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Synthesis of Bi2O3 by controlled transformation rate thermal analysis: a new route for this oxide?
Solid State Ionics 157 (2003) 163 – 169 www.elsevier.com/locate/ssi
Synthesis of Bi2O3 by controlled transformation rate thermal analysis: a new route for this oxide? O. Monnereau*, L. Tortet, P. Llewellyn, F. Rouquerol, G. Vacquier Elaboration Group, MADIREL Laboratory (UMR 6121), Provence University and CNRS, Saint-Charles Centre (Case 26), 3, Place V. Hugo, 13331 Marseille Cedex 3, France Received 19 September 2001; received in revised form 16 January 2002; accepted 5 February 2002
Abstract Several kinds of bismuth (III) oxalate have been prepared, depending on the experimental conditions during their precipitation. Decomposition of these precursors is followed by CRTA techniques. Under vacuum, the nature of decomposition product depends on the nature of oxalate precursor whereas in air, h-Bi2O3 oxide is systematically obtained between 250 and 300 jC. This h-Bi2O3 phase is stable from ambient temperature to its transition into a-phase near 300 jC. Ionic and electronic conductivities of the obtained bismuth (III) oxide is measured by Complex Impedance Spectroscopy between 25 and 820 jC. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Anionic conduction; Bismuth oxide; Bismuth oxalate; Thermal analysis; Soft chemistry
1. Introduction Chemical routes have been intensively investigated during the last decade in connection with the development of high-Tc superconducting ceramics because these chemical techniques improve the homogeneity of the final product [1]. Bi-based superconducting ceramics have been widely obtained by using the oxalate coprecipitation techniques [2– 5]. Methods involving precursor like metal oxalates continue to attract considerable interest as an original approach to generate homogeneously dispersed solid state materials such as Bi4 Ti3O12
* Corresponding author. Tel.: +33-491-63-71-44. E-mail address: [email protected] (O. Monnereau).
[6] or nanocrystalline oxides such as, e.g. BaTiO3 [7] and PbZrO3 [8]. To our knowledge, only Polla et al. [9] give structural information on bismuth oxalates. They have clearly characterised two different varieties: Bi2 (C2O4)37H2O and Bi2(C2O4)3H2C2O4. Cell parameters have been proposed (ICCD files 38-548 and 38549) and thermal behaviour of these compounds has been studied by means of thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC). Bi2O3 exists under two stable polymorphic forms (monoclinic a-Bi2O3 and cubic y-Bi2O3), with a transition temperature a ! y at 730 jC. Two other metastable phases (tetragonal h-Bi2O3 and bcc gBi2O3) have been also characterized, depending on the cooling procedures [10]. High temperature yphase (730 < T < 825 jC) is well known for its very high ionic conductivity from oxide anionic mobility
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 2 0 4 - 7
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O. Monnereau et al. / Solid State Ionics 157 (2003) 163–169
[11]. Recently, Switzer et al. [12] have stabilised this fcc-form y-Bi2O3 at room temperature by electrodeposition on a gold substrate. The present work deals with the study of the decomposition of different bismuth (III) oxalate pre-
cursors by an original thermal technique, so-called Controlled transformation Rate Thermal Analysis (CRTA) [13]. These CRTA investigations permit us to obtain the h-Bi2O3 phase at temperature above 300 jC.
˚ ) of Ox(I) (a) and Ox(II) (b) bismuth oxalate compounds. Fig. 1. XRD diagrams (k = 1.541 A
O. Monnereau et al. / Solid State Ionics 157 (2003) 163–169
2. Synthesis and characterisation of Bismuth (III) oxalates 2.1. Synthesis Bi(NO3)35H2O and H2C2O42H2O initial compounds are commercial (Aldrich) powders. Bismuth nitrate (0.2 M) in nitric acid (1 M) and oxalic acid (0.3 M) solutions are strarting reactants. Both solutions are progressively added, with constant stirring, to an aqueous solution where pH is maintained to desired value during the precipitation. Precipitates are filtered and washed with ethanol, then carefully dried over anhydrous air at ambient temperature for two days before investigation. Under pH < 0.5 and ambient temperature, a white and very low soluble precipitate is obtained, named Ox(I) hereafter . Under 2 < pH < 3 and with an excess of bismuth (III) solution, a white and very fine precipitate is also obtained, named Ox(II). With an excess of oxalate solution during precipitation and with an heating up to 75 jC of the mixture during one hour (0.5 < pH < 1.0), Bi2(C2O4)3 H2C2O4 is obtained similarly to Polla et al. [9]. This product will be named Ox(III). 2.2. Characterisation Products have been chemically analysed and characterised by X-ray powder diffraction (XRD, Siemens D5000), scanning electron microscopy (SEM, Philips XL30 ESEM) and Fourier transform infrared spectroscopy (FTIR, Nicolet 630). Their thermal decomposition was studied by classical thermogravimetry and differential thermal analysis (TG-DTA, Setaram TGDTA 92). These global results will be published soon and we present here only results essential for the purposes of this article. Neither XRD diagrams for Ox(I) and Ox(II) correspond to known ICCD files. Both are presented in
165
Fig. 1. Ox(III) XRD diagram is similar to Polla’s one for Bi2(C2O4)3H2C2O4 [9]. Chemical analysis of these products is realised by colorimetry with thiourea for bismuth and by titration with KMnO4 for oxalate ion. Total weight loss is determined after 6 h at 350 jC. All the results are reported in Table 1 for the two obtained unknown oxalates and a formula is proposed for each compound, based on these results and corroborated by FTIR and TG-DTA measured and complete XRD studies [14].
3. Controlled transformation rate thermal analysis The CRTA method is an original technique initially developed by Rouquerol in 70’s [15], which consists in measuring of a property directly linked to the transformation rate of the sample and controlling the sample heating by the variation of this property. In our case, the pressure of the gaseous phase is this monitoring property. Also, this technique authorises a finer approach of the thermal decomposition. Here, CRTA experiences have been made under a vacuum and under air atmospheric pressure in order to survey and to control the precursor decomposition. The complete study of the thermal decomposition of Bi2(C2O4)3 7H2O bismuth oxalate (so-called Ox-I here) by CRTA in air and under vacuum has been recently published [16]. 3.1. CRTA under vacuum The three bismuth oxalates have been first investigated under vacuum. Oxalate powder (c 80 mg) is introduced in a silica cell into a furnace. After slowly degassing of the cell up to secondary vacuum, investigation can begin when the system is stable. An automatic system permits to control the pressure and
Table 1 Chemical analysis of the obtained bismuth (III) oxalates and proposed formula Bi (wt.%)
Ox(I) Ox(II)
C2O4 (wt.%)
Total weight loss (wt.%)
Proposed formula
Exp.
Theor.
Exp.
Theor.
Exp.
Theor.
48 F 2 64 F 2
51.7 64.7
32.5 F 0.5 27.6 F 0.5
32.7 27.2
43.8 F 0.5 28.0 F 0.5
42.2 27.8
Bi2(C2O4)37H2O (BiOHC2O4)2H2O
166
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Fig. 2. CRTA curves corresponding to the thermal decomposition under vacuum of the bismuth oxalates: (a) Ox(I), (b) Ox(II) and (c) Ox(III).
O. Monnereau et al. / Solid State Ionics 157 (2003) 163–169
167
˚ ) of the h-Bi2O3 oxide obtained at 268 jC after Ox(I) decomposition during CRTA under air. It corresponds Fig. 3. XRD diagram (k = 1.541 A exactly with the ICCD file 27-0050.
monitors the temperature in order to keep a constant pressure of 510 3 mbar above the sample. When the final temperature is reached, the sample weight loss is measured and its characterisation is realised by XRD and FTIR. Fig. 2 presents the CRTA curves of the three oxalates: they show the critical decomposition temperatures and two different decomposition domains (I and II) can be defined in each case. These results allow to propose a thermal decomposition in two steps: (i) domain (I) corresponds to the dehydration of Ox(I) and Ox(II) and to the departure of H2C2O4 entities in the case of Ox(III). (ii) domain (II) corresponds to the gradual transformation of the dehydrated oxalate into Bi by loss of carbon dioxide in the case of Ox(I) and Ox (III), following the chemical reaction Bi2(C2O4)3 ! 2Bi + 6CO2(g). But in the case of Ox (II), the thermolysis leads to a mixture of Bi and h-Bi2O3.
CRTA, the basic idea is to control the sample temperature so as to keep constant a parameter related to the decomposition rate. As a reactive gas is introduced in the cell, the controlled parameter has to be a DP between test and sample cells. In previous works about Bi2(C2O4)37H2O compound [16], we have shown in a first step the oxalate dehydration. In a second step, hBi2O3 was obtained at 269 jC after 30 h. CRTA investigations under air atmosphere for Ox(II) and Ox(III) have been realised in the same conditions and the same results have been obtained: h-Bi2O3 oxide is formed at temperature lower than 300 jC. XRD diagram of this: h-Bi2O3 oxide is reported on Fig. 3: its corresponds exactly with the ICCD file 27-0050.
3.2. CRTA under air atmospheric pressure
Electrical properties have been investigated by complex impedance spectroscopy, using a 6310EGG impedance analyzer. Measurements have been realized on frequency range 10 –105 Hz, and between 25 and 810 jC under air. The electrical properties of bismuth (III) oxide are well known [10,11], the conductivity of the h-phase
Under air, the thermal decomposition mechanism will be quite different because O2 will be here a reactive gas. A more complex CRTA apparatus able to work under corrosive gases has been realised by Fulconis [17] in our laboratory. As for the classical
4. Electrical properties of the obtained bismuth (III) oxide
168
O. Monnereau et al. / Solid State Ionics 157 (2003) 163–169
presents a mainly electronic conduction, with Ea = 0.3 eV. We have not be able to observe here the low temperature behavior of the h-Bi2O3 phase : below 300 jC (on heating direction) the sample is insulator. We can suppose that the applied sintering temperature (270 jC) is too weak to allow sufficient sintering and accurate electrical measurements. We hope to be successful in these measurements by hot pressed sintering this h-Bi2O3 oxide under 270 jC and 10 kbar. 5. Conclusion
Fig. 4. Arrhenius plots of h-Bi2O3 oxide obtained at 268 jC by oxalate decomposition in air.
appearing only in cooling direction, between 650 and 350 jC. Here, a new experiment has been tried: we begin the electrical measurements on a pellet of hBi2O3 obtained after decomposition under air of the Ox(I) precursor. This h-Bi2O3 powder is compacted under a pressure of 10 kbar, then sintered at only 270 jC in order to keep the h structure. By this procedure, we hoped to observe the electrical transition in the heating direction between the electrical properties of the a (Ea = 0.64 eV) and h (Ea = 0.3 eV) phases of this oxide. Fig. 4 shows Arrhenius plots of conductivity on heating and cooling, where we can observe the classical electrical behavior of Bi2O3: (i) on heating, the sample is insulator up the h ! a transition around 300 jC, then up to the a ! y transition (730 jC) the conduction of a-Bi2O3 is electronic with an activation energy Ea = 0.64 eV. Above 730 jC, the well known ionic conductivity of y-Bi2O3 appears. (ii) on cooling, the y-Bi2O3 phase remains down to 650 jC. Between 650 and 350 jC, the h-Bi2O3
Three different Bismuth (III) oxalates have been prepared, depending on the experimental conditions during their precipitation. Decomposition of these precursors was followed by CRTA technique. Under vacuum, the nature of the decomposition products depends on the precursor: we have obtained pure bismuth with the Ox(I) and Ox(III) oxalates, and hBi2O3 with Ox(II). In air, the h-Bi2O3 oxide is systematically obtained between 250 and 300 jC, whatever the starting oxalate compound. This hBi2O3 phase is stable from ambient temperature to its transition into the a phase near 300 jC. Ionic and electronic conductivities of the obtained bismuth (III) oxide is measured by Complex Impedance Spectroscopy between 25 and 820 jC. We have not observed the low temperature behavior of the hBi2O3 phase: below 300 jC (on heating) the sample is insulator. We can suppose that the sintering temperature (270 jC) is too weak to permit a good sintering and accurate electrical measurements. In this work, we have shown the applicability of the CRTA technique to elucidate the different steps involved during a decomposition process. By a choice of different precursors and by a good knowledge of the decomposition steps, it is possible to control precisely the process and to promote new phases or stabilise metastable ones, like h-Bi2O3 in this case. References [1] Yu.G. Metlin, Yu.D. Tretyakov, J. Mater. Chem. 4 (1994) 1659. [2] C.Y. Shei, R.S. Liu, C.T. Chang, P.T. Wu, Mater. Lett. 9 (1990) 105.
O. Monnereau et al. / Solid State Ionics 157 (2003) 163–169 [3] H. Kitaguchi, F. Shitani, T. Egi, K. Oda, J. Takada, A. Osaka, Y. Miura, Y. Ikeda, M. Tanako, Mol. Cryst. Liq. Cryst. 184 (1990) 129. [4] M. Mansori, P. Satre, C. Breandon, M. Roubin, A. Sebaoun, Ann. Chim. (Fr) 18 (1993) 537. [5] C. Lang, S. Gauss, W. Becker, M. Rupich, W. Carter, in: U. Balachandra, E.W. Collins, A. Goyal (Eds.), Process. Long Lenghts Supercond. Proc. Symp., 1994, pp. 175. [6] M. Villegas, C. Moure, J.F. Fernandez, P. Duran, J. Mater. Sci. 31 (1996) 949. [7] M. Loue¨r, D. Loue¨r, F.J. Gotor, J.M. Criado, J. Solid State Chem. 92 (1991) 565. [8] C. Boudaren, J.P. Auffre´dic, M. Loue¨r, D. Loue¨r, Chem. Mater. 12 (2000) 2324. [9] G. Polla, R.F. Baggio, E. Manghi, P.K. De Perzzo, J. Cryst. Growth 67 (1984) 68.
169
[10] H.A. Harwig, A.G. Gerards, Thermochim. Acta 28 (1979) 121. [11] T. Takahashi, H. Iwahara, Y. Nagai, J. Appl. Electrochem. 2 (1972) 97. [12] J.A. Switzer, M.G. Shumsky, E.W. Bohnnan, Science 284 (1999) 293. [13] J. Rouquerol, Thermochem. Acta 144 (1989) 209. [14] O. Monnereau, L. Tortet, P. Conflant, G. Vacquier, submitted for publication to J. Mater. Chem. [15] J. Rouquerol, J. Therm. Anal. 2 (1970) 123. [16] E. Diez, O. Monnereau, L. Tortet, G. Vacquier, P. Llewellyn, F. Rouquerol, J. Optoelectron. Adv. Mater. 2 (2000) 552. [17] T. Gilardi, Thesis (1992); J.M. Fulconis, Thesis (1996), Universite´ de Provence, Marseille.
Synthesis of Bi2O3 by controlled transformation rate thermal analysis: a new route for this oxide? O. Monnereau*, L. Tortet, P. Llewellyn, F. Rouquerol, G. Vacquier Elaboration Group, MADIREL Laboratory (UMR 6121), Provence University and CNRS, Saint-Charles Centre (Case 26), 3, Place V. Hugo, 13331 Marseille Cedex 3, France Received 19 September 2001; received in revised form 16 January 2002; accepted 5 February 2002
Abstract Several kinds of bismuth (III) oxalate have been prepared, depending on the experimental conditions during their precipitation. Decomposition of these precursors is followed by CRTA techniques. Under vacuum, the nature of decomposition product depends on the nature of oxalate precursor whereas in air, h-Bi2O3 oxide is systematically obtained between 250 and 300 jC. This h-Bi2O3 phase is stable from ambient temperature to its transition into a-phase near 300 jC. Ionic and electronic conductivities of the obtained bismuth (III) oxide is measured by Complex Impedance Spectroscopy between 25 and 820 jC. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Anionic conduction; Bismuth oxide; Bismuth oxalate; Thermal analysis; Soft chemistry
1. Introduction Chemical routes have been intensively investigated during the last decade in connection with the development of high-Tc superconducting ceramics because these chemical techniques improve the homogeneity of the final product [1]. Bi-based superconducting ceramics have been widely obtained by using the oxalate coprecipitation techniques [2– 5]. Methods involving precursor like metal oxalates continue to attract considerable interest as an original approach to generate homogeneously dispersed solid state materials such as Bi4 Ti3O12
* Corresponding author. Tel.: +33-491-63-71-44. E-mail address: [email protected] (O. Monnereau).
[6] or nanocrystalline oxides such as, e.g. BaTiO3 [7] and PbZrO3 [8]. To our knowledge, only Polla et al. [9] give structural information on bismuth oxalates. They have clearly characterised two different varieties: Bi2 (C2O4)37H2O and Bi2(C2O4)3H2C2O4. Cell parameters have been proposed (ICCD files 38-548 and 38549) and thermal behaviour of these compounds has been studied by means of thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC). Bi2O3 exists under two stable polymorphic forms (monoclinic a-Bi2O3 and cubic y-Bi2O3), with a transition temperature a ! y at 730 jC. Two other metastable phases (tetragonal h-Bi2O3 and bcc gBi2O3) have been also characterized, depending on the cooling procedures [10]. High temperature yphase (730 < T < 825 jC) is well known for its very high ionic conductivity from oxide anionic mobility
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 2 0 4 - 7
164
O. Monnereau et al. / Solid State Ionics 157 (2003) 163–169
[11]. Recently, Switzer et al. [12] have stabilised this fcc-form y-Bi2O3 at room temperature by electrodeposition on a gold substrate. The present work deals with the study of the decomposition of different bismuth (III) oxalate pre-
cursors by an original thermal technique, so-called Controlled transformation Rate Thermal Analysis (CRTA) [13]. These CRTA investigations permit us to obtain the h-Bi2O3 phase at temperature above 300 jC.
˚ ) of Ox(I) (a) and Ox(II) (b) bismuth oxalate compounds. Fig. 1. XRD diagrams (k = 1.541 A
O. Monnereau et al. / Solid State Ionics 157 (2003) 163–169
2. Synthesis and characterisation of Bismuth (III) oxalates 2.1. Synthesis Bi(NO3)35H2O and H2C2O42H2O initial compounds are commercial (Aldrich) powders. Bismuth nitrate (0.2 M) in nitric acid (1 M) and oxalic acid (0.3 M) solutions are strarting reactants. Both solutions are progressively added, with constant stirring, to an aqueous solution where pH is maintained to desired value during the precipitation. Precipitates are filtered and washed with ethanol, then carefully dried over anhydrous air at ambient temperature for two days before investigation. Under pH < 0.5 and ambient temperature, a white and very low soluble precipitate is obtained, named Ox(I) hereafter . Under 2 < pH < 3 and with an excess of bismuth (III) solution, a white and very fine precipitate is also obtained, named Ox(II). With an excess of oxalate solution during precipitation and with an heating up to 75 jC of the mixture during one hour (0.5 < pH < 1.0), Bi2(C2O4)3 H2C2O4 is obtained similarly to Polla et al. [9]. This product will be named Ox(III). 2.2. Characterisation Products have been chemically analysed and characterised by X-ray powder diffraction (XRD, Siemens D5000), scanning electron microscopy (SEM, Philips XL30 ESEM) and Fourier transform infrared spectroscopy (FTIR, Nicolet 630). Their thermal decomposition was studied by classical thermogravimetry and differential thermal analysis (TG-DTA, Setaram TGDTA 92). These global results will be published soon and we present here only results essential for the purposes of this article. Neither XRD diagrams for Ox(I) and Ox(II) correspond to known ICCD files. Both are presented in
165
Fig. 1. Ox(III) XRD diagram is similar to Polla’s one for Bi2(C2O4)3H2C2O4 [9]. Chemical analysis of these products is realised by colorimetry with thiourea for bismuth and by titration with KMnO4 for oxalate ion. Total weight loss is determined after 6 h at 350 jC. All the results are reported in Table 1 for the two obtained unknown oxalates and a formula is proposed for each compound, based on these results and corroborated by FTIR and TG-DTA measured and complete XRD studies [14].
3. Controlled transformation rate thermal analysis The CRTA method is an original technique initially developed by Rouquerol in 70’s [15], which consists in measuring of a property directly linked to the transformation rate of the sample and controlling the sample heating by the variation of this property. In our case, the pressure of the gaseous phase is this monitoring property. Also, this technique authorises a finer approach of the thermal decomposition. Here, CRTA experiences have been made under a vacuum and under air atmospheric pressure in order to survey and to control the precursor decomposition. The complete study of the thermal decomposition of Bi2(C2O4)3 7H2O bismuth oxalate (so-called Ox-I here) by CRTA in air and under vacuum has been recently published [16]. 3.1. CRTA under vacuum The three bismuth oxalates have been first investigated under vacuum. Oxalate powder (c 80 mg) is introduced in a silica cell into a furnace. After slowly degassing of the cell up to secondary vacuum, investigation can begin when the system is stable. An automatic system permits to control the pressure and
Table 1 Chemical analysis of the obtained bismuth (III) oxalates and proposed formula Bi (wt.%)
Ox(I) Ox(II)
C2O4 (wt.%)
Total weight loss (wt.%)
Proposed formula
Exp.
Theor.
Exp.
Theor.
Exp.
Theor.
48 F 2 64 F 2
51.7 64.7
32.5 F 0.5 27.6 F 0.5
32.7 27.2
43.8 F 0.5 28.0 F 0.5
42.2 27.8
Bi2(C2O4)37H2O (BiOHC2O4)2H2O
166
O. Monnereau et al. / Solid State Ionics 157 (2003) 163–169
Fig. 2. CRTA curves corresponding to the thermal decomposition under vacuum of the bismuth oxalates: (a) Ox(I), (b) Ox(II) and (c) Ox(III).
O. Monnereau et al. / Solid State Ionics 157 (2003) 163–169
167
˚ ) of the h-Bi2O3 oxide obtained at 268 jC after Ox(I) decomposition during CRTA under air. It corresponds Fig. 3. XRD diagram (k = 1.541 A exactly with the ICCD file 27-0050.
monitors the temperature in order to keep a constant pressure of 510 3 mbar above the sample. When the final temperature is reached, the sample weight loss is measured and its characterisation is realised by XRD and FTIR. Fig. 2 presents the CRTA curves of the three oxalates: they show the critical decomposition temperatures and two different decomposition domains (I and II) can be defined in each case. These results allow to propose a thermal decomposition in two steps: (i) domain (I) corresponds to the dehydration of Ox(I) and Ox(II) and to the departure of H2C2O4 entities in the case of Ox(III). (ii) domain (II) corresponds to the gradual transformation of the dehydrated oxalate into Bi by loss of carbon dioxide in the case of Ox(I) and Ox (III), following the chemical reaction Bi2(C2O4)3 ! 2Bi + 6CO2(g). But in the case of Ox (II), the thermolysis leads to a mixture of Bi and h-Bi2O3.
CRTA, the basic idea is to control the sample temperature so as to keep constant a parameter related to the decomposition rate. As a reactive gas is introduced in the cell, the controlled parameter has to be a DP between test and sample cells. In previous works about Bi2(C2O4)37H2O compound [16], we have shown in a first step the oxalate dehydration. In a second step, hBi2O3 was obtained at 269 jC after 30 h. CRTA investigations under air atmosphere for Ox(II) and Ox(III) have been realised in the same conditions and the same results have been obtained: h-Bi2O3 oxide is formed at temperature lower than 300 jC. XRD diagram of this: h-Bi2O3 oxide is reported on Fig. 3: its corresponds exactly with the ICCD file 27-0050.
3.2. CRTA under air atmospheric pressure
Electrical properties have been investigated by complex impedance spectroscopy, using a 6310EGG impedance analyzer. Measurements have been realized on frequency range 10 –105 Hz, and between 25 and 810 jC under air. The electrical properties of bismuth (III) oxide are well known [10,11], the conductivity of the h-phase
Under air, the thermal decomposition mechanism will be quite different because O2 will be here a reactive gas. A more complex CRTA apparatus able to work under corrosive gases has been realised by Fulconis [17] in our laboratory. As for the classical
4. Electrical properties of the obtained bismuth (III) oxide
168
O. Monnereau et al. / Solid State Ionics 157 (2003) 163–169
presents a mainly electronic conduction, with Ea = 0.3 eV. We have not be able to observe here the low temperature behavior of the h-Bi2O3 phase : below 300 jC (on heating direction) the sample is insulator. We can suppose that the applied sintering temperature (270 jC) is too weak to allow sufficient sintering and accurate electrical measurements. We hope to be successful in these measurements by hot pressed sintering this h-Bi2O3 oxide under 270 jC and 10 kbar. 5. Conclusion
Fig. 4. Arrhenius plots of h-Bi2O3 oxide obtained at 268 jC by oxalate decomposition in air.
appearing only in cooling direction, between 650 and 350 jC. Here, a new experiment has been tried: we begin the electrical measurements on a pellet of hBi2O3 obtained after decomposition under air of the Ox(I) precursor. This h-Bi2O3 powder is compacted under a pressure of 10 kbar, then sintered at only 270 jC in order to keep the h structure. By this procedure, we hoped to observe the electrical transition in the heating direction between the electrical properties of the a (Ea = 0.64 eV) and h (Ea = 0.3 eV) phases of this oxide. Fig. 4 shows Arrhenius plots of conductivity on heating and cooling, where we can observe the classical electrical behavior of Bi2O3: (i) on heating, the sample is insulator up the h ! a transition around 300 jC, then up to the a ! y transition (730 jC) the conduction of a-Bi2O3 is electronic with an activation energy Ea = 0.64 eV. Above 730 jC, the well known ionic conductivity of y-Bi2O3 appears. (ii) on cooling, the y-Bi2O3 phase remains down to 650 jC. Between 650 and 350 jC, the h-Bi2O3
Three different Bismuth (III) oxalates have been prepared, depending on the experimental conditions during their precipitation. Decomposition of these precursors was followed by CRTA technique. Under vacuum, the nature of the decomposition products depends on the precursor: we have obtained pure bismuth with the Ox(I) and Ox(III) oxalates, and hBi2O3 with Ox(II). In air, the h-Bi2O3 oxide is systematically obtained between 250 and 300 jC, whatever the starting oxalate compound. This hBi2O3 phase is stable from ambient temperature to its transition into the a phase near 300 jC. Ionic and electronic conductivities of the obtained bismuth (III) oxide is measured by Complex Impedance Spectroscopy between 25 and 820 jC. We have not observed the low temperature behavior of the hBi2O3 phase: below 300 jC (on heating) the sample is insulator. We can suppose that the sintering temperature (270 jC) is too weak to permit a good sintering and accurate electrical measurements. In this work, we have shown the applicability of the CRTA technique to elucidate the different steps involved during a decomposition process. By a choice of different precursors and by a good knowledge of the decomposition steps, it is possible to control precisely the process and to promote new phases or stabilise metastable ones, like h-Bi2O3 in this case. References [1] Yu.G. Metlin, Yu.D. Tretyakov, J. Mater. Chem. 4 (1994) 1659. [2] C.Y. Shei, R.S. Liu, C.T. Chang, P.T. Wu, Mater. Lett. 9 (1990) 105.
O. Monnereau et al. / Solid State Ionics 157 (2003) 163–169 [3] H. Kitaguchi, F. Shitani, T. Egi, K. Oda, J. Takada, A. Osaka, Y. Miura, Y. Ikeda, M. Tanako, Mol. Cryst. Liq. Cryst. 184 (1990) 129. [4] M. Mansori, P. Satre, C. Breandon, M. Roubin, A. Sebaoun, Ann. Chim. (Fr) 18 (1993) 537. [5] C. Lang, S. Gauss, W. Becker, M. Rupich, W. Carter, in: U. Balachandra, E.W. Collins, A. Goyal (Eds.), Process. Long Lenghts Supercond. Proc. Symp., 1994, pp. 175. [6] M. Villegas, C. Moure, J.F. Fernandez, P. Duran, J. Mater. Sci. 31 (1996) 949. [7] M. Loue¨r, D. Loue¨r, F.J. Gotor, J.M. Criado, J. Solid State Chem. 92 (1991) 565. [8] C. Boudaren, J.P. Auffre´dic, M. Loue¨r, D. Loue¨r, Chem. Mater. 12 (2000) 2324. [9] G. Polla, R.F. Baggio, E. Manghi, P.K. De Perzzo, J. Cryst. Growth 67 (1984) 68.
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