Synthesis and characterization of Li2ZnTi3O8 spinel using the modified polymeric precursor method

Materials Chemistry and Physics 82 (2003) 68–72

Synthesis and characterization of Li2 ZnTi3O8 spinel using the modified polymeric precursor method M.S.C. Cˆamara a , P.N. Lisboa-Filho a , M.D. Cabrelon a , L. Gama b , W.A. Ortiz c , C.O. Paiva-Santos d , E.R. Leite a , E. Longo a,∗ a

d

Departamento de Qu´ımica, Laboratório Interdisciplinar de Eletroqu´ımica e Cerˆamica, Universidade Federal de São Carlos, São Carlos, SP, Brazil b Departamento de Engenharia de Materiais, Laboratório de Cerˆ amica, Centro de Ciˆencias e Tecnologia, Universidade Federal da Para´ıba, Campina Grande, PB, Brazil c Grupo de Supercondutividade e Magnetismo, Departamento de F´ısica, Universidade Federal de São Carlos, São Carlos, SP, Brazil Laboratório Computacional de Análise Cristalográficas e Cristalinas, Instituto de Qu´ımica, Universidade Estadual Paulista, Araraquara, SP, Brazil Received 10 April 2002; received in revised form 29 July 2002; accepted 20 February 2003

Abstract In this work we report the synthesis procedure, crystallographic, structural and magnetic properties of the Li2 ZnTi3 O8 spinel obtained using a modified polymeric precursor method. This synthesis method generates very reactive and property-controlled nanoparticles. The samples were characterized using X-ray powder diffraction (XRD) associated to the Rietveld refinement method, thermogravimetric analysis (TG), specific surface area, scanning electron microscopy (SEM) and magnetic susceptibility measurements. The phase formation temperature of the lithium zinc titanate spinel was observed to decrease due to the homogeneity and highly controlled nanometric particle size. © 2003 Elsevier B.V. All rights reserved. Keywords: Ceramic; Electronic materials; Chemical synthesis; Crystal structure

1. Introduction In these last years there is an increasing interest in the study of spinel structured materials due to their rich catalytic, electrical and magnetic properties. Furthermore, these materials are also known to be very useful in the ceramic pigment industry [1–4]. The spinel structure, whose general formula is AB2 O4 , where A refers to cations in tetrahedral sites and B represents the cations in the octahedral positions is a cubic structure with space group of symmetry Fd3m [5]. This elaborate crystallographic structure, which can accommodate significant cation disorder, has a unique perspective for studies of substitutions and their relations with the chemical and physical properties [6,7]. In the spinel crystallographic structure two cation configurations are possible. The normal structure presents the A(B2 )O4 distribution and the inverse one the B(AB)O4 . In both cases the parentheses represent the octahedral site position [8–10]. In fact, there are many possible intermediary distributions represented by the for∗ Corresponding author. Tel.: +55-16-260-8214; fax: +55-16-260-8350. E-mail address: [email protected] (E. Longo).

0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0254-0584(03)00144-5

mula (Ai−1 Bi )[Ai B2−i ]O4 , where i denotes the inversion degree. In the system Li2 O–ZnO–TiO2 several spinel-like structures can be formed and the phase Li2 ZnTi3 O8 is one of the most complex being synthesized using conventional ceramic routes at temperatures of 1150 ◦ C [11]. Chemical preparation methods have been used to advantage over conventional ceramic processing methods to produce powders with highly controlled stoichiometry and morphology [12–14]. Chemical synthesis based on polyesters obtained from citrates was first developed by Pechini [15] and employs the dissolution of cation precursors in an aqueous a citric acid solution. After that, ethylene-glycol addition promotes metallic citrate polymerization. Due to the formation of a high viscosity polyester after polymerization, cation segregation during thermal decomposition is minimal [16]. This paper studies the Li2 ZnTi3 O8 phase formation using samples prepared by a modified polymeric precursor route based on the Pechini method. X-ray powder diffraction (XRD) associated to the Rietveld refinement procedure, thermal analysis (TG–DTA), nitrogen adsorption isotherms (BET), scanning electron microscopy (SEM) and magnetic

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measurements were used to study the synthesis and better characterize this phase and correlated samples.

2. Experimental procedure In the first step titanium citrate was prepared dissolving titanium(IV) isoproproxide Ti[OCH(CH3 )2 ]4 (Aldrich) in an aqueous solution, following the fixed citric acid/metal molar ratio of 3:1. After this procedure, thermogravimetric analysis (TG) was done in order to ascertain the Ti concentration in solution. At once, a citric acid aqueous solution was prepared using the same molar ratio of 3:1. Both solutions were kept at the temperature of 70 ◦ C, at which lithium carbonate (Li2 CO3 , Merck) and zinc acetate dihydrate (CH3 CO2 )2 ·2H2 O (Aldrich) salts were dissolved, one by one. Subsequently, ethylene glycol (HOCH2 CH2 OH, Merck) in mass ratio of 40:60 was added to the solution. At last, the temperature was gradually elevated to 100 ◦ C, in order to promote esterification and the formation of a polymeric resin. The pyrolysis of the resin was then completed in a heat treatment at 400 ◦ C for 1 h. After this the obtained powder was ground in a mortar and calcined in air at different temperatures from 500 to 1000 ◦ C for 4 h. The crystallographic phases were followed by XRD using a Siemens D5000. The data were collected in the range 5–100◦ (2θ), with step size = 0.02◦ and step time = 10 s. The Rietveld refinements were performed with the program SIZE2K, which is a locally modified version of the program DBWS-9807a [17], The Thompson–Cox–Hastings pseudo-Voigt function was used to fit the peak profiles and their Gauss and Lorentz FWHM were used to determine the crystallite size and microstrain of the spinel phase, according to the weighted size-strain method proposed by Paiva-Santos et al. [18]. The initial spinel crystal structure is cubic, with space group P43 32 [11]. The tetrahedral 8c sites are shared by the Li1+ and Zn2+ ions; Li1+ is in the octahedral 4b sites and Ti4+ is in the octahedral 12d sites. The occupancy of the cations at the 8c sites were refined and constrained to have their sum equal to 1.0. Thermal analysis was done in a TA Instruments unit using 10 mg of powder at a heating rate of 10 ◦ C min−1 in air flow. BET analysis was completed using an ASAP 2000 (Micromeritics). The scanning electron microscopy images of samples prepared at temperatures of 500 and 900 ◦ C were acquired with a Zeiss-DSM940A apparatus. For the magnetic measurements, a set of samples was prepared in pellet form and sintered in air at 1050 ◦ C for 8 h. Magnetic measurements were conducted in a Quantum Design MPMS-5 SQUID magnetometer, at temperatures (T) from 2 to 300 K, with an applied magnetic field H = 10 Oe. The equipment measures the magnetic moment, m, of the sample, which we divided by the applied field and

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by the sample mass to obtain the DC magnetic susceptibility, ␹. These measurements as a function of the temperature, ␹(T), for different applied fields, were carried out using the zero-field-cooled (ZFC) procedure. In this measurement the sample is cooled in zero magnetic field down to the starting temperature, at which time the field is applied and the magnetic response is measured on warming.

3. Results and discussion Fig. 1 shows the XRD patterns obtained for the powders calcined for 4 h at temperatures from 500 to 1000 ◦ C. It is also possible to observe that the desirable spinel phase is the only phase present for heat treatment conditions, even at 500 and 600 ◦ C. This was obtained, using a chemical method based on the Pechini method. The analysis of these results indicates that this method produces highly crystalline materials. The Rietveld refinement study, displayed in Fig. 2, shows that the Li2 ZnTi3 O8 phase presents a P43 32 space group. The structure is a spinel derivative, in which the octahedral 12d and 4b sites are occupied by Ti and Li, respectively and the tetrahedral crystallographic 8c sites are shared by Zn and Li, suggesting an intermediary spinel structure. After the refinement of Li and Zn occupancies at 8c sites the unit formula may be written in expanded form as (Li0.480(3) Zn0.520(3) )tet [Ti1.50 Li0.50 ]oct O4 . The obtained refinement index RBRAGG (=2.87%) confirms the good quality of the structure model. The Rietveld refinement data, obtained for the Li2 ZnTi3 O8 , are displayed in Table 1. A small amount (2.7 wt.%) of a secondary phase, identified as the rutile TiO2 , was observed and only its scale factor could be refined. In Fig. 3 the precursor powder thermogravimetric analysis (TG) is revealed. As can be observed, a first weight

Fig. 1. XRD patterns for the Li2 ZnTi3 O8 phase calcined for 4 h at the temperature range of 500–1000 ◦ C.

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Fig. 2. Rietveld refinement plot for the Li2 ZnTi3 O8 phase. Table 1 Rietveld refinement data obtained for the Li2 ZnTi3 O8 Atom

Site

x

y

z

Biso

Occupancy

Li(1) Zn(1) Li(2) Ti O(1) O(2)

8c 8c 4b 12d 24e 8c

−0.0021(2) −0.0021(2) 5/8 0.368(1) 0.1062(4) 0.3878(4)

−0.0021(2) −0.0021(2) 5/8 −0.118(1) 0.1268(3) 0.3878(4)

−0.0021(2) −0.0021(2) 5/8 1/8 0.3900(3) 0.3878(4)

0.480(3) 0.480(3) 1 1 1 1

0.480(3) 0.520(3) 1 1 1 1

Rwp = 7.08, S (Goodness-of-fit) = 1.22, RB =2.87%, crystallite size = 870 Å, microstrain = 0.07%, a = 8.3738(1) Å, V = 587.17(1) Å3 .

Fig. 3. TG–DTA curves for the Li2 ZnTi3 O8 precursor powder.

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Fig. 4. Plot of specific surface area vs. temperature of calcination for Li2 ZnTi3 O8 .

loss related to water appears at 100 ◦ C. The pyrolysis of organic material occurs at temperatures between 300 and 530 ◦ C. Also shown in Fig. 3, the corresponding DTA study, presents the endothermic peak related to the water elimination. Other endothermic peaks are present in the temperature range of 330–410 ◦ C and an exothermic peak at the temperature range of 410–510 ◦ C is observed. The first peak is related to the decomposition of lithium carbonate and the other one to the zinc acetate decomposition [19]. At higher temperatures, spinel crystals start to be formed, as showed by XRD results. The specific surface area data can be seen in Fig. 4. The data obtained by BET showed an exponential reduction in the specific area (SBET ) as the heat treatment temperature increases and consequently there is a growth in the particles. This fact can be related to the occurrence of agglomerated material for heat treatment temperatures higher than 600 ◦ C. A scanning electron microscopy study con-

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Fig. 6. Magnetic susceptibility data for sintered ceramic samples of Li2 ZnTi3 O8 .

firms this behavior, as can be observed in Fig. 5A and B, which present the onset (500 ◦ C) and the end (900 ◦ C) of the agglomeration process, respectively. At 500 ◦ C some particles have already started to agglomerate, whereas other particles remain unchanged, giving rise to a bimodal distribution. At 900 ◦ C, once the agglomeration process is concluded, the particles were already grown, yielding therefore a higher mean particle size with monomodal size distribution. Another appropriate motivation to study the Li2 ZnTi3 O8 phase is related to previous studies on the magnetic behavior of titanium based systems [20,21] and the occurrence of superconductivity in related LiTi2−x Mx O4 spinel structures [22]. Fig. 6 presents the χ(T) experimental data for the Curie–Weiss (CW) fitting. Using a CW law modified to include a residual constant, χ = χ0 + C/(θ − T), we have calculated χ0 = 3.6 × 10−8 emu g−1 , C = 5.24 emu cal mol−1

Fig. 5. Scanning electron microscopy images at temperatures of 500 and 900 ◦ C.

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Acknowledgements The authors gratefully acknowledge the financial support of the Brazilian research agencies FAPESP, PRONEX, FINEP and CNPq. References

Fig. 7. Magnetization vs. applied field plot at 2 K for sintered ceramic samples of Li2 ZnTi3 O8 .

and θ = −9.5 × 10−3 K. The observed reversible low-field paramagnetic behavior indicates the absence of magnetic correlations in this system. It was already observed [20,21] that the Ti rutile and anatase phases also have a paramagnetic signal at low fields [20,21] and no magnetic correlation are expected for the Li ions [23]. Furthermore, no indication of superconducting behavior was observed at the temperature range of 1.8–200 K. The study of the magnetic hysteresis measured in a magnetization versus applied field procedure at 2 K is shown in Fig. 7 and indicates a signature characteristic of paramagnetic behavior.

4. Conclusions This article reports the use of the modified polymeric precursor method, known as the Pechini method, in the synthesis of Li2 ZnTi3 O8 ceramic samples. A reasonable decrease in the phase formation temperature was observed in contrast with conventional synthesis procedures. The obtained precursor powders were nanometric in size and allowed homogeneous and controlled phase samples. Magnetic measurements revealed reversible paramagnetic behavior and no indication of superconductivity was observed.

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