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Effect of isovalent doping on the high temperature thermopower and resistivity properties of Ba2BiInO6
Solid State Communications 149 (2009) 1735–1738
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Effect of isovalent doping on the high temperature thermopower and resistivity properties of Ba2 BiInO6 Krishnendu Biswas, U.V. Varadaraju ∗ Materials Science Research Centre and Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
article
info
Article history: Received 28 March 2009 Received in revised form 30 June 2009 Accepted 21 July 2009 by D.D. Sarma Available online 24 July 2009
abstract Ba2 BiInO6 is a semiconductor which can be derived from Ba2 Bi3+ Bi5+ O6 by substituting all the Bi3+ ions. Presently we report on the isovalent substitution of Sb5+ at Bi5+ site. Sb acts as a sintering aid as well as a dopant. Doping results in an increase in the resistivity as well as thermopower. All the doped compositions show degenerate semiconducting behavior at high temperature. The highest figure of merit obtained is 2.4 × 10−5 K−1 at 770 K for the x = 0.06 composition. © 2009 Elsevier Ltd. All rights reserved.
PACS: 72.15.Jf Keywords: A. Oxides A. Semiconductors C. Double perovskites D. Resistivity
1. Introduction Thrust on unconventional energy sources is increasing due to the increase in demand of energy in excess of production. Thermoelectric materials form one of the best sources in this category since they convert heat energy to electrical energy directly and vice versa. The research on these materials has been given immense importance during the last two decades as they can be used to employ vast resource of waste heat, to produce electrical energy in a ‘‘green’’ way. The efficiency of such materials is governed by the figure of merit Z = S 2 σ /λ, where S is the Seebeck coefficient, σ is the electrical conductivity and λ is the thermal conductivity. To obtain greater conversion efficiencies the figure of merit should be as high as possible which should be greater than unity for commercial applications. Recently, oxide compounds have attracted attention as promising thermoelectric materials because of their high temperature stability and nontoxic nature. Ternary oxides with layered structure like NaCo2 O4 [1], misfit compounds like Ca3 Co4 O9 [2] and Bi2 Sr2 Co2 Oy [3], have been reported as particularly strong candidates. Among oxides, perovskites (ABO3 ) are an interesting class of compounds because of their versatile physical properties brought about by facile crystal chemical substitutions. Androulaki, et al., have recently
∗
Corresponding author. Tel.: +91 44 22574215. E-mail address: [email protected] (U.V. Varadaraju).
0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.07.031
reported a perovskite La0.95 Sr0.05 CoO3 [4] which has a ZT value of 0.2 at room temperature. The electrical transport properties of the perovskites depend, mainly on the B–O–B interactions. An interesting semiconducting phase is BaBiO3 which has a perovskite structure with Bi disproportionating into Bi5+ and Bi3+ . These ions due to their large difference in ionic radii form an ordered arrangement which is equivalent to a commensurate charge density wave [5]. Due to the difference in Bi3+ –O and Bi5+ –O bond lengths, the BiO6 octahedra are tilted which gives rise to low Bi–O–Bi bond angles and therefore, the insulating nature of the compound. Recently, Fu et al. [6] observed that with increasing In concentration in BaBi1−x Inx O3 there is a phase transition from monoclinic (for x = 0) to cubic (for x = 0.5) which is accompanied by an increase in the electrical conductivity. We have taken up the study of the compound Ba2 BiInO6 for thermoelectric applications and the effect of doping isovalent ions like Sb5+ at the Bi5+ site on the high temperature thermoelectric and resistivity properties. 2. Experimental Polycrystalline phases of Ba2 Bi1−x Sbx InO6 (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.3, 0.4) are synthesized by conventional high temperature solid state reaction. High purity BaCO3 , In2 O3 , Sb2 O3 and Bi2 O3 were taken in stoichiometric amounts and finely ground in a mortar and pestle for one hour. The powders were then heated slowly (to minimize volatilization of Sb2 O3 ) in alumina crucibles to 900 ◦ C and kept at this temperature for 24 h followed by oxygen
1736
K. Biswas, U.V. Varadaraju / Solid State Communications 149 (2009) 1735–1738
x = 0.3 x = 0.2
x = 0.1
Intensity (a.u.)
x = 0.08
x = 0.06
x = 0.04
10
20
30
40 2θ (degrees)
50
Fig. 2. Lattice parameter variation in the compositions Ba2 Bi1−x Sbx InO6 . (440)
(422)
(420)
(400)
(200)
x = 0.0
(220)
x = 0.02
60
70
Fig. 1. XRD patterns of Ba2 Bi1−x Sbx InO6 (* indicates secondary phase of Ba2 InSbO6 ).
treatment at 1050 ◦ C for 12 h. The powders were made into 8 mm dia pellets by applying pressure of 60 bars and sintered at 1050 ◦ C for 10 h under oxygen flow. The phase purity was checked by powder XRD patterns (Rigaku, Miniflex). The high temperature thermopower in the range 400 K–800 K was measured by a steady state method wherein the sample was kept between two Pt plates welded to two Pt, Pt-13%Rh thermocouples. The thermo-emf generated and the temperature difference were measured using a Keithley 182 nVm connected through a Keithley 705 scanner. The resistivity was measured in the same temperature range by Van der Pauw four probe method. 3. Results and discussion All the compositions Ba2 Bi1−x Sbx InO6 form a single phase for x ≤ 0.1 as shown in Fig. 1. The powder XRD patterns are indexed based on the JCPDS file no. 480061. The plot of lattice parameter vs. Sb concentration (Fig. 2) shows a linear decrease in the cubic lattice parameter with increasing Sb concentration following Vegard’s law. This indicates that Sb is doped in the lattice as Sb5+ since ionic radius of Bi5+ (0.76 Å) > Sb5+ (0.61 Å). The solubility limit of Sb is found to be 10 mol% above which a secondary phase formation takes place. This secondary phase is identified as Ba2 InSbO6 (indicated in the XRD pattern as *). The resistivity vs. temperature plots shown in Fig. 3 indicates a degenerate semiconducting nature for all the compositions at high temperatures. The resistivity reduces with 2 mol% Sb doping. It is known that Sb2 O3 can act as a sintering aid, and hence minor doping facilitates elimination of grain boundaries thereby decreasing resistivity. SEM pictures (Fig. 4) reveal the fusion of the grain boundaries in the 2 mol% doped phase vis a vis the undoped phase. Further increase in Sb content results in increase in resistivity, (with reference to the 2 mol% composition) which is also indicated by the increase in activation energy with Sb concentration as shown in Fig. 5. A possible explanation can be the existence of overlap of empty Bi 6s orbital and the O 2p orbital at the Fermi level in the parent compound. This is drawn from the explanation for the metallic behavior of BaPbO3 by Sleight et al. [7]. Sb5+ is devoid of 6s orbitals and hence with increasing Sb content more Bi5+ are being substituted as evidenced
Fig. 3. Resistivity vs. temperature plots for the compositions Ba2 Bi1−x Sbx InO6 .
by the variation in lattice parameters which leads to reduction in overlapping orbitals and hence the resistivity increases. An interesting feature is that the conduction mechanism is thermally activated type for lower doping level of Sb and changes to 3D variable range hopping type for higher concentrations of Sb (Fig. 6). Such high temperature VRH conduction is known for layered copper oxides [8]. It is well known that in heavily doped systems considerable number of inter band levels are created which gives rise to the VRH mechanism of conduction. In the present case since the dopant is distributed randomly and the unit cell is 3D in character it is only to be expected that the VRH will be in 3 dimensions (3D VRH). The high temperature thermopower plots for all the compositions (Fig. 7) show negative Seebeck coefficients indicating that electrons are the majority carriers. The variation in thermopower with Sb doping is similar to that of the resistivity behavior. Among the doped compositions, there is a systematic increase in the thermopower values with increasing concentration of Sb. This can be attributed to the increase in effective mass of the electrons as the empty 6s overlapping orbitals of Bi5+ decreases with increasing Sb5+ concentration. In all cases, the change in thermopower with temperature is not significant, a feature necessary for a thermoelectric material. It is known that in oxide materials the thermal conductivity is dominated by almost 60%–70% contribution from phonons whereas 30%–40% is from the electrons. Thus, since the lattice predominantly
K. Biswas, U.V. Varadaraju / Solid State Communications 149 (2009) 1735–1738
1737
Fig. 4. SEM images for the compositions Ba2 Bi1−x Sbx InO6 for x = 0.0 and x = 0.02.
Fig. 5. Activation energy vs. Sb concentration plots for the compositions Ba2 Bi1−x Sbx InO6 .
Fig. 7. Thermopower vs. temperature plots for the compositions Ba2 Bi1−x Sbx InO6 .
remains unchanged after doping, we can assume the thermal conductivity value not to be very different for the various Sb doped compositions. The usual ball park value of thermal conductivity of many perovskite related oxides is 1 Wm−1 K−1 . Using this, the Z values (Fig. 8) are calculated and the highest figure of merit is found to be 2.4 × 10−5 K−1 for the x = 0.06 composition at 770 K.
a
4. Conclusions We have studied the effect of Sb doping in the double perovskite Ba2 Bi1−x Sbx InO6 on the high temperature thermoelectric and resistivity properties. We find that the solubility limit of Sb at
b
1.2
0.8
x = 0.04 x = 0.0
0.6
0.5
0.4
log σ
log σ
x = 0.06 x = 0.1
1.0
1.0
0.2
0.0
0.0 -0.5
-0.2 -0.4
-1.0 -0.6 1.2
1.4
1.6
1.8 1000/T
2.0
2.2
2.4
2.6
0.185 0.190 0.195 0.200 0.205 0.210 0.215 0.220 0.225 1/T1/4
Fig. 6. Arrhenius plots for the compositions Ba2 Bi1−x Sbx InO6 (a) x = 0.0 and 0.04 (b) x = 0.06 and 0.1.
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K. Biswas, U.V. Varadaraju / Solid State Communications 149 (2009) 1735–1738
sintering aid at low concentration and for higher concentrations, lattice doping takes place leading to increase in resistivity. This is attributed to the reduction in empty Bi5+ 6s orbitals with increasing Sb5+ ion concentration. High temperature resistivity measurements show a degenerate semiconducting behavior and with increasing Sb concentration the conduction mechanism changes from activation to VRH type. The highest figure of merit obtained is 2.4 × 10−5 K−1 at 770 K for the x = 0.06 composition. References
Fig. 8. Z vs. T plots for the compositions Ba2 Bi1−x Sbx InO6 .
the Bi site is 10 mol%. On further Sb doping we observe that phase segregation takes place. Sb addition has the effect of a
[1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685. [2] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, J. Hejtmanek, Phys. Rev. B 62 (2000) 166. [3] R. Funahashi, I. Matsubara, S. Sodeoka, Appl. Phys. Lett. 76 (2000) 2385. [4] J. Androulaki, P. Migiakis, J. Giapintzakis, App. Phys. Lett. 84 (7) (2004) 1099. [5] Shiyou Pei, J.D. Jorgensen, B. Dabrowski, D.G. Hinks, D.R. Richards, A.W. Michell, J.M. Newsam, S.K. Sinha, D. Vaknin, A.J. Jacobson, Phys. Rev. B 41 (1990) 4126. [6] W.T. Fu, M.J. Polderman, F.M. Mulder, Mat. Res. Bul. 35 (8) (2000) 1205. [7] A.W. Sleight, J.L. Gillson, P.E. Bierstedt, Solid State Comm. 17 (1975) 27–28. [8] V. Ponnambalam, U.V. Varadaraju, Phys. Rev. B 52 (1995) 16213.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Effect of isovalent doping on the high temperature thermopower and resistivity properties of Ba2 BiInO6 Krishnendu Biswas, U.V. Varadaraju ∗ Materials Science Research Centre and Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
article
info
Article history: Received 28 March 2009 Received in revised form 30 June 2009 Accepted 21 July 2009 by D.D. Sarma Available online 24 July 2009
abstract Ba2 BiInO6 is a semiconductor which can be derived from Ba2 Bi3+ Bi5+ O6 by substituting all the Bi3+ ions. Presently we report on the isovalent substitution of Sb5+ at Bi5+ site. Sb acts as a sintering aid as well as a dopant. Doping results in an increase in the resistivity as well as thermopower. All the doped compositions show degenerate semiconducting behavior at high temperature. The highest figure of merit obtained is 2.4 × 10−5 K−1 at 770 K for the x = 0.06 composition. © 2009 Elsevier Ltd. All rights reserved.
PACS: 72.15.Jf Keywords: A. Oxides A. Semiconductors C. Double perovskites D. Resistivity
1. Introduction Thrust on unconventional energy sources is increasing due to the increase in demand of energy in excess of production. Thermoelectric materials form one of the best sources in this category since they convert heat energy to electrical energy directly and vice versa. The research on these materials has been given immense importance during the last two decades as they can be used to employ vast resource of waste heat, to produce electrical energy in a ‘‘green’’ way. The efficiency of such materials is governed by the figure of merit Z = S 2 σ /λ, where S is the Seebeck coefficient, σ is the electrical conductivity and λ is the thermal conductivity. To obtain greater conversion efficiencies the figure of merit should be as high as possible which should be greater than unity for commercial applications. Recently, oxide compounds have attracted attention as promising thermoelectric materials because of their high temperature stability and nontoxic nature. Ternary oxides with layered structure like NaCo2 O4 [1], misfit compounds like Ca3 Co4 O9 [2] and Bi2 Sr2 Co2 Oy [3], have been reported as particularly strong candidates. Among oxides, perovskites (ABO3 ) are an interesting class of compounds because of their versatile physical properties brought about by facile crystal chemical substitutions. Androulaki, et al., have recently
∗
Corresponding author. Tel.: +91 44 22574215. E-mail address: [email protected] (U.V. Varadaraju).
0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.07.031
reported a perovskite La0.95 Sr0.05 CoO3 [4] which has a ZT value of 0.2 at room temperature. The electrical transport properties of the perovskites depend, mainly on the B–O–B interactions. An interesting semiconducting phase is BaBiO3 which has a perovskite structure with Bi disproportionating into Bi5+ and Bi3+ . These ions due to their large difference in ionic radii form an ordered arrangement which is equivalent to a commensurate charge density wave [5]. Due to the difference in Bi3+ –O and Bi5+ –O bond lengths, the BiO6 octahedra are tilted which gives rise to low Bi–O–Bi bond angles and therefore, the insulating nature of the compound. Recently, Fu et al. [6] observed that with increasing In concentration in BaBi1−x Inx O3 there is a phase transition from monoclinic (for x = 0) to cubic (for x = 0.5) which is accompanied by an increase in the electrical conductivity. We have taken up the study of the compound Ba2 BiInO6 for thermoelectric applications and the effect of doping isovalent ions like Sb5+ at the Bi5+ site on the high temperature thermoelectric and resistivity properties. 2. Experimental Polycrystalline phases of Ba2 Bi1−x Sbx InO6 (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.3, 0.4) are synthesized by conventional high temperature solid state reaction. High purity BaCO3 , In2 O3 , Sb2 O3 and Bi2 O3 were taken in stoichiometric amounts and finely ground in a mortar and pestle for one hour. The powders were then heated slowly (to minimize volatilization of Sb2 O3 ) in alumina crucibles to 900 ◦ C and kept at this temperature for 24 h followed by oxygen
1736
K. Biswas, U.V. Varadaraju / Solid State Communications 149 (2009) 1735–1738
x = 0.3 x = 0.2
x = 0.1
Intensity (a.u.)
x = 0.08
x = 0.06
x = 0.04
10
20
30
40 2θ (degrees)
50
Fig. 2. Lattice parameter variation in the compositions Ba2 Bi1−x Sbx InO6 . (440)
(422)
(420)
(400)
(200)
x = 0.0
(220)
x = 0.02
60
70
Fig. 1. XRD patterns of Ba2 Bi1−x Sbx InO6 (* indicates secondary phase of Ba2 InSbO6 ).
treatment at 1050 ◦ C for 12 h. The powders were made into 8 mm dia pellets by applying pressure of 60 bars and sintered at 1050 ◦ C for 10 h under oxygen flow. The phase purity was checked by powder XRD patterns (Rigaku, Miniflex). The high temperature thermopower in the range 400 K–800 K was measured by a steady state method wherein the sample was kept between two Pt plates welded to two Pt, Pt-13%Rh thermocouples. The thermo-emf generated and the temperature difference were measured using a Keithley 182 nVm connected through a Keithley 705 scanner. The resistivity was measured in the same temperature range by Van der Pauw four probe method. 3. Results and discussion All the compositions Ba2 Bi1−x Sbx InO6 form a single phase for x ≤ 0.1 as shown in Fig. 1. The powder XRD patterns are indexed based on the JCPDS file no. 480061. The plot of lattice parameter vs. Sb concentration (Fig. 2) shows a linear decrease in the cubic lattice parameter with increasing Sb concentration following Vegard’s law. This indicates that Sb is doped in the lattice as Sb5+ since ionic radius of Bi5+ (0.76 Å) > Sb5+ (0.61 Å). The solubility limit of Sb is found to be 10 mol% above which a secondary phase formation takes place. This secondary phase is identified as Ba2 InSbO6 (indicated in the XRD pattern as *). The resistivity vs. temperature plots shown in Fig. 3 indicates a degenerate semiconducting nature for all the compositions at high temperatures. The resistivity reduces with 2 mol% Sb doping. It is known that Sb2 O3 can act as a sintering aid, and hence minor doping facilitates elimination of grain boundaries thereby decreasing resistivity. SEM pictures (Fig. 4) reveal the fusion of the grain boundaries in the 2 mol% doped phase vis a vis the undoped phase. Further increase in Sb content results in increase in resistivity, (with reference to the 2 mol% composition) which is also indicated by the increase in activation energy with Sb concentration as shown in Fig. 5. A possible explanation can be the existence of overlap of empty Bi 6s orbital and the O 2p orbital at the Fermi level in the parent compound. This is drawn from the explanation for the metallic behavior of BaPbO3 by Sleight et al. [7]. Sb5+ is devoid of 6s orbitals and hence with increasing Sb content more Bi5+ are being substituted as evidenced
Fig. 3. Resistivity vs. temperature plots for the compositions Ba2 Bi1−x Sbx InO6 .
by the variation in lattice parameters which leads to reduction in overlapping orbitals and hence the resistivity increases. An interesting feature is that the conduction mechanism is thermally activated type for lower doping level of Sb and changes to 3D variable range hopping type for higher concentrations of Sb (Fig. 6). Such high temperature VRH conduction is known for layered copper oxides [8]. It is well known that in heavily doped systems considerable number of inter band levels are created which gives rise to the VRH mechanism of conduction. In the present case since the dopant is distributed randomly and the unit cell is 3D in character it is only to be expected that the VRH will be in 3 dimensions (3D VRH). The high temperature thermopower plots for all the compositions (Fig. 7) show negative Seebeck coefficients indicating that electrons are the majority carriers. The variation in thermopower with Sb doping is similar to that of the resistivity behavior. Among the doped compositions, there is a systematic increase in the thermopower values with increasing concentration of Sb. This can be attributed to the increase in effective mass of the electrons as the empty 6s overlapping orbitals of Bi5+ decreases with increasing Sb5+ concentration. In all cases, the change in thermopower with temperature is not significant, a feature necessary for a thermoelectric material. It is known that in oxide materials the thermal conductivity is dominated by almost 60%–70% contribution from phonons whereas 30%–40% is from the electrons. Thus, since the lattice predominantly
K. Biswas, U.V. Varadaraju / Solid State Communications 149 (2009) 1735–1738
1737
Fig. 4. SEM images for the compositions Ba2 Bi1−x Sbx InO6 for x = 0.0 and x = 0.02.
Fig. 5. Activation energy vs. Sb concentration plots for the compositions Ba2 Bi1−x Sbx InO6 .
Fig. 7. Thermopower vs. temperature plots for the compositions Ba2 Bi1−x Sbx InO6 .
remains unchanged after doping, we can assume the thermal conductivity value not to be very different for the various Sb doped compositions. The usual ball park value of thermal conductivity of many perovskite related oxides is 1 Wm−1 K−1 . Using this, the Z values (Fig. 8) are calculated and the highest figure of merit is found to be 2.4 × 10−5 K−1 for the x = 0.06 composition at 770 K.
a
4. Conclusions We have studied the effect of Sb doping in the double perovskite Ba2 Bi1−x Sbx InO6 on the high temperature thermoelectric and resistivity properties. We find that the solubility limit of Sb at
b
1.2
0.8
x = 0.04 x = 0.0
0.6
0.5
0.4
log σ
log σ
x = 0.06 x = 0.1
1.0
1.0
0.2
0.0
0.0 -0.5
-0.2 -0.4
-1.0 -0.6 1.2
1.4
1.6
1.8 1000/T
2.0
2.2
2.4
2.6
0.185 0.190 0.195 0.200 0.205 0.210 0.215 0.220 0.225 1/T1/4
Fig. 6. Arrhenius plots for the compositions Ba2 Bi1−x Sbx InO6 (a) x = 0.0 and 0.04 (b) x = 0.06 and 0.1.
1738
K. Biswas, U.V. Varadaraju / Solid State Communications 149 (2009) 1735–1738
sintering aid at low concentration and for higher concentrations, lattice doping takes place leading to increase in resistivity. This is attributed to the reduction in empty Bi5+ 6s orbitals with increasing Sb5+ ion concentration. High temperature resistivity measurements show a degenerate semiconducting behavior and with increasing Sb concentration the conduction mechanism changes from activation to VRH type. The highest figure of merit obtained is 2.4 × 10−5 K−1 at 770 K for the x = 0.06 composition. References
Fig. 8. Z vs. T plots for the compositions Ba2 Bi1−x Sbx InO6 .
the Bi site is 10 mol%. On further Sb doping we observe that phase segregation takes place. Sb addition has the effect of a
[1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685. [2] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, J. Hejtmanek, Phys. Rev. B 62 (2000) 166. [3] R. Funahashi, I. Matsubara, S. Sodeoka, Appl. Phys. Lett. 76 (2000) 2385. [4] J. Androulaki, P. Migiakis, J. Giapintzakis, App. Phys. Lett. 84 (7) (2004) 1099. [5] Shiyou Pei, J.D. Jorgensen, B. Dabrowski, D.G. Hinks, D.R. Richards, A.W. Michell, J.M. Newsam, S.K. Sinha, D. Vaknin, A.J. Jacobson, Phys. Rev. B 41 (1990) 4126. [6] W.T. Fu, M.J. Polderman, F.M. Mulder, Mat. Res. Bul. 35 (8) (2000) 1205. [7] A.W. Sleight, J.L. Gillson, P.E. Bierstedt, Solid State Comm. 17 (1975) 27–28. [8] V. Ponnambalam, U.V. Varadaraju, Phys. Rev. B 52 (1995) 16213.