Effect of the composition on the thermal behaviour of

J Therm Anal Calorim DOI 10.1007/s10973-013-3051-1

Effect of the composition on the thermal behaviour of the SrSn12xTixO3 precursor prepared by the polymeric precursor method A. L. M. de Oliveira • M. R. S. Silva • H. Sales • E. Longo • A. S. Maia • A. G. Souza I. M. G. Santos

•

Received: 8 November 2012 / Accepted: 8 February 2013 Ó Akade´miai Kiado´, Budapest, Hungary 2013

Abstract Strontium stannate titanate Sr(Sn, Ti)O3 is a solid solution between strontium stannate (SrSnO3) and strontium titanate (SrTiO3). In the present study, it was synthesized at low temperature by the polymeric precursor method, derived from the Pechini process. The powders were calcined in oxygen atmosphere in order to eliminate organic matter and to decrease the amount of SrCO3 formed during the synthesis. The powders were annealed at different temperatures to crystallize the samples into perovskites-type structures. All the compositions were studied by thermogravimetry (TG) and differential thermal analysis (DTA), infrared spectroscopy (IR) and X-ray diffraction (XRD). The lattice former, Ti4? and Sn4?, had a meaningful influence in the mass loss, without changing the profile of the TG curves. On the other hand, DTA curves were strongly modified with the Ti4?:Sn4? proportion in the system indicating that intermediate compounds may be formed during the synthesis being eliminated at different temperature ranges, while SrCO3 elimination occurs at higher temperature as shown by XRD and IR spectra. Keywords Perovskite
Strontium stannate titanate
Crystallization and thermal analysis

A. L. M. de Oliveira
M. R. S. Silva
H. Sales
A. S. Maia
A. G. Souza
I. M. G. Santos (&) LACOM/INCTMN, DQ-CCEN-Universidade Federal de Paraiba, Joa˜o Pessoa, Paraı´ba, Brazil e-mail: [email protected] E. Longo LIEC/INCTMN, Instituto de Quı´mica, UNESP, Araraquara, Sa˜o Paulo, Brazil

Introduction Metal oxides based on perovskite structure with general formula ABO3 are one of the most important classes in the field of materials science and have attracted considerable attention in the past decades due to their simple crystalline structure [1, 2]. Moreover, its versatile structure allows modifications in different degrees by the selection of an adequate cation for substitution into the A and B sites, which lead to a great scope to tailor different properties [1–3]. Among these materials, strontium stannate (SrSnO3) and strontium titanate (SrTiO3) have been investigated much due to their interesting properties leading to different technological applications. For instance, SrSnO3 can be applied as dielectric material for use in capacitors [4], as insulating layer for high-temperature superconductor (HTS) single-flux-quantum (SFQ) circuits [5], and as humidity sensor [6]. On the other hand, SrTiO3 can be employed in tunable microwave devices [7], as oxygen-gas sensor [8], as photocatalytic material [9] and so on. Therefore, considering the interesting properties of these two perovskites, SrTiO3 and SrSnO3, they were combined with each other to obtain strontium stannate titanate Sr(Sn1-xTix)O3 solid solution. SrSnO3 and SrTiO3 have been extensively studied so far, but the solid solution between these two perovskitetype oxides, Sr(Sn, Ti)O3, has not been studied as much. Wu et al. [10] reported the synthesis of strontium stannate titanate SrSn1-xTixO3 0.25 B x B 0.75 by solid state reaction, in the temperature range from 1,250 to 1,400 °C for 2–4 h in air. The authors have observed that SrSn0.50Ti0.50O3 composition was a promising humidity sensor. Singh et al. [11] have synthesized SrSn1-xTixO3 with the 0 B x B 0.50 compositions by the conventional solid state reaction and showed that the single phase solid solution was formed only up to x = 0.40 with two heat

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A. L. M. de Oliveira et al.

(a) 100 95 90

Mass/%

85 80 75 70

SrSn0.75 Ti0.25 O3

65

SrSnO3

SrTiO3

60

SrSn0.50Ti0.50 O3

SrSn0.25Ti

O 0.75 3

55 50 45 0

100

200

300

400

500

600

700

800

900

Temperature/°C SrSnO3

(b)

SrSn0.75Ti O3 0.25 SrSn0.50Ti0.50O3 SrSn0.25 Ti0.75 O3

DTG/a.u.

SrTiO3

0

100 200 300 400 500 600 700 800 900

Temperature/°C

(c)

exo SrSnO3 SrSnO0.75Ti0.75 O3

DTA/a.u.

treatments-above 1,250 °C for 12 h and then at 1,500 °C for 12 h. All of the compositions were indexed as cubic perovskites. As the crystallization process of the oxides by solid state reaction occurs at higher temperatures, it often leads to powders of large and varied grain sizes and varying impurity contents. Differently, Stanulis et al. [12] have studied the structural and microstructural characteristics of the same solid solution SrSn1-xTixO3 (x = 0.05–0.5) obtained by the gel to crystalline conversion method (G–C) at temperature from 700 to 1,100 °C. Considering that the chemical and physical properties of metal oxides depend on the synthesis method, we have chosen the polymeric precursor one, which is characterized by high reproducibility and low cost, compared to other chemical methods of synthesis for the preparation of the SrSn1-xTixO3 ceramic oxides [13]. This method has been successfully used in the synthesis of nanoparticles and thin films of different oxides and is based on the chelation of cations by a hydrocarboxylic acid (normally citric acid), followed by polyesterification using a glycol (normally ethylene glycol) [10]. The formed precursor resins contain cations randomly distributed throughout the polymer [11]. Our research group has successfully obtained SrTiO3 [14] and SrSnO3 [15] powders by the polymeric precursor method below 700 °C. Up to our knowledge, no study reporting the synthesis of Sr(Sn,Ti)O3 powders using this method was found in the literature. In this sense, this study aims to study the thermal behavior of the precursors of the SrSn1-xTixO3 (x = 0; 0.25; 0.50; 0.75 and 1.0) perovskites obtained by the polymeric precursor method. These results were compared to infrared spectra and XRD patterns. The influence of the lattice former, Ti4? and Sn4?, in the structural characteristics of this solid solution was also evaluated.

SrSn0.50Ti0.50 O3

SrSn0.25Ti0.75 O3

Experimental details SrTiO3

SrSn1-xTixO3 (SST) solid solution was prepared by the precursor polymeric method, as already reported in literature for pure SrTiO3 [13] and SrSnO3 [14]. For the synthesis of the various compositions in the system, SrSn1-xTixO3 (x = 0; 0.25; 0.50; 0.75 and 1.0), strontium nitrate (Sr(NO3)2 Vetec), tin chloride (SnCl2
2H2O J. T. Backer), liquid titanium isopropoxide (Ti[OCH(CH3)2]4 Hulls-AG), monohydrated citric acid (C6H8O7
H2O Cargill), and ethylene glycol (C2H6O2 Vetec) were used as raw materials. The solutions of each salt were prepared separately to synthesize the polymeric resin. The syntheses of titanium citrate and tin citrate were done similarly to the methodology described in [16] and [17, 18], respectively. For the preparation of the polymeric resin, citric acid was dissolved in

123

0

100

200

300

400

500

600

700

800

900

Temperature/°C Fig. 1 Thermal analysis of the powder precursors a TG curves; b DTG curves; c DTA curves

distilled water at 60 °C, followed by the addition of strontium salt (3:1 citric acid:strontium molar ratio) under constant stirring to obtain strontium citrate. This solution was added into the titanium citrate solution at the same temperature, followed by the addition of the tin citrate previously dissolved in concentrated nitric acid (HNO3 Fmaia). Finally, ethylene glycol was added with 40:60 mass ratio (ethylene glycol:citric

Effect of the composition on the thermal behavior Table 1 Results of TG/DTA analyses for the SrSn1-xTixO3 powder precursors calcined in O2 atmosphere Samples

Steps

Mass loss/%

Temperature range/°C

DTA peak temperature/°C

DTG peak temperature/°C

SrSnO3

1

6.5

34–262

endo: 87

82

2

23.2

262–584

exo: 540

544

3

6.5

584–779

endo: 662/exo: 675

695

1

9.4

26–286

endo: 87

80

2

28.4

286–608

exo: 510/544

512 and 548

3

5.7

608–757

endo: 668

565 and 672

1

8.2

25–273

endo: 88

83

2

25.9

273–583

exo: 472/515/570

485 and 520

3

8.1

583–868

endo: 616

613

1 2

11.1 33.2

27–250 250–554

endo: 89 exo: 475

83 484

9.2

544–860

exo: 589

591

37–286

endo: 87

80 516

endo: 696 SrSn0.75Ti0.25O3

SrSn0.50Ti0.50O3

SrSn0.25Ti0.75O3

3 SrTiO3

1

11

2

29.2

286–580

exo: 509

Mass gain

?0.04

580–609

exo: 596

602

7.4

609–875

endo: 629

627

3

acid) under constant stirring and the temperature was raised to about 90 °C to eliminate water and obtain the polymeric resin. The pH of the solution was below 1. The polymeric resin was calcined at 300 °C for 1 h to obtain the powder precursors which were deagglomerated, dry milled and then calcined at 300 °C in an oxidizing atmosphere for 12 h to eliminate the organic matter. Dry milling was carried out in a spex planetary miller using two balls of tungsten carbide (6 mm of diameter) for each 0.8 g of the powder precursor, during 10 min. The powders were analyzed by thermogravimetry (TG) and differential thermal analysis (DTA) using a thermobalance (SDT 2960-TA Instruments analyzer) with heating rate of 10 °C min-1 up to 900 °C, in a synthetic air atmosphere with a flow rate of 100 mL min-1, using alumina crucibles. The powders precursors were subsequently annealed in air at different temperatures (from 400 to 700 °C) for 4 h and then characterized by infrared spectroscopy (IR) performed using an IRPrestige-21 Shimadzu spectrophotometer scanned in the range from 2,000 to 400 cm-1 using KBr pellets. X-ray diffraction (XRD) was carried out in a Siemens D-5000 diffractometer with a step size of 0.03° and step time of 1 s, and CuKa1 radiation in the range between 20 and 80°.

Results and discussion The TG curves with their derivative form (DTG) and the DTA curves of the powder precursors after heat treatment in O2 atmosphere are presented in Fig. 1a, b and c,

respectively and the results are shown in the Table 1, where temperatures of peaks are maximum of exothermic effects and minimum for endothermic ones. The TG curves show three thermal decomposition steps for the samples with 0 B x B 0.75 in the SrSn1-xTixO3 solid solution. For SrTiO3 (x = 1), one gain mass step was also observed. The first decomposition step at lower temperature is related to the powders dehydration besides the elimination of gases adsorbed on the surface of the precursors. This process is associated to an endothermic peak in the DTA curves. The second one at higher temperatures is assigned to an exothermic combustion of the organic matter leading to the formation of CO, CO2, and H2O characteristic of the synthesis method, as already reported in [14, 19–22]. As the polymeric precursor method is characterized by the formation of a metal–citrate complex, a high amount of organic matter is eliminated during the thermal treatment. Thus, a release of a high quantity of energy is observed. Furthermore, the formation of carbonates is favored, especially when alkaline-earth metals are present [15]. Comparing the SrSn1-xTixO3 (0 B x B 1) decompositions, it is noticed that the peak temperatures are higher for Sn4?-richer samples indicating that tin makes carbon elimination more difficult. The intense exothermic peaks are related to the combustion of esters [15]. Moreover, the high amount of organic matter may induce the formation of intermediate compounds as pointed out by literature data [23–27]. According to Fang and Tsay [23], during the synthesis, a high number of titanium ions are chelated with an even higher quantity of citric acid facilitating the Ti–citrate

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A. L. M. de Oliveira et al.

(a)

Transmittance/a.u.

700 °C

600 °C 500 °C 400 °C 300 °C

2000 1800 1600 1400 1200 1000

800

600

400

Wavenumber/cm–1

(b)

Transmittance/a.u.

700 °C

600 °C 500 °C 400 °C 300 °C

2000 1800 1600 1400 1200

1000

800

600

400

Wavenumber/cm–1

Fig. 2 IR spectra of the powders after annealing at different temperatures. a SrSnO3; b SrTiO3

complex formation. These authors also studied the effect of pH on the chemistry of the barium titanium citrate gel and its thermal decomposition for synthesis by the Pechini

method, with the evaluation of the precursor solutions by C13 NMR spectroscopy and of the powders by XRD. They observed the formation of the metal-mixed oxocarbonate as an intermediate compound originated at 600 °C from the thermal decomposition of a mixed–metal citric acid complex. This intermediate, Ba2Ti2O5CO3, decomposed at higher temperature (700 °C) into BaTiO3 and BaCO3. On the other hand, Zhou et al. [24] synthesized and studied the crystal structures of KMg0.5[Ti(H2cit)3]
6H2O and (NH4)Mg0.5 [Ti(H2cit)3]
6H2O mixed-cation compounds and also observed the formation of Ba2Ti2O5
CO3 as intermediate compound originated during the thermal decomposition (214–494 °C). The decomposition of this intermediate occurred in the range of 494–700 °C leading to the formation of BaTiO3. In the present study, the shape and the position of the most intense exothermic peak (Fig. 1c) are influenced by the Ti4?:Sn4? ratio in the system, their position shift from 540 °C for Sn4?-richer samples to 509 °C for Ti4?-richer ones. This behavior may be attributed to the formation of different metal–citrate complexes leading to the different energies of decomposition. Moreover, the superposed exothermic peaks for the SrSn0.75Ti0.25O3 and SrSn0.50 Ti0.50O3 samples indicate that the organic matter is not completely eliminated in one step. Based on these results, we believe that a possible formation of a mixed–metal citrate complex as an intermediate phase takes place at these temperatures. This is in good agreement with the literature data as previously mentioned [23–25]. In the present study, the third thermal decomposition step is observed above 583 °C being associated to an endothermic transition in the DTA curves (Fig. 1c). According to the literature, this temperature range corresponds to the decomposition of the metal oxocarbonates intermediates, (Ba0.75Sr0.25)2Ti2O5
CO3 (690–750 °C) [25] and Sr2Ti2O5
CO3 (570–730 °C) [26], which were formed

Table 2 Assignments of the infrared spectra of the SrSnO3 and SrTiO3 samples annealed at different temperatures SrSnO3

SrTiO3

Assignments

300 °C

400 °C

500 °C

600 °C

700 °C

300 °C

400 °C

500 °C

600 °C

700 °C

–

1765s

1,768

1,779

–

–

1,771

1,772

–

–

1,564

1580sh

–

–

–

1,564

1568sh

–

–

–

m/C=O of COO–

–

1,447

1,452

1,456

1,448

–

1,448

1,445

1,468

–

m/CO23

1,385/1,318sh

1398sh

1408sh

–

–

1,379

1388sh

–

–

–

m/C–O of COO–

1,061

1,062

1,065

–

–

1,062

1,065

1,068

–

–

m/CO23

–

858

862

871

864

–

856

857

859vs

860vs

m/CO23

672sh

701sh

694sh

662

662

778sh

–

–

–

–

m/M–O-st

554br

554br

554br

–

–

579br

579br

573br

567

581

m/M–O-st

412vs

410vs

432sh

488s

–

–

–

443

443sh

443sh/409s

d/M–O-bd

The peak wavenumbers are presented in cm-1 Band characteristics s small, vs very small, sh shoulder, br broad. Vibration modes st stretching mode, bd bending mode

123

m/CO23

Effect of the composition on the thermal behavior Table 3 Assignments of the infrared spectra of the SrSn0.25Ti0.75O3, SrSn0.50Ti0.50O3, and SrSn0.75Ti0.25O3 samples annealed at different temperatures SrSn0.25Ti0.75O3 300 °C

400 °C

SrSn0.50Ti0.50O3 500 °C

600 °C

700 °C

300 °C

SrSn0.75Ti0.25O3

400 °C

500 °C

600 °C

700 °C

300 °C

Assignments

400 °C

500 °C

600 °C

700 °C

–

1,775

1,768

1,794

1,792

–

1,795s

1,770s

1,775s

1,775s

–

1773s

1772s

–

1770s

m/CO23

1,568

1547s

–

–

–

1564s

1548sh

–

–

–

1,562

1568sh

–

–

–

m/C=O of COO–

1,452

1,452

1,464

1,462

–

1,452

1,448

1,464

1,464

–

1,448

1,449

1,462

1,462

m/CO23

1,389/ 1318sh

1391sh

–

–

–

1,387/ 1319sh

1396sh

–

–

–

1,391/ 1322sh

1391sh

–

–

–

m/C–O of COO–

1,061

1,065

1,059

1,065

–

1,061

1,061

1,069

1,069

1,065

1,062

1,061

1,065

1,055

1,068

m/CO23

–

858

858

860

857

–

858

860

860

860

–

858

858

858

860

m/CO23

–

–

–

m/M–O-st

698sh

685

687

m/M–O-st

536br

581sh

581sh

m/M–O-st

540sh

537sh

–

–

–

–

700sh

665sh

700sh

–

–

–

700

700

–

–

672sh

–

630

629

635

642br

631br

638sh

630sh

629sh

635sh

673

584br

575br

552

559

550

540sh

559br

537sh

549sh

550sh

578br

573br

412

496s

422s

420br

–

The peak wavenumbers are presented in cm

423s

419sh

423s

407

401

–

sh

–

–

d/M–O-bd

-1

Band characteristics s small, vs very small, sh shoulder, br broad. Vibration modes st stretching mode, bd bending mode

Table 4 Dm values for SrSn1-xTixO3 samples 300 °C

(a)

400 °C

* SrCO3

Dm1

Dm2

Dm1

Dm2

SrSnO3

180

246

133

Not exist

SrSn0.75Ti0.25O3

171

244

120

Not exist

SrSn0.50Ti0.50O3

177

245

102

Not exist

SrSn0.25Ti0.75O3

176

248

105

Not exist

SrTiO3

185

244

133

Not exist

Intensity/a.u.

Samples

Dm1 = m(C=O) - m1(C-O) and Dm2 = m(C=O) - m2(C–O)

600 °C

* * *

500 °C 400 °C 300 °C

15 20 25

30 35

40 45

50

2θ /°

(b)

55 60

65

70 75 80

* SrCO3

700 °C

Intensity/a.u.

from the thermal decomposition of oxalates. In our study, we believe that the formation of these kinds of intermediates occurs followed by their decomposition leading to the formation of the respective perovskite-type oxide. It is also worthy to note that a small mass gain (0.04 %) is observed for SrTiO3 sample (Fig. 1a), confirmed by the base line deviation observed in DTG curve (Fig. 1b). This event is associated to the exothermic peak in the DTA curve (Fig. 1c). This behavior also was reported by Silva et al. [14] for SrTiO3 powders. Mass gain associated to an exothermic reaction is usually assigned to an oxidation process [28]. Moreover, the literature has reported the reduction of Ti4? in the lattice SrTiO3 at low temperatures. For instance, Y0.08Sr0.88TiO3 was synthesized by Puengjinda et al. [29] and the Ti3? presence was deduced from the temperature-programmed reduction (TPR) profile at 300 °C, due to the reduction of Ti4?. Khunrattanaphon et al. [30] synthesized novel mesoporous-assembled SrTixZr1-xO3 nanocrystals by a sol–gel process, and the TPR results for this system showed a reduction peak associated to Ti4? ? Ti3? at temperatures higher than

700 °C

600 °C

* *

500 °C 400 °C 300 °C

15 20 25 30 35

40 45

50 55 60 65

70 75 80

2θ /° Fig. 3 XRD patterns of the powders after annealing at different temperatures. a SrSnO3; b SrTiO3

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A. L. M. de Oliveira et al.

# perovskite

#

#

SrTiO3

Intensity/a.u.

SrSn0.25Ti0.75 O3

#

SrSn0.50Ti0.50O3

SrSn0.75Ti0.25 O3 #

SrSnO3 29

30 31

32 33

34 35

2θ /° Fig. 4 XRD patterns of all compositions after annealing at 500 °C, showing the main peak of the perovskite

400 °C. In another paper, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) confirmed the presence of Ti3? associated to oxygen vacancies in SrTiO32d samples [31]. According to the authors, the adsorption of oxygen leads to the oxidation of surface Ti3? to Ti4?, which was also confirmed by XPS results. When titanates are synthesized by polymeric precursor method [32, 33], as in the present study, techniques as XANES and EPR confirmed the presence of complex clusters [TiO5
V..O] and [SrO11
V..O] for samples heat treated at low temperatures. For higher temperatures of synthesis, these complex vacancies disappear or decrease. Considering the literature data, the mass gain observed for the SrTiO3 synthesized in our study is probably correlated to the elimination of oxygen vacancies. The IR spectra of the SrSnO3 and SrTiO3 powder precursors before (300 °C) and after annealing at different temperatures are presented in Fig. 2a and b, respectively. The assignments of the vibration bands are displayed in the Tables 2 and 3. The IR spectra of all powder precursors after calcinations at 300 and 400 °C show intense bands related to the ester groups (COO-) bonded to metals as a consequence of the transesterification reaction characteristic of the method. These vibrations are observed between 1,680 and 1,318 cm-1 indicating that different chelating complexes are formed [19, 20, 34, 35].

123

According to Nakamoto [34], these configurations can be determined by calculating the difference (Dm) between the m(C=O) and m(C–O) frequencies of the symmetric COO– groups. For values of Dm much greater than 164 cm-1 (which is the Dm value observed for the ionic CH3COO-), the carboxyl group is considered to present an unidentate coordination; for values of Dm smaller than 164 cm-1, a bidentate configuration is considered; if the values of Dm are close to the ionic CH3COO– one (164 cm-1), a bridging configuration seems to occur. Calculations of Dm values for the SrSn1-xTixO3 solid solution heat treated at 300 and 400 °C are listed in the Table 4. Based on the calculated values of Dm (Table 4), two types of coordination may be present in the metal-complexes formed between COO– groups and the cations (Sr2?, Sn4?, and/or Ti4?) at 300 °C: the unidentate and the bridging configurations, with a higher amount of the bridging one as m2(C–O) vibration corresponds to a shoulder in the IR spectra of all samples. After heat treatment at 400 °C, the profiles of the spectra change and the shoulder ascribed to the m2 vibration disappears indicating that the unidentate configuration does not occur anymore. A decrease in the calculated values of Dm1 was also observed, and for perovskites with the concomitant presence of Ti4? and Sn4?, a bidentate configuration may occur. A similar behavior was observed by Cho et al. [27] in (Sr, Ti)-organic precursors, where the nature of the bonding varies: unidentate ? bridging ? ionic. The bands assigned to esters are not observed after heat treatment at and above 500 °C. Comparing this result with the TG/DTA curves, it can be observed that the most intense exothermic peak observed in the DTA curves between 470 and 575 °C is assigned to the combustion of esters as suggested by [15, 22]. After annealing at 400 °C, bands assigned to carbonate groups (CO23 ) are observed at around 1,770, 1,440, 1,060, and 860 cm-1, being more intense for SrSnO3 sample. The intensity of these bands decreases with temperature increase, disappearing for SrTiO3 after heat treatment at 700 °C. As shown in the DTA curves, the elimination of these groups takes place at higher temperatures being in good agreement with these results. The different behavior of the carbonate bands in SrTiO3 and SrSnO3 is another indication of the formation of intermediate compounds in these systems. Broad bands associated to the MO23 (M = Sn and/or Ti) were observed from 780 to 400 cm-1 for all the precursors. This behavior indicates the presence of regions with short-range order besides others with shortrange disorder, i.e., differences in the crystal structures of the perovskites or in the symmetry of the octahedra, indicating the presence of symmetric and asymmetric MO23 [36, 39].

Effect of the composition on the thermal behavior

The vibrations for SrSnO3 are observed in the range of 600–700 and 300–400 cm-1, with Sn–O stretching vibrations centered at about 674 and 530 cm-1 [15, 37]. For Moreira et al. [38], asymmetric Sn–O–Sn stretching vibrations are observed at 593 and 576 cm-1. In the present study, these bands are observed between 782 and 548 cm-1 for samples heat treated at low temperatures. With the increasing temperature, these bands become more intense with a higher resolution leading to one band centered at 662 cm-1 indicating that a higher short range order occurs. When Ti4? replaces Sn4? in the SrSn1-xTixO3 solid solution, bands assigned to the M–O stretching vibrations are shifted to lower frequencies which characterize a change of the symmetry of the octahedra. The same behavior is observed by Last [39] for titanates and niobates. In the present study, for Ti4?-richer samples, these bands appear around 570 cm-1 indicating the presence of TiO23 with a higher symmetry, characteristic of the Ti4? cubic compounds. In addition, a higher definition of these bands is observed after heat treatment at 600 °C for SrTiO3 powders, indicating that a higher short range order is reached. Other bands associated to the M–O3 (M = Sn, Ti) vibrations are also observed at lower wavenumbers (440–300 cm-1), as shown in Tables 2 and 3. All these results are in good agreement with the literature [15, 16, 22, 40]. The XRD patterns of SrSnO3 and SrTiO3 powder precursors before and after annealing at different temperatures are presented in Fig. 3. XRD patterns show the presence of the amorphous phase for the precursors calcined at 300 °C, while a peak ascribed to the SrCO3 is observed after annealing at 400 °C as confirmed in the IR spectra. This peak is more evident for the Sn4?-richer samples. Moreover, the crystalline carbonate is completely eliminated after calcinations at 600 °C for SrTiO3, while it is still observed even after calcinations at 700 °C for SrSnO3. These results are in agreement with IR spectra (Fig. 2a, b). After annealing at higher temperatures, the desired perovskite phase is formed and the beginning of the crystallization depends on the material composition (Fig. 4). For Ti4?-richer samples, the crystallization begins in the temperature range of 400–500 °C and well defined peaks are already observed after heat treatment at 500 °C (Fig. 3). For SrSnO3 (Fig. 3) and SrSn0.5Ti0.5O3, small peaks are observed after calcination at 500 °C, indicating that crystallization had just begun. For SrSn0.75Ti0.25O3, no peaks assigned to perovskite are observed at this temperature while well defined ones are noticed after calcinations at 600 °C (not shown), indicating that crystallization started between these two temperatures—500 and 600 °C. For all of the samples, a high amount of mass loss is observed between 250 and 600 °C, with exothermic peaks due to the combustion of the chelating agents. We believe that the

crystallization peak is overlapped to these exothermic ones not being observed in the DTA curves. According to the literature, at room temperature the crystalline structures of SrSnO3 and SrTiO3 perovskites are orthorhombic with space group Pbnm [41] (JCPDS 22-1442) and cubic with space group Pm 3m [42] (JCPDS 35-0734), respectively.

Conclusions SrSn1-xTixO3 solid solution was successfully obtained at low temperature by the polymeric precursor method. In spite of the similarity of the TG curves for all the compositions, their thermal behavior had important differences according to the powder precursor composition. The increase of titanium content decreased the temperature of the organic matter combustion probably due to the formation of intermediate compounds prior to the perovskite crystallization. The combustion of the esters was more exothermic for Ti4?-richer samples. Furthermore, considering the literature data on SrTiO3, the mass gain observed in the TG curve of the powder precursor of this material was assigned to oxygen entrapment. All these results confirmed the importance of correlating the thermal analysis with other techniques in order to understand the thermal behavior and crystallization process of these perovskites. Acknowledgments The authors acknowledge CNPq/MCT, INCT/ CNPq/MCT, PROINFRA/FINEP and CAPES for supporting this work.

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