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The influence of Fe, Cu, Al -doping on the crystal structure, thermal and electrical properties of calcium titanate
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Solid State Ionics 179 (2008) 1615 – 1619 www.elsevier.com/locate/ssi
The influence of Fe, Cu, Al -doping on the crystal structure, thermal and electrical properties of calcium titanate A.A. Murashkina a , A.N. Demina a , A.K. Demin a , V.I. Maragou b , P.E. Tsiakaras b,⁎ a
b
Institute of High Temperature Electrochemistry, 22 S. Kovalevskoy, 620219 Yekaterinburg, Russia School of Engineering, Dept. of Mechanical Engineering, University of Thessaly, Pedion Areos, 383 34 Volos, Greece Received 11 July 2007; received in revised form 23 November 2007; accepted 7 December 2007
Abstract СаTi1 − xMxO3 − δ (M = Fe, Al, Cu) samples were synthesized by the standard ceramic technology. The stability areas and the crystal structure of the solid solutions were determined by the X-ray Diffraction technique. CaTi1 − xFexO3 − δ system in the interval from 0 up to 0.5 had a perovskite-like homogeneous phase. In CaTi1 − xAlxO3 − δ system in the range of 0 ≤ х ≤ 0.2, a solid solution with orthorhombic structure was found. In CaTi1 − xCuxO3 − δ system a solid solution existed in the interval of structures 0 ≤ х ≤ 0.1. When the content of copper was more than 0.15 (х N 0.15) other phases were observed. Unit cell parameters were defined by Rietveld method. Additionally, the electrical and thermal properties of the single phase compounds were investigated. It was concluded that all of the compounds are semiconductors with different main types of charge carriers, depending on the partial pressure field. Usually they present p-type conductivity in the high oxygen pressure region, ionic conductivity in middle pressures and n-type in the low oxygen pressure field. Finally, the thermal expansion coefficients of the samples in different temperature regions were determined. © 2008 Elsevier B.V. All rights reserved. Keywords: Calcium titanates; Thermal and electrical properties
1. Introduction АTiO3 — compounds (A: alkali-earth metal) with perovskite structure are characterized by high mixed ionic-electronic conductivity and stability in a wide oxygen partial pressure range. According to the defects theory, charge carriers in such oxides are oxygen vacancies and electrons (holes). Replacing ions in A-sublattice and Ti ions by various metals makes it possible to vary the electrochemical properties of these compounds widely. АTi1 − хМхO3 titanates (where A: alkaline metal, M: 3d-metal) have been recently investigated more intensively. CaTiO3-based oxides with perovskite structure show mixed ionic-electronic conductivity that enables their use as catalysts, electrode materials for high temperature solid oxide fuel cells and other electrochemical devices, such as membranes for oxygen separation and hydrogen production, etc. ⁎ Corresponding author. E-mail addresses: [email protected] (A.K. Demin), [email protected] (P.E. Tsiakaras). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.12.050
[1]. The doping of calcium titanate with different impurities allows the variation of ionic and electronic structure and therefore the electrical properties of these compounds in a wide range. It has been reported [2–4], that the partial substitution by Fe in Ti-sites improves the electrical conductivity of the CaTiO3 perovskite; however, the ionic conductivity does not increase monotonously with iron concentration. The maximum ionic conductivity is observed at intermediate values of x (x = 0.2), while decreases at higher iron contents. It has been also shown, that the introduction of iron, aluminum and gallium in Ti-sublattice of calcium titanate increases conductivity in the sequence: Fe N Ga N Al. The influence of various additives on the electric properties of calcium, strontium and barium titanate has been widely investigated [5–8]. It was concluded that calcium, strontium and barium titanate doping in the B-site by acceptorion, increases the concentration of oxygen vacancies in a crystal structure, and thus increases the whole conductivity. The present work is devoted to the study of the influence of B-site doping by iron, copper and aluminum on the crystal structure, the electrical properties and the thermal expansion of calcium titanate.
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2. Experimental The samples CaTi1 −xMxO3 −δ (M=Fe, Al, Cu) (х=0–0.5) were synthesized according to the standard ceramic processing technology. The initial reactants were preliminary calcined for the removal of the adsorbed gases and adhesive water at 773 K for 2 h. CaTi1 −x FexO3 −δ synthesis was carried out at 1523 K for 5 h. Then the samples were stirred in ethyl alcohol, pressed at 500 MPa into 3 × 4 × 26 mm bars and fired at 1723 K for 2 h. Samples of nominal structure of CaTi1 − xAlxO3 − δ were synthesized at 1523 K for 3 h, pressed and fired at 1723 K for 2 h. The CaTi1 − xCuxO3 − δ samples were synthesized at 1523 K for 10 h and then fired at 1623 K for 10 h. The XRD analysis was carried out using a DronUm1 device in CuKα-radiation in 20о ≤ 2θ ≤ 80о. The unit cell parameters were defined by Rietveld method in a Fullprof program. The main idea of the method is that the calculated diffractogramm is compared with the experimental one. By this comparison the profile (half-widths, diffractogramm shear, unit cell parameters, asymmetry parameters, scale coefficient, profile parameter, background coefficient and heat factor) and the structural parameters (atoms coordinates, individual isotropic or anisotropic heat factors) are refined. The criteria of refinement quality are the following R-factors: profile Rp ¼
X
X jyie k yie j = yie
weight profile − Rwp ¼
X 1=2 X wi ðyie k yit Þ2 = wi y2ie
Bragg − factor − RBr ¼ structural − Rf ¼
X
X jIit cIkt j = Ike
X X jFit cFkt j = Fke
1=2 X expected − Rexp ¼ ð N PÞ= wi ; y2ie
samples were placed in a holder with Pt pressure contacts mounted in a Y2O3 stabilized ZrO2 tube. Platinum paste stripes, which were deposited on the inner and outer surface of the YSZ tube and connected with platinum wires, were used as the electrochemical oxygen pump and the oxygen sensor. The tube was placed inside a quartz tube and was tightly sealed. The temperature was monitored with a Pt/Pt–Rh thermocouple. The temperature and the oxygen partial pressure were varied by a Zirconia-318 microprocessor controller. The thermal expansion was measured in air atmosphere during heating from 298 to 1170 K using an AF-1 dilatometer. The bar-shaped ceramic samples were prepared in the same way
ð1aÞ ð1bÞ ð1cÞ ð1dÞ ð1eÞ
where yie, yit, denote the experimental and theoretical intensities on each angle; Iit, Ikt the experimental and theoretical integral reflexes intensities; Fit, Fkt the experimental and theoretical structural amplitudes; k, c the scale multipliers; N the experimental points number; Р the refined parameters number; and wi = 1/yie the weight multipliers. The profile fitting procedure (XRFIT calculation) uses pseudoVoigt functions with a global FWHM (Full Width at Half Maximum), a global eta (proportion of Lorentzian) and a linear background. Each peak is characterized by its position, intensity, FWHM and eta shifts with respect to the global parents. At least one of the peaks must have zero shifts to avoid singular matrix error. The Chi2 value is calculated as follows: Chi2 ¼ ðSiðwi:ðyoi yciÞ⁎⁎2ÞÞ=ð N PÞ
ð1f Þ
where Si denotes the summation of the N points, wi the counting weight (wi = 1/sigma(Yoi)), yoi the observed counting, yci the calculated counting and P the refined parameters number. Conductivity was investigated by using the standard four probe method (DC) in a wide range of oxygen partial pressures (0.21–10− 17 atm) and temperatures (Troom − 1273 K). The
Fig. 1. Raw XRD data (continuous line) and calculated profile (points) of the (a) CaTi0.8Fe0.2O3 − δ (b) CaTi0.9Al0.1O3 − δ and (c) CaTi0.9Cu0.1O3 − δ solid solution.
A.A. Murashkina et al. / Solid State Ionics 179 (2008) 1615–1619
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as those for the electrical conductivity measurements. The heating/cooling rate was 200 K/h. The linear thermal expansion coefficient at constant pressure, α, was determined using the well known relation [6]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
ffiffiffiffiffi p 1 ∂V 1 ∂L 3 3 aP ¼ ¼ ð2Þ V0 ∂T P L0 ∂T P where L0 is the sample's length at room temperature and L is the change in sample's length during heating to temperature T.
3. Results and discussion The XRD analysis of the CaTi1 − xFexO3 − δ system revealed the existence of a perovskite-like homogeneous phase (sp.gr. Pbnm) in the interval from 0 up to 0.5 and that all samples were single phase. Fig. 1(a) shows a typical XRD pattern (raw XRD data and calculated profile by Rietveld method) of the CaTi0.8Fe0.2O3 − δ sample. In CaTi1 − xAlxO3 − δ system in the range of 0 ≤ х ≤ 0.2, a solid solution with an orthorhombic structure was found (sp.gr Pnma). The samples with aluminium content х N 0.2 are not single-phase. Fig. 1(b) shows a typical XRD pattern of the CaTi0.9Al0.1O3 − δ sample. In CaTi1 − xCuxO3 − δ system a solid solution exists in the interval of structures 0 ≤ х ≤ 0.1 (sp.gr. Pnma). When the content of copper is more than 0.15 (х N 0.15) other phases are observed. Finally, Fig. 1(c) shows a typical XRD pattern of CaTi0.9Cu0.1O3 − δ sample. The respective lattice parameters for all samples are listed in Table 1. The dependence of conductivity on the oxygen partial pressure for the CaTi1 − xMxO3 − х system (M = Fe, Al, Cu) is presented in Fig. 2(a). The partial replacement of B-metal in a АВО3 perovskite by a cation of lower valence leads to the
Table 1 Lattice parameters of CaTi1 − xMxO3 − δ (M = Fe, Al, Cu) solid solutions а, Å
b, Å
c, Å
V, Å3
Rp
CaTiO3 5.379(5) 7.641(5) 5.441(8) 222.63(3) 22.9 CaTi0.8Fe0.2O3 − δ 5.3927 5.4371
7.6483
224.25
Rwp
Rexp Chi2 RBr Rf
26.3 15.0 3.1
4.1 3.1
14.32 16.4 25.1 2.1
3.2 2.4
Fig. 2. Conductivity of CaTi1 − xMxO3 − δ (M = Fe, Cu, Al) vs. (a) oxygen partial pressure and (b) vs. inverse temperature.
formation of oxygen vacancies [6,7]. The equation of defects formation in Kroger–Vink signs may be the following [8]: =
2ABO3 þ M2 O3 Y2MB þ VBB O þ 2AA þ 9OO þ 2BB
ð3Þ
While the equation of electro-neutrality: BB h = i 2 V O ¼ MB
ð4Þ
In the high oxygen partial pressure field, p-type conductivity prevails: B 1=2O2 þ VBB O ²OO þ 2h
ð5Þ
The balance constant of the Eq. (5) is: p2 K1 ¼ 1=2 VBB O pO2
ð6Þ
CaTi0.75Fe0.25O3 − δ 5.3922 5.4284 7.6559
224.09(5) 5.9
7.0
19.4 2.0
2.8 2.6
CaTi0.7Fe0.3O3 − δ 5.3921 5.4280
224.36
10.9
10.1 18.0 2.2
2.6 2.3
CaTi0.9Al0.1O3 − δ 5.382(3) 7.625(4) 5.434(4) 223.04(1) 24.8
33.6 22.9 2.3
1.8 2.5
CaTi0.8Al0.2O3 − δ 5.385(3) 7.619(3) 5.427(4) 222.72(1) 26.5
33.1 22.8 2.1
2.1 2.6
As hole conductivity is directly proportional to the concentration of charge carriers it is possible to write:
CaTi0.9Cu0.1O3 − δ 5.379(9) 7.633(2) 5.435(9) 22.63(3)
31.1 50.0 0.4
2.4 2.3
rp cpO2
7.6561
26.4
The hole concentration in acceptor doped oxide depends on pressure according to the following equation: h i1=2 1=2 = 1=4 p ¼ K1 0:51=2 MTi pO 2 ð7Þ
1=4
ð8Þ
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Table 2 Activation energy for conduction in CaTi1 − xMxO3 − δ (M = Fe, Al, Cu) solid solutions Ea, eV CaTi0.9Fe0.1O3 − δ CaTi0.8Fe0.2O3 − δ CaTi0.7Fe0.3O3 − δ CaTi0.5Fe0.5O3 − δ CaTi0.9Al0.1O3 − δ CaTi0.9Cu0.1O3 − δ
0.9 2.49 1.43 2.03 1.84 0.81
In the low oxygen partial pressure field n-type conductivity prevails. Vacancies of oxygen are in balance with electronic carriers and oxygen in gas phase: O O ²1=2O2
þ
VBB O
þ 2e
=
The reaction constant is: 1=2 p n2 VBB O O2 K2 ¼ OO Hence, from Eqs. (9) and (10): h i1=2 1=2 = 1=4 n ¼ K2 0:51=2 MTi pO2
ð9Þ
ð10Þ
ð11Þ
Conductivity dependence on oxygen partial pressure may be presented as: 1=4
ð12Þ
rn cpO2
By replacing В4+ ions with М3+ in a perovskite lattice, oxygen vacancies are formed, whose concentration is equal to the defects. In this case, the change of oxygen vacancies' concentration with the oxygen partial pressure change is very small and it is possible to assume that VOBB is constant, and hence, σi is constant, in a specific interval of oxygen partial pressures. Conductivity dependence on oxygen partial pressure may be divided into two areas: in the intermediate oxygen partial pressure area (pO2 = 10− 4–10− 11 atm) where CaTiO3 conductivity is proportional to p1/6O2, and the low partial pressures area (b 10− 11 atm), where the obtained experimental dependences increase by the law ~ p− 1/6O2. The process of defect creation at intermediate pressures of oxygen in CaTiO3 lattice is the following: ==
1=2O2 YOi þ 2hB h i == Oi p2 K¼ 1=2 PO2 h i == Oi ¼ 1=2p
ð13Þ
pf pO 2
ð16Þ
1=6
in oxidizing conditions are increased proportionally. At low oxygen pressure oxygen vacancies in CaTiO3 lattice and electrons according to Eq. (9) are formed. Using the equation of electro-neutrality 2 VBB O ¼ n: BB 1=6 1=6 VO pO2 ; nepO2 ð17Þ e Hence, ionic and electronic conductivity are proportional. Change of the inclination of log σ dependence on log Р for calcium titanate systems with 1/6 on 1/4 at 10− 11 аtm, can be caused by localization of a part of free electrons on Ti4+ions and the formation of negatively charged defects (Ti/Ti) by the reaction: =
BB Ti Ti þ OO Y1=2O2 þ VO þ e þ TiTi
1=2 h = i VBB O nPO2 TiTi K¼ OO
ð18Þ
ð19Þ
If it is assumed, that the concentration depends weakly on Ро2, then n is proportional to Ро2− 1/4. By replacing part of Ti4+ ions with Fe3+in a perovskite lattice, oxygen vacancies are formed which concentration is equal to ion defects. In this case of oxygen vacancies concentration is very small, theBBchange VO is constant, and hence, σi is constant, in a specific range of oxygen partial pressures. As CaTi1 − xMxO3 − δ (M = Fe, Al, Cu) present mixed oxygen ion and electron conductivity in air atmosphere, conductivity measurements vs. temperature were conducted at 10− 7 atm, where they are pure ionic conductors (Fig. 2(b)). The activation energy values (Ea) of ionic conductivity for the examined solid solutions are listed in Table 2. The conductivity increases with temperature, which attests the semi conducting behavior:
A Ea r ¼ exp ð20Þ T kT Apparently from the cited dependences, the increase of Fe content results in the increase of ionic conductivity, reaching the maximum value at х = 0.2 at 1273 K. Further increase of х
ð14Þ ð15Þ
Hence, if the mobility of charge carriers does not depend on their concentration, both ionic and hole conductivity of CaTiO3
Fig. 3. Relative linear expansion as a function of temperature for CaTi1 − xМ xO3 − δ (M = Fe, Al, Cu) system.
A.A. Murashkina et al. / Solid State Ionics 179 (2008) 1615–1619 Table 3 Thermal properties of the samples x
α × 10− 6, K− 1
α × 10− 7, K− 1
CaTi1 − xFexO3 − δ 0.1 293–770 K 770–1273 K 0.2 293–770 K 770–1273 K 0.3 293–770 K 770–1273 K 0.4 293–770 K 770–1273 K 0.5 293–770 K 770–1273 K
14.5 10.6 16.4 11.7 15.4 10.5 14.6 11.2 15.5 11.1
0.83 3.3 2.7 3.7 1.9 2.0 0.53 3.7 1.3 3.2
CaTi1 − xAlxO3 − δ 0.1 293–570 K 570–800 K 800–1273 K
4.8 10.6 8.5
6.8 2.2 2.9
CaTi1 − xCuxO3 − δ 0.1 293–1273 K
10
0.95
results in the decrease of the ionic conductivity [7], thus activation energy is essentially increased. The received values (the deviation of the experimental values is 3%) of the latter are in good agreement with the literature data: х = 0.1: Еа = 0.90 and 0.87 eV, х = 0.2: Еа = 2.49 and 0.83 eV (1473–1273 K), and 2.9 (1273–1073 K), х = 0.3: Еа = 1.43 eV and 1.8 eV (1473– 1373 K), respectively [8]. Probably, the reduction of conductivity at х = 0.2 is related with ordering defects - formation of complexes of oxygen vacancies. The influence of the dopand's kind on the conductivity of CaTi0.9 M0.1O3 − δ (M = Fe, Al, Cu) system was also investigated. The cationic radii increase in the sequence Al3+–Ti4+–Fe3+–Cu2+. The greatest ionic conductivity is observed for dopant with radius close to radius of the host cation (Fe). For practical applications of doped calcium titanates, it is very important to know their behavior with the change of the temperature. As these materials may be applied in electrochemical devices, their compatibility with materials of other
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units of electrochemical systems plays a very significant role. The relative increase of the samples' size with the increase of temperature in air atmosphere (pO2 = 0.21 atm) was investigated, in a wide range of temperatures from 293 up to 1273 K. The dependence of relative linear expansion on temperature is shown in Fig. 3. The thermal expansion coefficients evaluated from the slope of the linear portions are listed in Table 3. 4. Conclusions СаTi1 − xMxO3 − δ (M = Fe, Al, Cu) samples were synthesized by the standard ceramic technique. The stability areas and the crystal structure of solid solutions were determined by the X-ray diffraction technique. It was found that all of the single phase samples have orthorhombic structure as initial CaTiO3. The unit cell parameters were defined by using the Rietveld method. The electrical and thermal properties of the single phase compounds were also investigated. It was concluded that all the compounds are semiconductors with different main types of charge carriers depending on the partial pressure field. Usually they are characterized by p-type conductivity in the high oxygen pressure region, ionic conductivity in middle pressures and ntype in the low oxygen pressure field. Finally, the thermal expansion coefficients of the samples in different temperature regions were also determined. References [1] W.L. George, R.E. Grace, J. Phys. Chem. Solids 30 (1969) 881. [2] H. Iwahara, T. Esaka, T. Mangahara, J. Appl. Electrochem. 18 (1988) 173. [3] D.P. Sutija, T. Norby, P.A. Osborg, P. Kofstad, Electrochem. Soc. Proc. 93 (1993) 552. [4] L.A. Dunyushkina, A.K. Demin, B.V. Zhuravlev, Solid State Ionics 116 (1999) 85. [5] L.A. Dunyushkina, V.A. Gorbunov, A.A. Babkina, N.O. Esina, Ionics 9 (2003) 67. [6] V.P. Gorelov, V.B. Balakireva, Electrochemistry, 33 (1997) 1450. [7] L.A. Dunyushkina, V.A. Gorbunov, Inorg. Mater. 57 (2001) 1165. [8] S. Marion, A.I. Becerro, T. Norby, Ionics 5 (1999) 385.
Solid State Ionics 179 (2008) 1615 – 1619 www.elsevier.com/locate/ssi
The influence of Fe, Cu, Al -doping on the crystal structure, thermal and electrical properties of calcium titanate A.A. Murashkina a , A.N. Demina a , A.K. Demin a , V.I. Maragou b , P.E. Tsiakaras b,⁎ a
b
Institute of High Temperature Electrochemistry, 22 S. Kovalevskoy, 620219 Yekaterinburg, Russia School of Engineering, Dept. of Mechanical Engineering, University of Thessaly, Pedion Areos, 383 34 Volos, Greece Received 11 July 2007; received in revised form 23 November 2007; accepted 7 December 2007
Abstract СаTi1 − xMxO3 − δ (M = Fe, Al, Cu) samples were synthesized by the standard ceramic technology. The stability areas and the crystal structure of the solid solutions were determined by the X-ray Diffraction technique. CaTi1 − xFexO3 − δ system in the interval from 0 up to 0.5 had a perovskite-like homogeneous phase. In CaTi1 − xAlxO3 − δ system in the range of 0 ≤ х ≤ 0.2, a solid solution with orthorhombic structure was found. In CaTi1 − xCuxO3 − δ system a solid solution existed in the interval of structures 0 ≤ х ≤ 0.1. When the content of copper was more than 0.15 (х N 0.15) other phases were observed. Unit cell parameters were defined by Rietveld method. Additionally, the electrical and thermal properties of the single phase compounds were investigated. It was concluded that all of the compounds are semiconductors with different main types of charge carriers, depending on the partial pressure field. Usually they present p-type conductivity in the high oxygen pressure region, ionic conductivity in middle pressures and n-type in the low oxygen pressure field. Finally, the thermal expansion coefficients of the samples in different temperature regions were determined. © 2008 Elsevier B.V. All rights reserved. Keywords: Calcium titanates; Thermal and electrical properties
1. Introduction АTiO3 — compounds (A: alkali-earth metal) with perovskite structure are characterized by high mixed ionic-electronic conductivity and stability in a wide oxygen partial pressure range. According to the defects theory, charge carriers in such oxides are oxygen vacancies and electrons (holes). Replacing ions in A-sublattice and Ti ions by various metals makes it possible to vary the electrochemical properties of these compounds widely. АTi1 − хМхO3 titanates (where A: alkaline metal, M: 3d-metal) have been recently investigated more intensively. CaTiO3-based oxides with perovskite structure show mixed ionic-electronic conductivity that enables their use as catalysts, electrode materials for high temperature solid oxide fuel cells and other electrochemical devices, such as membranes for oxygen separation and hydrogen production, etc. ⁎ Corresponding author. E-mail addresses: [email protected] (A.K. Demin), [email protected] (P.E. Tsiakaras). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.12.050
[1]. The doping of calcium titanate with different impurities allows the variation of ionic and electronic structure and therefore the electrical properties of these compounds in a wide range. It has been reported [2–4], that the partial substitution by Fe in Ti-sites improves the electrical conductivity of the CaTiO3 perovskite; however, the ionic conductivity does not increase monotonously with iron concentration. The maximum ionic conductivity is observed at intermediate values of x (x = 0.2), while decreases at higher iron contents. It has been also shown, that the introduction of iron, aluminum and gallium in Ti-sublattice of calcium titanate increases conductivity in the sequence: Fe N Ga N Al. The influence of various additives on the electric properties of calcium, strontium and barium titanate has been widely investigated [5–8]. It was concluded that calcium, strontium and barium titanate doping in the B-site by acceptorion, increases the concentration of oxygen vacancies in a crystal structure, and thus increases the whole conductivity. The present work is devoted to the study of the influence of B-site doping by iron, copper and aluminum on the crystal structure, the electrical properties and the thermal expansion of calcium titanate.
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2. Experimental The samples CaTi1 −xMxO3 −δ (M=Fe, Al, Cu) (х=0–0.5) were synthesized according to the standard ceramic processing technology. The initial reactants were preliminary calcined for the removal of the adsorbed gases and adhesive water at 773 K for 2 h. CaTi1 −x FexO3 −δ synthesis was carried out at 1523 K for 5 h. Then the samples were stirred in ethyl alcohol, pressed at 500 MPa into 3 × 4 × 26 mm bars and fired at 1723 K for 2 h. Samples of nominal structure of CaTi1 − xAlxO3 − δ were synthesized at 1523 K for 3 h, pressed and fired at 1723 K for 2 h. The CaTi1 − xCuxO3 − δ samples were synthesized at 1523 K for 10 h and then fired at 1623 K for 10 h. The XRD analysis was carried out using a DronUm1 device in CuKα-radiation in 20о ≤ 2θ ≤ 80о. The unit cell parameters were defined by Rietveld method in a Fullprof program. The main idea of the method is that the calculated diffractogramm is compared with the experimental one. By this comparison the profile (half-widths, diffractogramm shear, unit cell parameters, asymmetry parameters, scale coefficient, profile parameter, background coefficient and heat factor) and the structural parameters (atoms coordinates, individual isotropic or anisotropic heat factors) are refined. The criteria of refinement quality are the following R-factors: profile Rp ¼
X
X jyie k yie j = yie
weight profile − Rwp ¼
X 1=2 X wi ðyie k yit Þ2 = wi y2ie
Bragg − factor − RBr ¼ structural − Rf ¼
X
X jIit cIkt j = Ike
X X jFit cFkt j = Fke
1=2 X expected − Rexp ¼ ð N PÞ= wi ; y2ie
samples were placed in a holder with Pt pressure contacts mounted in a Y2O3 stabilized ZrO2 tube. Platinum paste stripes, which were deposited on the inner and outer surface of the YSZ tube and connected with platinum wires, were used as the electrochemical oxygen pump and the oxygen sensor. The tube was placed inside a quartz tube and was tightly sealed. The temperature was monitored with a Pt/Pt–Rh thermocouple. The temperature and the oxygen partial pressure were varied by a Zirconia-318 microprocessor controller. The thermal expansion was measured in air atmosphere during heating from 298 to 1170 K using an AF-1 dilatometer. The bar-shaped ceramic samples were prepared in the same way
ð1aÞ ð1bÞ ð1cÞ ð1dÞ ð1eÞ
where yie, yit, denote the experimental and theoretical intensities on each angle; Iit, Ikt the experimental and theoretical integral reflexes intensities; Fit, Fkt the experimental and theoretical structural amplitudes; k, c the scale multipliers; N the experimental points number; Р the refined parameters number; and wi = 1/yie the weight multipliers. The profile fitting procedure (XRFIT calculation) uses pseudoVoigt functions with a global FWHM (Full Width at Half Maximum), a global eta (proportion of Lorentzian) and a linear background. Each peak is characterized by its position, intensity, FWHM and eta shifts with respect to the global parents. At least one of the peaks must have zero shifts to avoid singular matrix error. The Chi2 value is calculated as follows: Chi2 ¼ ðSiðwi:ðyoi yciÞ⁎⁎2ÞÞ=ð N PÞ
ð1f Þ
where Si denotes the summation of the N points, wi the counting weight (wi = 1/sigma(Yoi)), yoi the observed counting, yci the calculated counting and P the refined parameters number. Conductivity was investigated by using the standard four probe method (DC) in a wide range of oxygen partial pressures (0.21–10− 17 atm) and temperatures (Troom − 1273 K). The
Fig. 1. Raw XRD data (continuous line) and calculated profile (points) of the (a) CaTi0.8Fe0.2O3 − δ (b) CaTi0.9Al0.1O3 − δ and (c) CaTi0.9Cu0.1O3 − δ solid solution.
A.A. Murashkina et al. / Solid State Ionics 179 (2008) 1615–1619
1617
as those for the electrical conductivity measurements. The heating/cooling rate was 200 K/h. The linear thermal expansion coefficient at constant pressure, α, was determined using the well known relation [6]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
ffiffiffiffiffi p 1 ∂V 1 ∂L 3 3 aP ¼ ¼ ð2Þ V0 ∂T P L0 ∂T P where L0 is the sample's length at room temperature and L is the change in sample's length during heating to temperature T.
3. Results and discussion The XRD analysis of the CaTi1 − xFexO3 − δ system revealed the existence of a perovskite-like homogeneous phase (sp.gr. Pbnm) in the interval from 0 up to 0.5 and that all samples were single phase. Fig. 1(a) shows a typical XRD pattern (raw XRD data and calculated profile by Rietveld method) of the CaTi0.8Fe0.2O3 − δ sample. In CaTi1 − xAlxO3 − δ system in the range of 0 ≤ х ≤ 0.2, a solid solution with an orthorhombic structure was found (sp.gr Pnma). The samples with aluminium content х N 0.2 are not single-phase. Fig. 1(b) shows a typical XRD pattern of the CaTi0.9Al0.1O3 − δ sample. In CaTi1 − xCuxO3 − δ system a solid solution exists in the interval of structures 0 ≤ х ≤ 0.1 (sp.gr. Pnma). When the content of copper is more than 0.15 (х N 0.15) other phases are observed. Finally, Fig. 1(c) shows a typical XRD pattern of CaTi0.9Cu0.1O3 − δ sample. The respective lattice parameters for all samples are listed in Table 1. The dependence of conductivity on the oxygen partial pressure for the CaTi1 − xMxO3 − х system (M = Fe, Al, Cu) is presented in Fig. 2(a). The partial replacement of B-metal in a АВО3 perovskite by a cation of lower valence leads to the
Table 1 Lattice parameters of CaTi1 − xMxO3 − δ (M = Fe, Al, Cu) solid solutions а, Å
b, Å
c, Å
V, Å3
Rp
CaTiO3 5.379(5) 7.641(5) 5.441(8) 222.63(3) 22.9 CaTi0.8Fe0.2O3 − δ 5.3927 5.4371
7.6483
224.25
Rwp
Rexp Chi2 RBr Rf
26.3 15.0 3.1
4.1 3.1
14.32 16.4 25.1 2.1
3.2 2.4
Fig. 2. Conductivity of CaTi1 − xMxO3 − δ (M = Fe, Cu, Al) vs. (a) oxygen partial pressure and (b) vs. inverse temperature.
formation of oxygen vacancies [6,7]. The equation of defects formation in Kroger–Vink signs may be the following [8]: =
2ABO3 þ M2 O3 Y2MB þ VBB O þ 2AA þ 9OO þ 2BB
ð3Þ
While the equation of electro-neutrality: BB h = i 2 V O ¼ MB
ð4Þ
In the high oxygen partial pressure field, p-type conductivity prevails: B 1=2O2 þ VBB O ²OO þ 2h
ð5Þ
The balance constant of the Eq. (5) is: p2 K1 ¼ 1=2 VBB O pO2
ð6Þ
CaTi0.75Fe0.25O3 − δ 5.3922 5.4284 7.6559
224.09(5) 5.9
7.0
19.4 2.0
2.8 2.6
CaTi0.7Fe0.3O3 − δ 5.3921 5.4280
224.36
10.9
10.1 18.0 2.2
2.6 2.3
CaTi0.9Al0.1O3 − δ 5.382(3) 7.625(4) 5.434(4) 223.04(1) 24.8
33.6 22.9 2.3
1.8 2.5
CaTi0.8Al0.2O3 − δ 5.385(3) 7.619(3) 5.427(4) 222.72(1) 26.5
33.1 22.8 2.1
2.1 2.6
As hole conductivity is directly proportional to the concentration of charge carriers it is possible to write:
CaTi0.9Cu0.1O3 − δ 5.379(9) 7.633(2) 5.435(9) 22.63(3)
31.1 50.0 0.4
2.4 2.3
rp cpO2
7.6561
26.4
The hole concentration in acceptor doped oxide depends on pressure according to the following equation: h i1=2 1=2 = 1=4 p ¼ K1 0:51=2 MTi pO 2 ð7Þ
1=4
ð8Þ
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A.A. Murashkina et al. / Solid State Ionics 179 (2008) 1615–1619
Table 2 Activation energy for conduction in CaTi1 − xMxO3 − δ (M = Fe, Al, Cu) solid solutions Ea, eV CaTi0.9Fe0.1O3 − δ CaTi0.8Fe0.2O3 − δ CaTi0.7Fe0.3O3 − δ CaTi0.5Fe0.5O3 − δ CaTi0.9Al0.1O3 − δ CaTi0.9Cu0.1O3 − δ
0.9 2.49 1.43 2.03 1.84 0.81
In the low oxygen partial pressure field n-type conductivity prevails. Vacancies of oxygen are in balance with electronic carriers and oxygen in gas phase: O O ²1=2O2
þ
VBB O
þ 2e
=
The reaction constant is: 1=2 p n2 VBB O O2 K2 ¼ OO Hence, from Eqs. (9) and (10): h i1=2 1=2 = 1=4 n ¼ K2 0:51=2 MTi pO2
ð9Þ
ð10Þ
ð11Þ
Conductivity dependence on oxygen partial pressure may be presented as: 1=4
ð12Þ
rn cpO2
By replacing В4+ ions with М3+ in a perovskite lattice, oxygen vacancies are formed, whose concentration is equal to the defects. In this case, the change of oxygen vacancies' concentration with the oxygen partial pressure change is very small and it is possible to assume that VOBB is constant, and hence, σi is constant, in a specific interval of oxygen partial pressures. Conductivity dependence on oxygen partial pressure may be divided into two areas: in the intermediate oxygen partial pressure area (pO2 = 10− 4–10− 11 atm) where CaTiO3 conductivity is proportional to p1/6O2, and the low partial pressures area (b 10− 11 atm), where the obtained experimental dependences increase by the law ~ p− 1/6O2. The process of defect creation at intermediate pressures of oxygen in CaTiO3 lattice is the following: ==
1=2O2 YOi þ 2hB h i == Oi p2 K¼ 1=2 PO2 h i == Oi ¼ 1=2p
ð13Þ
pf pO 2
ð16Þ
1=6
in oxidizing conditions are increased proportionally. At low oxygen pressure oxygen vacancies in CaTiO3 lattice and electrons according to Eq. (9) are formed. Using the equation of electro-neutrality 2 VBB O ¼ n: BB 1=6 1=6 VO pO2 ; nepO2 ð17Þ e Hence, ionic and electronic conductivity are proportional. Change of the inclination of log σ dependence on log Р for calcium titanate systems with 1/6 on 1/4 at 10− 11 аtm, can be caused by localization of a part of free electrons on Ti4+ions and the formation of negatively charged defects (Ti/Ti) by the reaction: =
BB Ti Ti þ OO Y1=2O2 þ VO þ e þ TiTi
1=2 h = i VBB O nPO2 TiTi K¼ OO
ð18Þ
ð19Þ
If it is assumed, that the concentration depends weakly on Ро2, then n is proportional to Ро2− 1/4. By replacing part of Ti4+ ions with Fe3+in a perovskite lattice, oxygen vacancies are formed which concentration is equal to ion defects. In this case of oxygen vacancies concentration is very small, theBBchange VO is constant, and hence, σi is constant, in a specific range of oxygen partial pressures. As CaTi1 − xMxO3 − δ (M = Fe, Al, Cu) present mixed oxygen ion and electron conductivity in air atmosphere, conductivity measurements vs. temperature were conducted at 10− 7 atm, where they are pure ionic conductors (Fig. 2(b)). The activation energy values (Ea) of ionic conductivity for the examined solid solutions are listed in Table 2. The conductivity increases with temperature, which attests the semi conducting behavior:
A Ea r ¼ exp ð20Þ T kT Apparently from the cited dependences, the increase of Fe content results in the increase of ionic conductivity, reaching the maximum value at х = 0.2 at 1273 K. Further increase of х
ð14Þ ð15Þ
Hence, if the mobility of charge carriers does not depend on their concentration, both ionic and hole conductivity of CaTiO3
Fig. 3. Relative linear expansion as a function of temperature for CaTi1 − xМ xO3 − δ (M = Fe, Al, Cu) system.
A.A. Murashkina et al. / Solid State Ionics 179 (2008) 1615–1619 Table 3 Thermal properties of the samples x
α × 10− 6, K− 1
α × 10− 7, K− 1
CaTi1 − xFexO3 − δ 0.1 293–770 K 770–1273 K 0.2 293–770 K 770–1273 K 0.3 293–770 K 770–1273 K 0.4 293–770 K 770–1273 K 0.5 293–770 K 770–1273 K
14.5 10.6 16.4 11.7 15.4 10.5 14.6 11.2 15.5 11.1
0.83 3.3 2.7 3.7 1.9 2.0 0.53 3.7 1.3 3.2
CaTi1 − xAlxO3 − δ 0.1 293–570 K 570–800 K 800–1273 K
4.8 10.6 8.5
6.8 2.2 2.9
CaTi1 − xCuxO3 − δ 0.1 293–1273 K
10
0.95
results in the decrease of the ionic conductivity [7], thus activation energy is essentially increased. The received values (the deviation of the experimental values is 3%) of the latter are in good agreement with the literature data: х = 0.1: Еа = 0.90 and 0.87 eV, х = 0.2: Еа = 2.49 and 0.83 eV (1473–1273 K), and 2.9 (1273–1073 K), х = 0.3: Еа = 1.43 eV and 1.8 eV (1473– 1373 K), respectively [8]. Probably, the reduction of conductivity at х = 0.2 is related with ordering defects - formation of complexes of oxygen vacancies. The influence of the dopand's kind on the conductivity of CaTi0.9 M0.1O3 − δ (M = Fe, Al, Cu) system was also investigated. The cationic radii increase in the sequence Al3+–Ti4+–Fe3+–Cu2+. The greatest ionic conductivity is observed for dopant with radius close to radius of the host cation (Fe). For practical applications of doped calcium titanates, it is very important to know their behavior with the change of the temperature. As these materials may be applied in electrochemical devices, their compatibility with materials of other
1619
units of electrochemical systems plays a very significant role. The relative increase of the samples' size with the increase of temperature in air atmosphere (pO2 = 0.21 atm) was investigated, in a wide range of temperatures from 293 up to 1273 K. The dependence of relative linear expansion on temperature is shown in Fig. 3. The thermal expansion coefficients evaluated from the slope of the linear portions are listed in Table 3. 4. Conclusions СаTi1 − xMxO3 − δ (M = Fe, Al, Cu) samples were synthesized by the standard ceramic technique. The stability areas and the crystal structure of solid solutions were determined by the X-ray diffraction technique. It was found that all of the single phase samples have orthorhombic structure as initial CaTiO3. The unit cell parameters were defined by using the Rietveld method. The electrical and thermal properties of the single phase compounds were also investigated. It was concluded that all the compounds are semiconductors with different main types of charge carriers depending on the partial pressure field. Usually they are characterized by p-type conductivity in the high oxygen pressure region, ionic conductivity in middle pressures and ntype in the low oxygen pressure field. Finally, the thermal expansion coefficients of the samples in different temperature regions were also determined. References [1] W.L. George, R.E. Grace, J. Phys. Chem. Solids 30 (1969) 881. [2] H. Iwahara, T. Esaka, T. Mangahara, J. Appl. Electrochem. 18 (1988) 173. [3] D.P. Sutija, T. Norby, P.A. Osborg, P. Kofstad, Electrochem. Soc. Proc. 93 (1993) 552. [4] L.A. Dunyushkina, A.K. Demin, B.V. Zhuravlev, Solid State Ionics 116 (1999) 85. [5] L.A. Dunyushkina, V.A. Gorbunov, A.A. Babkina, N.O. Esina, Ionics 9 (2003) 67. [6] V.P. Gorelov, V.B. Balakireva, Electrochemistry, 33 (1997) 1450. [7] L.A. Dunyushkina, V.A. Gorbunov, Inorg. Mater. 57 (2001) 1165. [8] S. Marion, A.I. Becerro, T. Norby, Ionics 5 (1999) 385.