only cobalt, the cations are forced to get into less favorable positions and this makes crystallization more difficult.

The discontinuity in curve linearity, observed in the unit cell volume versus cobalt content, is also observed in the plots of relative crystallinity as a function of cobalt content, at 3 < x < 4 (Figure 7a). This is probably due to the occupation of tetrahedral sites by Co²⁺, as explained before.

Increase in temperature results in higher crystallinity, as shown in Figure 7b. This is due to the elimination of defects, and is a consequence of higher diffusion rates. Relative crystallinity of the sample Zn₇Sb₂O₁₂ is higher at 800 °C, probably due to phase decomposition, as stated before. In the other samples (x = 3, 5 and 7), the highest crystallinity is observed at 1000 °C.

Variation of crystallite size as a function of cobalt content is presented in Figure 8a. In samples with x = 0 and 7, the crystallization process is more difficult, as shown earlier. This could lead to the formation fewer nuclei, which grow faster and result in bigger crystallites. When x = 3 and 4, Zn²⁺ and Co²⁺ are in positions of lower energy, making crystallization easier. In this case, more crystallites nucleate, grow slowly and result in smaller crystallites.

The difference in crystallite size can be also attributed to cation mobility, as cation coordination changes diffusion rate. Colinas and Aréan¹² observed that preference of Zn²⁺ for tetrahedral sites and preference of Ni²⁺ for octahedral sites makes cation migration more difficult, due to energy loss of the preferential site, when cation diffuses from a tetrahedral to an octahedral site. Behavior of Co²⁺ is similar to that of Ni²⁺.

On the basis of this theory, when x = 0, Zn²⁺ has higher mobility between sites and the crystallites are bigger. When 1 < x < 3, cobalt substitutes zinc in the octahedral site. With 3 < x < 4, cobalt also occupies the tetrahedral sites. At this point, cation migration becomes more difficult and crystallites are smaller. For x > 4, Co²⁺ substitution increases, leading to higher mobility between sites. At x = 7 (Co₇Sb₂O₁₂), it attains its highest mobility, leading to highest crystallite size. Ion mobility in this composition is higher than in the sample Zn₇Sb₂O₁₂, due to the smaller size of Co²⁺ and lower bonding energy.

Figure 8b shows the variation of crystallite size as a function of temperature. Crystallite size increases with temperature. An inflexion point can be observed at 900 °C. The same result was observed by Gama¹⁰, for the spinel Zn₃Ni₄Sb₂O₁₂. This inflexion point can be attributed to a change in mass transport mechanism.

The surface area results are presented in Figure 9. The surface area increases linearly with temperature (Figure 9a). Two different behaviors can be observed in terms of cobalt content (Figure 9b). When x = 0 - 3, a decrease in surface area is observed. At x = 4, an increase is observed, followed by a linear decrease between x = 4 and 7. When the number of crystallites per particle is evaluated, the behavior is just the opposite. This result indicates that particle size increase is due to sintering among particles.

Infrared spectroscopy results of samples with x = 0 to 7, calcined at 1000 °C, are shown in Table 3. Thermal evolution results are shown in Figures 10 and 11.

Data from literature were used to associate the four vibrational bands, characteristic of the spinel phase, to cobalt and zinc in octahedral and tetrahedral sites and to antimony in octahedral sites^13,14,15.

According to these data, n₁ vibration frequency can be related only to antimony in octahedral sites - [SbO₆]. According to Preudhome and Tarte¹⁶, if spinel contains different cations in the tetrahedral position, vibrational frequencies of the groups XO₄ and YO₄ are similar. In this case, splitting may occur¹⁷. Therefore n₂ e n₂^' are probably related to zinc and cobalt in tetrahedral sites – (ZnO₄) and (CoO₄). On the other hand, the cation position cannot be commented about, due to band enlargement. The low frequency band n₃ was related to [CoO₆], while n₄ was attributed to (ZnO₆) and (CoO₆)¹⁶. In the Zn₇Sb₂O₁₂ spectrum n₃ is not observed.

Figures 10 and 11 reveal the effect of calcination temperature. The bands tend to dislocate to lower frequencies, with increase in temperature. No absorption band was observed at 700 °C, indicating the disordered behavior of the material. This result is in agreement with XRD that indicated an amorphous material until 700 °C. With increase in temperature, the n₁, n₂ and n₄ bands appear, indicating the formation of polyhedral coordination. After calcination at 1000 °C, n₂' appears at 485 cm^-1, due to n₂ splitting (Figure 10).

The sample Co₇Sb₂O₁₂ (Figure 11) presented four bands that can be attributed to the spinel structure. These were in the regions: n₁ = 623 - 656 cm^-1, n₂ = 433 - 456 cm^-1, n₃ = 199 - 309 cm^-1 and n₄ = 162 - 187 cm^-1. Splitting of n₂^' indicates absorption in the range 485-504 cm^-1. Additional bands were observed at 653 cm^-1 and 555 cm^-1 for samples with x = 7, calcined at 600 and 700 °C. For samples calcined at 800 °C, another band was observed at 212 cm^–1. Lenglet and Lefez¹⁸, Djambazov et al.¹⁹ and Jun et al.²⁰ observed that Co₃O₄ absorbs in the regions near 210, 216, 560 and 660 cm^-1. Thus, the additional bands can be related to the presence of Co₃O₄, which was not observed by X-ray diffraction.

These bands disappear with increase in temperature, probably due to decomposition of Co₃O₄, as observed in the thermogravimetric results (Figure 1). Co₃O₄ formation probably delays the entrance of cobalt into tetrahedral sites. These results are in agreement with lattice parameter data, where an inflexion was observed at 900 °C, for samples with x > 3, associated with the entrance of cobalt into tetrahedral sites.

4. Conclusions

Synthesis of the spinel phase Zn_7-xSb₂O₁₂ (x = 0 - 7) was done using the Pechini method. All the samples containing zinc and cobalt presented a substitutional solid solution. The different analyses showed an inflexion point at 3 < x < 4, probably due to the entrance of cobalt into tetrahedral sites. Infrared spectroscopy results indicated the presence of Co₃O₄, which decomposes above 800 °C, and enters the spinel unit cell.

Acknowledgments

The authors acknowledge CAPES and CNPq for the financial support.

Received: November 23, 2003; Revised: December 28, 2004

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Publication Dates

• Publication in this collection
  06 Feb 2006
• Date of issue
  June 2005

History

• Accepted
  28 Dec 2004
• Received
  23 Nov 2003