Journal of Alloys and Compounds Influence of the ... - CiteSeerX

0 downloads 0 Views 267KB Size Report
for the synthesis of these compounds, such as sol–gel, poly- meric precursors ... The infrared spectra of CaSnO3 and SrSnO3 (Fig. 1a and b) ..... [13] R. Nyquist, R. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press. Inc., London ...
Journal of Alloys and Compounds 476 (2009) 507–512

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Influence of the modifier on the short and long range disorder of stannate perovskites Mary C.F. Alves a,∗ , Soraia C. Souza a , Hebert H.S. Lima a , Marcelo R. Nascimento a,b , Márcia R.S. Silva a , José Waldo M. Espinosa a , Severino J.G. Lima c , E. Longo d , P.S. Pizani e , Luiz E.B. Soledade a , Antonio G. Souza a , Iêda M.G. Santos a a

LACOM, Departamento de Química/CCEN, Universidade Federal da Paraíba, Campus I, CEP 58059-900, João Pessoa, PB, Brazil EAFS-PB – Escola Agrotécnica Federal de Sousa, Sousa, PB, Brazil c LSR, Departamento de Engenharia Mecânica/CT, UFPB, Campus I, João Pessoa, PB, Brazil d CMDMC-LIEC, Instituto de Química, UNESP, Araraquara, SP, Brazil e Departamento de Física, Universidade Federal de São Carlos, São Carlos, SP, Brazil b

a r t i c l e

i n f o

Article history: Received 2 July 2008 Accepted 5 September 2008 Available online 23 October 2008 Keywords: Chemical synthesis Crystal structure Phase transitions X-ray diffraction Order–disorder effects

a b s t r a c t Ca1−x Srx SnO3 powders (x = 0, 0.25, 0.50, 0.75 and 1) were synthesized by the polymeric precursor method. This method is derived from the Pechini one, being quite simple, yielding a high chemical homogeneity. This work aims at evaluating the influence of the modifier (Sr2+ and/or Ca2+ ) on the properties of stannates with perovskite structure. The powders were characterized by XRD, Raman and FTIR. The increase of the Sr2+ concentration in the Ca1−x Srx SnO3 system promotes a high short range organization between the polyhedra, diminishing the high degree of distortions of CaSnO3 orthorhombic perovskite. On the other hand, it was verified that the samples that with a higher concentration of Ca2+ show a higher short range organization within the polyhedra (SnO6 ). As a consequence pure SrSnO3 presents higher long range order, crystallizing at lower temperature. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The compounds CaSnO3 and SrSnO3 are alkaline-earth stannates, displaying the perovskite structure. These materials have been currently investigated for their attractive dielectric characteristics, finding application as thermally stable capacitors in electronic industries, as well as ceramic capacitors, sensors, battery electrode materials, etc. [1–3]. Despite such technological importance, these stannates have not been thoroughly and systematically studied yet. For example, no sound and reliable data are available on the thermodynamic stability of these compounds or on other compounds in MO–SnO2 pseudo–binary systems [1]. Azad et al. reported the preparation of CaSnO3 and SrSnO3 by solid-state reaction [1]. Several other methods were also used for the synthesis of these compounds, such as sol–gel, polymeric precursors, peroxide precursors, etc. A great advantage of the polymeric precursor method, in relation to other chemical synthesis methods, is its low cost, once the reagents used

∗ Corresponding author. E-mail address: [email protected] (M.C.F. Alves). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.034

in larger amounts are relatively cheap, besides working at relatively low temperatures in order to obtain single-phase systems [4]. BaSnO3 , SrSnO3 and CaSnO3 structures have been recently described in literature. The three compounds have classic perovskite structures [5–7]. The BaSnO3 has a cubic structure, while SrSnO3 and CaSnO3 were characterized by distorted cubes, due to the inclination of the octahedra. In these compounds, the coordination around Sn4+ does not change, as well as the tridimensional arrangements of the octahedra. On the other hand, different inclinations of the octahedra lead to meaningful changes in the coordination of the A cation (Ba, Sr or Ca) [5]. According to Mountstevens et al. [7], the average Sn4+ O2− bond distance does not change systematically with the decrease in the size of the A cation, but there is a continuous decrease in the Sn O Sn angle, indicating a change in the way by which the octahedra are bonded, with a higher distortion. As a consequence, a change in the optical properties is observed when different modifiers are added to the stannates. The CaSnO3 SrSnO3 system has been synthesized by solid-state reaction. All samples are orthorhombic, but a difference in the XRD patterns has been observed, indicating that subgroups exist inside the orthorhombic structure [7].

508

M.C.F. Alves et al. / Journal of Alloys and Compounds 476 (2009) 507–512

Thus, this work aims at evaluate the CaSnO3 –SrSnO3 system, synthesized by the polymeric precursor method. The disorder was evaluated in relation to the short and long range, according to XRD, Raman and IR results. 2. Experimental In order to guarantee the chelation of the metals during the synthesis by the polymeric precursor method, all metals were separately dissolved as citrate solutions. A 3:1 citric acid to metal molar ratio was used. Calcium citrate and strontium citrate were prepared by the addition of citric acid (99.5% purity, from Cargill) into distilled water, at 70 ◦ C, followed by the slow addition of calcium acetate (99% purity, from Vetec) or strontium nitrate (99% purity, from Vetec), up to the complete dissolution. Tin citrate was prepared from tin chloride dehydrate, as previously described [8]. In a beaker kept under stirring, calcium citrate and/or strontium citrate were added to the tin citrate solution, in order to achieve the target composition. Finally, ethylene glycol was added to the solution, to yield a 40:60 ethylene glycol:citric acid mass ratio. The temperature was raised to about 100 ◦ C, to promote the polyesterification reaction and the formation of the polymeric resin. The resins were heat treated at 300 ◦ C for 2 h, leading to the formation of the powder precursors. These precursors were milled until passing through a 100-mesh sieve. High energy milling in an alcoholic medium was carried out, using an attritor mill for 4 h. After drying, the powder precursors were calcined at 250 ◦ C for 24 h, in an oxygen atmosphere. A second heat treatment was performed in air at 700 ◦ C for 2 h. The X-ray diffraction (XRD) patterns were obtained by means of a Siemens D-5000 diffractometer, using the monochromatic Cu K␣ radiation. The lattice parameters were calculated using the REDE 93 program [9], developed at the Institute of Chemistry of UNESP, Araraquara, SP, Brazil. The infrared spectra were obtained in a Bomem MB 102 spectrometer, using KBr pellets. The Raman spectroscopy was conducted in a FT-Raman Bruker spectrophotometer, model RFS/100/S, with a Nd: YAG laser, 60 mW of power, with a wavelength of 1064 nm and a resolution of 4 cm−1 , in wavenumber range from 10 to 1200 cm−1 .

3. Results and discussion The infrared spectra of CaSnO3 and SrSnO3 (Fig. 1a and b) present a similar behavior. When the precursors are calcined at 250 and 300 ◦ C, two bands are observed at about 1574 and 1390 cm−1 or 1581 and 1412 cm−1 for SrSnO3 and CaSnO3 , respectively. These bands are related to the stretching of carboxylate ions in bridging complexes. Bands at about 1450, 1088 and 850 cm−1 are also observed, being assigned to carbonates [10,11]. These bands are more intense in SrSnO3 . At higher calcination temperatures, the intensities of the carboxylate bands decrease, as the intensity of the carbonate bands increase. These results indicate that carbonates are formed during the calcination. For the CaSnO3 , carbonate bands increase up to 500 ◦ C, decreasing at higher temperatures, whereas for SrSnO3 , no further decrease is observed. Thus, for CaSnO3 , its elimination is easier than for the samples containing strontium. Literature data indicate that BaSnO3 , with cubic structure, has three F1u vibration modes (acoustic phonons), which are infrared active, being related to the following vibrations: Sn O (stretching), SnO3 (external vibration with barium atoms) and Sn O (angle) [12]. According to Nyquist and Kagel [13], the spectra of CaSnO3 and SrSnO3 are similar to that of BaSnO3 , in spite of the different structures. The only difference is related to the intensity of some bands, for instance, SrSnO3 presents more intense bands at 600 cm−1 . In the present work, the spectra have the same profile described by Nyquist and Kagel [13]. The metal–oxygen bands become more defined with the temperature increase. At 700 ◦ C, all samples present a similar behavior in relation to the bands at 484 and 666 cm−1 . The pure CaSnO3 sample presents thinner bands, indicating that a smaller short range disorder occurs. In relation to the band positions, a shift from 659 to 671 cm−1 is observed when Sr2+ is added to the material, while the low frequency band presents an opposite behavior (Fig. 1c). This occurs because each band has a different assignment, as shown by Licheron et al. [12].

Fig. 1. Infrared spectra of: (a) CaSnO3 after calcination at different temperatures, (b) SrSnO3 after calcination at different temperatures and (c) Ca1−x Srx SnO3 after calcination at 700 ◦ C.

Stannates with perovskite structure present different Bravais lattices, as a function of network modifier and synthesis temperature. The increase of the modifier promotes changes in the bond angles between the [SnO6 ] polyhedra, thus generating phase transitions, due to the decrease of orthorhombic perovskite distortions, promoting a higher system organization, both at the short and long ranges. For SrSnO3 , phase transitions are observed, with a structure change from orthorhombic to tetragonal and then to cubic (Eq.

M.C.F. Alves et al. / Journal of Alloys and Compounds 476 (2009) 507–512

509

Fig. 3. XRD patterns, between 55◦ and 58◦ , of the Ca1−x Srx SnO3 , calcined at 700 ◦ C: (a) CaSnO3 , (b) Ca0.75 Sr0.25 SnO3 , (c) Ca0.50 Sr0.50 SnO3 , (d) Ca0.25 Sr0.75 SnO3 and (e) SrSnO3 .

Fig. 2. XRD patterns, after calcination at different temperatures of: (a) CaSnO3 and (b) SrSnO3 .

(1)). The symbols in parenthesis are in agreement with the Glazer notation [7]. Pbnm(a+ b− b− ) → Imma(ao b− b− ) → 14/mcm(ao ao c − ) −

→ Pm3m(ao ao ao )

(1)

XRD data are presented in Fig. 2a and b. A small change in the XRD profile is observed when Sr2+ substitutes for Ca2+ , what is more easily observed at higher angles (Fig. 3). According to the literature, only transitions in the orthorhombic subgroup are observed in the system Ca1−x Srx SnO3 , obtained by solid-state reaction [7]. Literature data also report that SrSnO3 is orthorhombic, even after calcination at 800 ◦ C [14]. This information may not be only confirmed from the XRD patterns, because this characterization technique is not sensitive enough to assess the crystalline structure of SrSnO3 , due to the superposition of the peaks of the SrSnO3 cubic and orthorhombic phases.

Fig. 2a illustrates the XRD patterns of pure CaSnO3 , calcined at different temperatures. Well-defined peaks are only noticed after calcination at 700 ◦ C (JCPDS 07-1797). When the calcination occurs between 250 and 500 ◦ C, peaks assigned to calcium carbonate are observed at 26.7◦ , 32.8◦ and 51.4◦ (JCPDS 84-1778). At 600 ◦ C, these peaks disappear, whereas the peaks assigned to the perovskite phase are not yet observed (Fig. 2a). These results are in agreement with the infrared spectra, which indicate that the carbonate bands decrease above 500 ◦ C. For pure SrSnO3 perovskite, crystallization is observed at 600 ◦ C (Fig. 2b), with a higher definition at 700 ◦ C (JCPDS 77-1798). Broad peaks, at 25.3◦ , 36.3◦ , 44.0◦ and 50.0◦ , are observed, being assigned to strontium carbonate (JCPDS 01-1032), which is present in all samples with strontium addition. This secondary phase does not hamper crystallization, as CaSnO3 , with lower carbonate amount, only crystallizes at higher temperatures. We believe that the higher covalent character of Ca2+ makes crystallization more difficult, with the need of a higher heat treatment temperature. This is due to the higher distortion between the octahedra, as CaSnO3 has a lower Sn O Sn angle (160◦ ) than SrSnO3 (180◦ ) [15,16]. No meaningful change in the disorder is observed within the octahedra. As a consequence, it may be observed that the increase in Sr2+ concentration leads to a lower long range disorder, as indicated by the FWHM results, calculated using the XRD patterns of the samples heat treated at 700 ◦ C (Fig. 4). The only exception is the sample Ca0.5 Sr0.5 SnO3 , which presents a high long range disorder (FWHM of 0.47◦ ). The same behavior was observed by Mountstevens et al. [7] for the sample Ca0.60 Sr0.40 SnO3 . Another consequence of the Sr2+ addition into the Ca1−x Srx SnO3 system is the increase in the unit cell volume (Table 1). This increase may also be responsible for the lower long range disorder in this system. Results of Raman spectroscopy are presented in Fig. 5. According to Zhang et al. [3], SrSnO3 orthorhombic perovskite presents Table 1 Lattice parameters of the system Ca1−x Srx SnO3 . Sample

a (Å)

b (Å)

c (Å)

Volume (Å3 )

CaSnO3 Ca0.75 Sr0.25 SnO3 Ca0.50 Sr0.50 SnO3 Ca0.25 Sr0.75 SnO3 SrSnO3

5.524 5.682 5.630 5.714 5.698

5.665 5.589 5.692 5.676 5.738

7.940 8.018 8.150 8.112 8.070

248.47 254.61 261.18 263.08 263.82

510

M.C.F. Alves et al. / Journal of Alloys and Compounds 476 (2009) 507–512

Table 2 Frequencies (cm−1 ) of the Raman absorption bands, with the respective assignments. Modes

SrSnO3 (Zhang)

SrSnO3 (Udawatte)

Present work SrSnO3

CaSnO3

Sr SnO3

– 119 – 150 168

90(m) 109(w) – – –

93(w) 108(vw) 119(vw) 152(vs) –

– – – 157(m) –

O Sn O

220 257 – 305

225(vs) 259(m) – 308(vw)

227(m) 250(w) 261(w) –

211(w) – 282(m) 361(m)

Sn O3 (torsion)

403 511 596

403(w) – 573(vw)

406(vw) – 577(s)

– 448(vw) 580(m)

Sn O (symmetric stretching)

713 890



703(m) 986(vw)

709(w) 992(w)

171(w) 1076(s) –

186(s) 1074(vs) 1125(m)

186(w) 1087(m) 1125(w)

Sr CO3 – (vw) very weak; (w) weak; (m) medium; (s) strong; (vs) very strong.

active modes at 119, 150, 168, 220, 257, 305, 403, 511, 596, 713 and 890 cm−1 . Some differences are observed between the literature data [3,17] and Raman spectra of SrSnO3 obtained in the present work (93, 108, 119, 152, 186, 227, 250, 406, 577, 703, 1074, 1125 cm−1 ), as observed in Table 2. This difference is probably related to the carbonate anions. In listed literature works, the syntheses were performed by solidstate reaction, using strontium carbonate [3] or by the polymeric precursor method [17], known by the high amount of carbon compounds. In the present work, a meaningful elimination of carbon took place, by means of the preliminary heat treatment at 250 ◦ C for 24 h, in an oxygen atmosphere. As a consequence, a smaller amount of carbonate is formed. When the calcination in oxygen atmosphere was not performed, the same profile observed in literature was obtained. We believe that this carbonate is bonded to the Sr2+ , in the crystalline structure of perovskite, leading to a change in the Raman spectrum profiles. The higher change is observed in the low frequency region, which is related to the network modifier (Sr2+ ). Peaks at 189, 1074 and 1180 cm−1 are assigned to carbonates that are not bonded to the perovskite structure [17,18]. These peaks have a higher intensity in SrSnO3 perovskite, confirming that strontium

carbonate is more difficult to eliminate than calcium carbonate, as already indicated by the infrared spectra (Fig. 1a and b) and XRD patterns (Fig. 2a and b). The presence of a different modifier, Sr2+ and/or Ca2+ , in the Ca1−x Srx SnO3 system, also leads to a change in the Raman spectra, mainly in the range between 100 and 400 cm−1 (Fig. 5a and Table 2). SrSnO3 presents a higher amount of peaks, at around 93, 108 and 119 cm−1 , which are not observed in CaSnO3 . Moreover, the dislocation of the other peaks is also observed. One of the highest differences is observed in the peak around 361 cm−1 (O Sn O), which presents a smaller intensity as Sr2+ is added to the structure, until its complete disappearance in SrSnO3 . This change may be related to the lower short range disorder between the [SnO6 ] polyhedra, in SrSnO3 . In order to evaluate the short range disorder in the modifier region (between octahedra), deconvolution of the peak around 150 cm−1 (Me SnO3 stretching) was done. Table 3 lists the peak position, intensity and FWHM values of such peaks. It is noticed that higher concentrations of Sr2+ in the lattice lead to changes in the Raman peaks, increasing the intensity and decreasing both the FWHM peak values and the frequency region. These results indicate that weaker bonds are formed between Sr2+ and the [SnO6 ] polyhedra, with a higher short range order in the modifier region. This is due to the higher covalent character of Ca2+ , which leads to a higher distortion and inclination of the [SnO6 ] octahedra, as already reported in literature of Ref. [5]. It may also be observed that the presence of the two network modifiers in the lattice increases the short range disorder (higher FWHM values). The substitution of Sr2+ for Ca2+ is represented in Eq. (2), according to the Krogër-Vink notation [19]. CaO

SrO−→[SrCa O12 ]x + Oxo

(2)

Table 3 Results of deconvolution the peaks of the Raman spectra at around 150 cm−1 . Sample

Fig. 4. FWHM values of the XRD patterns of Ca1−x Srx SnO3 , as a function of Sr2+ amount.

CaSnO3 Ca0.75 Sr0.25 SnO3 Ca0.50 Sr0.50 SnO3 Ca0.25 Sr0.75 SnO3 SrSnO3

Absorption bands Peak position (cm−1 )

Intensity (a.u.)

FWHM (cm−1 )

158.90 153.82 152.53 153.90 152.71

2.72 2.11 3.21 6.25 18.25

9.23 12.67 11.86 12.00 8.62

M.C.F. Alves et al. / Journal of Alloys and Compounds 476 (2009) 507–512

511

Fig. 6. Results of the deconvolution of the peaks at around 580 cm−1 , in the Raman spectra, of the system Ca1−x Srx SnO3 .

The Raman peaks between 400 and 1000 cm−1 was related to the vibrations of the network former. Such region was deconvolved, what allowed to notice that sample displays a peak at around 580 cm−1 (torsional Sn O3 vibrations). The position and FWHM values of these peaks are illustrated in Fig. 6. It may be observed that the increase in the Sr2+ amount leads to an increase in the FWHM and to the shift of the peaks toward lower energy values. These results indicate that the Sr2+ addition leads to a higher disorder in relation to Sn O3 bonds, with weaker bonds. The only exception was the sample Ca0.75 Sr0.25 SnO3 , which presented the smallest FWHM value and the highest frequency. These results lead to an interesting conclusion about the Ca1−x Srx SnO3 system. When a modifier with a higher covalent character is present in the lattice (in this case, Ca2+ ), a higher disorder between the polyhedra occurs, while a lower disorder within the [SnO6 ] polyhedra is observed. In other words, a higher covalent character leads to a higher disorder in the modifier region. As a consequence, long range crystallization becomes more difficult and samples with higher long range disorder are obtained. This higher disorder between the octahedra is confirmed by literature data [5], which indicate that a smaller Sn O Sn angle is obtained for CaSnO3 . When two network modifiers are present in the Ca1−x Srx SnO3 system, the simultaneous presence of [CaO12 ] and [SrO12 ] coordination polyhedra leads to a higher disorder between them, besides a higher order within them.

4. Conclusions

Fig. 5. Raman spectra of the Ca1−x Srx SnO3 powders, calcined at 700 ◦ C/2 h, at different frequency ranges: (a) 0–400 cm−1 , (b) 400–1050 cm−1 and (c) 1000–1200 cm−1 .

The XRD results point out that the Ca1−x Srx SnO3 powder samples, obtained using the polymeric precursor method, are crystalline after calcination at 700 ◦ C for 2 h. Results indicate a different behavior, depending upon the network modifier. While SrSnO3 sample has a lower disorder between the [SnO6 ] polyhedra, CaSnO3 has a lower disorder within these polyhedra. This behavior may be related to the higher covalent character of Ca2+ , which leads to a higher inclination between the [SnO6 ] polyhedra and to a higher distortion. As a consequence, the crystallization of SrSnO3 occurs at lower temperatures, with a higher long range order, in spite of the higher amount of carbonate. The same behavior is observed when two modifiers, Ca2+ and Sr2+ are present in this perovskite – a higher disorder occurs between the [SnO6 ] polyhedra, with a lower disorder within them.

512

M.C.F. Alves et al. / Journal of Alloys and Compounds 476 (2009) 507–512

Acknowledgements The authors acknowledge CAPES, FAPESQ/PB and CNPq/MCT, for the financial support of this work. References [1] [2] [3] [4] [5] [6] [7] [8]

A.M. Azad, L.L.W. Shyan, P.T. Yen, J. Alloys Compd. 282 (1999) 109–124. Z. Lu, J. Liu, J. Tang, Y. Li, Inorg. Chem. Commun. 7 (2004) 731–733. W.F. Zhang, J. Tang, J. Ye, Chem. Phys. Lett. 418 (2006) 174–178. S.C. Souza, I.M.G. Santos, M.R.S. Silva, M.R.C. Santos, L.E.B. Soledade, A.G. Souza, S.J.G. Lima, E. Longo, J. Therm. Anal. Calorim. 79 (2005) 451–454. H. Mizoguchi, H.W. Eng, P.M. Woodward, Inorg. Chem. 43 (2004) 1667–6680. W. Zhang, J. Tang, J. Ye, J. Mater. Res. 22 (2007) 1858–1871. E.H. Mountstevens, J.P. Attfield, S.A.T. Redfern, J. Phys. 15 (2003) 8315–8326. F.R.C. Ciaco, F.M. Pontes, C.D. Pinheiro, E.R. Leite, S.R. Lazaro, J.A. Varela, P.S. Pizani, C.A. Paskocimas, A.G. Souza, E. Longo, Ceramica 50 (2004) 43–49.

[9] C.O. Paiva, P. Santos, D. Garcia, Y.P. Mascarenhas, J.A. Eiras, Assoc. Bras. Ceram. 35 (1989) 153. [10] S.G. Cho, P.F. Johnson, R.A.J. Condrate, Mater. Sci. 25 (1990) 4738–4740. [11] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, John Wiley & Sons, New York, 1986. [12] M. Licheron, G. Jouarf, E. Husson, J. Eur. Ceram. Soc. 17 (1997) 1453–1457. [13] R. Nyquist, R. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press Inc., London, 1971. [14] M. Glerup, K.S. Knight, F.W. Poulsen, Mater. Res. Bull. 40 (2005) 507–520. [15] M.A. Green, K. Prassides, P. Day, D.A. Neumann, Inter. J. Inorg. Mater. 2 (2000) 35–41. [16] S.K. Mishra, R. Ranjan, D. Pandey, H.T. Stokes, J. Phys. 18 (2006) 1885–1898. [17] C.P. Udawatte, M. Kakihana, M. Yoshimura, Solid State Ionics 128 (2000) 217–226. [18] J. Cerdà, J. Arbiol, G. Dezanneau, R. Díaz, J.R. Morant, Sens. Actuators B: Chem. 84 (2002) 21–25. [19] Y.M. Chiang, W.D. Kingery, D.P. Birnie III, Physical Ceramics, John Wiley & Sons, New York, 1997.