Article pubs.acs.org/cm
Anisotropic Chemical Expansion of La1−xSrxCoO3−δ Xinzhi Chen and Tor Grande* Department of Materials Science and Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway S Supporting Information *
ABSTRACT: The crystal structure and the thermal and chemical expansion of rhombohedral La1−xSrxCoO3−δ (x = 0.3, 0.4) perovskite were investigated by in situ high-temperature X-ray diffraction in pure oxygen and nitrogen atmospheres. The crystal structure was confirmed to change from rhombohedral (R3̅c) to cubic symmetry (Pm3̅m) with increasing temperature due to a second-order ferroelastic to paraelastic phase transition. The rhombohedral distortion at low temperature causes crystallographic anisotropy, and anisotropic thermal expansion, caused by rectification of the antiferrodistortive tilting and a minor decompression of the CoO6/2 octahedra is reported. The onset of chemical expansion due to reduction of the oxidation state of Co was observed below the phase transition temperature, which enabled investigation of the crystallographic anisotropy of the chemical expansion. The chemical expansion was significantly larger along the c-axis relative to the a-axis, demonstrating the strong damping of the octahedral tilting with increasing oxygen vacancies induced by the thermal reduction of Co. This phenomenon caused a strong depletion of the ferroelastic phase transition temperature with decreasing partial pressure of oxygen.
KEYWORDS: perovskite, LaCoO3, thermal expansion, chemical expansion, anisotropy
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INTRODUCTION LaCoO3-based perovskite oxide materials have been widely studied due to their combination of high oxygen ionic and electronic conductivity, which leads to the potential for applying these materials as cathodes in solid oxide fuel cells (SOFC) and as mixed ionic and electronic conducting (MIEC) membranes for oxygen separation or partial oxidation of natural gas.1−5 LaCoO3 is a rhombohedral perovskite with R3̅c space group at ambient temperature,6−8 and becomes cubic with space group Pm3̅m at the second-order phase transition temperature (Tc) at 1337 °C.8 The distortion from cubic symmetry is due to the antiferrodistortive tilting of CoO6/2 octahedra and a minor distortion of the octahedra.9,10 LaCoO3 is ferroelastic below the second-order phase transition temperature, and the ferroelasticity of La1−xSrxCoO3−δ (LSC) and similar materials have been confirmed by stress−strain measurements.11−14 The structural phase diagram of LSC has also been investigated,6−8 and the ferroelastic (rhombohedral) to paraelastic (cubic) phase transition temperature is reduced from 1337 °C for pure LaCoO3 to ambient by substitution of the La3+ site with about 55 mol % Sr2+.7,8 At low doping level and relatively low temperatures the aliovalent substitution is charge-compensated by oxidation of Co3+ to Co4+.6 Sr substitution in LSC is partly compensated by oxygen vacancies above 30 mol % Sr2+.6,14,15 The oxygen deficiency in LSC becomes pronounced also at lower substitution levels at elevated temperatures and decreasing partial pressure of oxygen.17,18 Above the onset temperature (To) of thermal reduction of Co, lattice expansion induced by the change in © 2013 American Chemical Society
valence of Co contributes significantly to the total thermal expansion coefficient.16,19,20 This phenomenon has been termed chemical expansion by Adler and co-workers.21,22 The substantial increase in the thermal expansion coefficient above To has been shown for both La1−xSrxCoO3−δ22 and a whole range of different perovskite materials including La1−xSrxMnO3±δ and La1−xSrxFeO3−δ.23−25 High thermal and chemical expansion is critical for the thermo-mechanical stability of electrochemical devices such as SOFC and MIEC membranes.26−29 Significant mismatch in thermal expansion between membranes and sealing materials or a cathode and electrolyte in a SOFC will induce stresses during thermal cycling. Moreover, chemical expansion gives additional stresses when the material is exposed to a gradient in the chemical potential of oxygen, which is detrimental for the mechanical stability of high temperature electrochemical devices.30 The isotropic chemical expansion of La1−xSrxCoO3−δ has been measured by Chen et al. by dilatometry in a controlled atmosphere.22 Here, we report on the anisotropic nature of chemical expansion in LSC observed by in situ hightemperature X-ray diffraction, which to the authors’ knowledge has not been reported previously for any oxide material. The anisotropic thermal and chemical expansions are reported in detail, and the crystallographic data are discussed in relation to Received: December 18, 2012 Revised: February 18, 2013 Published: February 25, 2013 927
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Table 1. Crystallographic Data of Rhombohedral La1−xSrxCoO3−δ (x = 0.3, 0.4) with Space Group R3̅c at Ambient Temperature: Lattice Parameters ah and ch, Atomic Positions, and the Displacement from the Cubic Position for Oxygen Ions (e) and Goodness of Fit (Rwp) atomic positions and occupancy x
lattice parameters
atom
x/a
y/b
z/c
occ.
0.3
ah = 5.4434(9) ch = 13.1954(3)
0.4
ah = 5.4367(4) ch = 13.2142(1)
La Sr Co O La Sr Co O
0 0 0 0.1179(3) 0 0 0 0.1330(9)
0 0 0 1/3 0 0 0 1/3
0.25 0.25 0 1/12 0.25 0.25 0 1/12
0.7 0.3 1.0 1.0 0.6 0.4 1.0 1.0
Rwp 4.207
0.04873(7) 4.166
0.03357(6)
449C TG-DSC in both O2 and N2 atmosphere. The materials were assumed to be stoichiometric with respect to oxygen prior to the analysis. The heating rate was 5 °C/min. Scanning electron microscopy (SEM) was performed using an Hitachi S-3400N instrument.
the ferroelastic to paraelastic phase transition of LSC and the point defect chemistry of LSC.
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O displ., e
EXPERIMENTAL SECTION
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La1−xSrxCoO3−δ (LSC) powders of compositions x = 0.3 (LSC-73), x = 0.4 (LSC-64) were prepared by solid-state reaction using La2O3 (VWR, >99.9%), SrO (Sigma-Aldrich, >99.9%), and Co3O4 (Sigma, >99.99%). The materials were dried at 800 °C prior to use to remove humidity. Stoichiometric amounts of the solid-state precursors were thoroughly mixed in an agate mortar and fired in air at 1200 °C for 24 h twice with intermediate grindings to ensure chemical homogeneity. The powders were further pressed into pellets, which were fired at 1200 °C for 12 h. The pellets were crushed and the powders were finally annealed at 800 °C for 24 h in air and slowly cooled (30 °C/h) to oxidize the materials to obtain a close to stoichiometric oxygen content. High-temperature X-ray diffraction (HTXRD) was performed with a θ−θ Bruker D8 ADVANCE diffractometer utilizing Cu Kα radiation and equipped with a VANTEC-1 position-sensitive detector. Powders for investigation were contained within an alumina sample holder and heated using a radiant heater mounted within an MRI Physikalische Geräte GmbH high-temperature camera. Prior to heating of the powders, the camera was evacuated and flushed three times with the appropriate sweep gas O2 or N2 (pO2 ∼ 10−5 atm), and a constant slow flow of gas was maintained for the duration of the experiment. An S-type thermocouple mounted in close proximity to the sample was used for temperature determination. Calibration of the system against an Al2O3 standard gave an estimated temperature error of ±15 °C. Patterns were collected from 100 to 900 °C (every 25 °C every step), across an angular range 15−75° 2θ, which was the 2θ range possible using a radiant heater. A step size of 0.016° was used. Prior to collecting the pattern, the sample was held for 30 min at the set temperature to establish equilibrium between the material and the atmosphere. Total collection time per scan at one temperature was approximately 70 min, which was sufficient collection time to obtain low signal-to-noise ratio and good accuracy of the diffraction patterns. The heating rate between each temperature was 5 °C/min. Rietveld refinements were carried out with the Topas academic software version 4.2.31 The structure of two phases was described using a rhombohedral model (R3̅c) at low temperatures and cubic model (Pm3̅m) at high temperatures as described in a previous study.6 The sample peak shapes were described using a Fundamental Parameters model, with broadening described according to a crystallite size type angular dependence. For all temperatures independent variables consisted of five parameters: Chebychev polynomial background function, lattice parameters, sample displacement, symmetry constrained atomic positions, and isotropic thermal displacement parameters. The atomic positions for the rhombohedral space group are (0, 0, 1/4) for La3+/Sr2+, (0, 0, 0) for Co3+/Co4+, and (1/6−e, 1/3, 1/12) for O2− where e is the displacement from the ideal cubic position. Thermogravimetric analysis (TGA) of the annealed La1−xSrxCoO3−δ (x = 0.3, 0.4) were performed using a Netzsch STA
RESULTS
The two materials were single phase according to XRD, and the ambient temperature crystal structure for both compositions, summarized in Table 1, are in excellent agreement with the literature.6 The particle size of the materials were found by SEM to be 3 ± 1 μm with a relatively narrow particle size distribution (SEM micrograph of the powders are provided in Supporting Information). The particle size is important for the discussion of the relaxation time for the thermal reduction occurring during heating. TGA of the two powders were performed to investigate the oxygen nonstoichiometry and the onset of thermal reduction of the two LSC materials during heating; see Supporting Information. The onset of thermal reduction is shifted downward with increasing Sr substitution and reducing pO2 in line with the literature.33−35 The two materials were close to stoichiometric with respect to oxygen after slow cooling in air since oxidation during heating could not be observed, also in agreement with the literature.6,42 All the diffraction patterns were refined with the space group R3̅c or Pm3̅m. Generally, a good fit (Rwp < 4.3%) was obtained for all the patterns, and a representative Rietveld refinement of the X-ray diffractogram for La0.6Sr0.4CoO3−δ at 575 °C in N2 is shown in the Supporting Information. The hexagonal unit cell parameters ah and ch of La1−xSrxCoO3−δ (x = 0.3, 0.4) below the phase transition temperature are shown as a function of temperature and atmosphere in Figure 1. Above the phase transition temperature the cubic unit cell parameter, ac, is displayed in Figure 1 as well. The lattice parameters and the atomic position of oxygen are summarized for each temperature and atmosphere in the Supporting Information. The thermal expansion of the lattice parameters of La0.7Sr0.3CoO3−δ below ∼300 °C was not dependent on the oxygen partial pressure; see Figure 1a. Above 300 °C the thermal expansion of ah and ch demonstrated a clear dependence on the oxygen partial pressure (pO2). While a close to linear dependence was observed in O2, both ah and ch deviate significantly from a linear behavior in N2, which is a clear signature of the onset of chemical expansion above ∼300 °C. The unit cell parameters of La0.6Sr0.4CoO3−δ demonstrated similar behavior as shown in Figure 1b. The onset of chemical expansion was observed from about the same temperature 928
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given temperature intervals, are summarized in Table 3. They are in good agreement with previous XRD studies.6,32 The isotropic thermal expansion calculated from the unit cell volume is in reasonable agreement with those reported by dilatometry,22 also given in Table 3 for comparison. Significant anisotropy in the thermal expansion of ah and ch is observed for both compositions. The TEC of ch is almost 2 times higher than the TEC of ah. The c-axis in the hexagonal unit cell is parallel to the [111] direction in the pseudocubic/rhombohedral unit cell. Chemical expansion contributes significantly to the TEC close to the phase transition, particularly for ch but also for ah above 400 °C for La0.6Sr0.4CoO3−δ. To illustrate the unit cell distortion and anisotropy of the thermal and chemical expansion, the normalized pseudocubic lattice parameters apc = ah/21/2 and cpc = ch/121/2 for the two LSC compositions are shown in Figure 2. The distortion mainly due to the antiferrodistortive tilting of the CoO6/2 octahedra, evidenced by the difference between pseudocubic lattice parameters apc and cpc, is clearly decreasing with increasing temperature, and the pseudocubic lattice parameters apc and cpc become equal at rhombohedral to the cubic phase transition temperature. The thermal evolution of the normalized lattice parameters reveal that the pO2 dependence of ch is significantly more pronounced than that for ah. The strong depression of the phase transition with decreasing pO2 is strongly coupled to an elongation of the c-axis. The thermal evolution of the unit cell volume for the two materials as a function of temperature and atmosphere is shown in Figure 3. The unit cell volume is continuous through the phase transition, providing clear evidence for the second-order nature of the phase transition. Despite the difference in composition between the two materials, the unit cell volume becomes equal in both atmospheres at elevated temperatures. This peculiar variation in the molar volume has been pointed out previously by Mastin et al.6
Figure 1. Hexagonal lattice parameters ah and ch below the rhombohedral to cubic phase transition temperature and cubic lattice parameter ac above the transition temperature as a function of temperature in pure O2 and N2 atmospheres for La1−xSrxCoO3−δ (x = 0.3, 0.4).
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(∼300 °C), but for this composition the nonlinear thermal expansion was more pronounced for both ah and ch. The thermal evolution of ah and ch are however qualitatively dissimilar, which is due to simultaneous occurrence of the second-order phase transition and the onset of chemical expansion as discussed in the following. The thermal expansion of the cubic cell parameter ac for the two materials displays a strong dependence of pO2 with a significantly higher expansion in N2 relative to the expansion in O2. This is in line with the significant chemical expansion reported for LSC.22 The rhombohedral to cubic phase transition temperature for the two materials was clearly dependent on pO2. The transition temperatures determined from the XRD data are summarized in Table 2, and the transition temperatures decrease with increasing Sr substitution and decreasing pO2. The linear thermal expansion coefficients (TEC) of the two unit cell lattice parameters, found by a linear fit to the data in
DISCUSSION Oxygen Nonstoichiometry and Defect Chemistry. Substitution of La3+ with Sr2+ at ambient temperature is mainly compensated by increasing the oxidation state of Co at low substitution level of Sr and/or by creation of oxygen vacancies at high substitution level and elevated temperatures. The two dominating point defect equilibria, using Kröger-Vink notation,36 are × CoCo + La ×La + SrO(s) +
⇄ Co•Co + Sr′La +
Sr content
O2
0.3 0.4 0.3 0.4
N2
± ± ± ±
(1)
1 O2 2
(2)
× where Co•Co and CoCo correspond to Co4+ and Co3+, Oo× and •• Vo are oxygen anion and a vacancy in the oxygen sublattice, respectively. The defect chemistry of LSC has been a subject of intense research, and it has been shown that a mass action type of model including these point defect equations cannot describe the defect chemistry and transport properties. The most successful model describing the oxygen stoichiometry in LSC as a function of T, pO2, and Sr content is the itinerant electron model,33−35 which is not based on a localization of the electrons on Co, a prerequisite in the mass action type of
Tc (°C)(R→C) 700 575 575 500
1 La 2O3 2
× × 2Co•Co + OO ⇄ 2CoCo + V •• O +
Table 2. Second-Order Phase Transition Temperature from Rhombohedral to Cubic Crystal Structure Observed for La1−xSrxCoO3−δ (x = 0.3, 0.4) in O2 and N2 Atmospheres atmosphere
1 O2 (g) 4
25 25 25 25 929
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Table 3. Thermal Expansion Coefficient (TEC) of the Unit Cell Parameters ah and ch and the Isotropic Linear Thermal Expansion of La1−xSrxCoO3−δ (x = 0.3, 0.4) Measured in O2 and N2 Atmospheresa temperature region [oC]
x 0.3
30−500 500−700 30−375 400−575
0.4 a
(30−500) (500−600) (30−375) (400−500)
TEC ah [10−6 K−1] 12.9 12.9 11.9 17.1
(13.5) (18.9) (11.6) (18.5)
TEC ch [10−6 K−1] 29.1 29.1 27.5 25.8
(30.4) (43.7) (28.6) (31.8)
TEC isotropic [10−6 K−1] 18.5 21.4 18.1 23.9
(19.5) (34.2) (18.2) (33.9)
TEC by dilatometry [10−6 K−1]
15.8 (25−450 °C)
Numbers in parentheses are measured in a N2 atmosphere. The corresponding TEC measured by dilatometry22 is given for comparison.
Figure 3. Unit cell volume of the pseudocubic unit cell of La1−xSrxCoO3−δ (x = 0.3, 0.4) as a function of temperature in pure O2 and N2 atmospheres.
level. The model parameters used here to estimate the oxygen deficiency are summarized in the Supporting Information. The model provided by eq 4 is used in the following to calculate the oxygen nonstoichiometry (δ) in O2 and N2 atmospheres (pO2 ∼ 5 × 10−5 atm), and the calculated oxygen nonstoichiometry are shown in the Supporting Information. The onset of thermal reduction in O2 and N2 is in reasonable agreement with the prediction of significant oxygen nonstoichiometry (Figure S3 in the Supporting Information), and it can be concluded that the materials were in equilibrium with the atmosphere during the XRD experiments. This is in clear contrast to a recent study of La1−xSrxMnO3+δ (LSM),23 where equilibrium was not achieved until above 600 °C despite 30 times smaller particle size. The difference can be explained by the redox kinetics, which is dominated by slow diffusion of cations in LSM.23,40,43,44 The kinetics of defect equilibrium, eq 2 or eq 3, is controlled by diffusion of oxygen vacancies and oxygen anions. During reduction, oxygen gas is released to the atmosphere and oxygen vacancies diffuse from the surface to the interior of the grains due to the chemical gradient introduced; simultaneously, oxygen anions are transported in the opposite direction, and vice versa for the oxidation process. Tracer diffusion of oxygen in LSC has been investigated in detail, and the oxygen vacancy diffusion is known to be relatively high in LSC.37−41 In this study the powders were annealed at 800 °C in air and cooled slowly to ambient in air prior to the analysis. The initial oxygen stoichiometry of the powders would depend on the size of particles (length scale of the diffusion process) and the cooling rate (time scale).23 Slow reheating of the materials in a higher and lower pO2 could then possibly lead to an oxidation in O2 and reduction in N2 to
Figure 2. Normalized lattice parameters apc = ah/21/2 and cpc = ch/ 121/2 and the cubic lattice parameter ac as a function of temperature in pure O2 and N2 atmospheres for La1−xSrxCoO3−δ (x = 0.3, 0.4).
model. The defect chemistry in this model is simply described by the equation 1 × OO ⇄ O2gas + V •• O + 2e′ (3) 2 × •• where Oo denotes regular lattice oxygen ions and Vo oxygen vacancies. The equilibrium oxygen nonstoichiometry can be calculated from the equation22,33−35 ⎛ −ΔF 0 − 4(2δ − x)/g (ε ) ⎞ ⎛ δ ⎞2 F ⎟ = exp⎜ ⎟ PO2⎜ ⎝3 − δ⎠ kBT ⎝ ⎠
(4)
where ΔF is the Helmholz free energy, which only depends on the temperature.35 g(εF) is the density of states at the Fermi 0
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Table 4. Calculated Isotropic Chemical Expansivity (ac) for La1−xSrxCoO3−δ (x = 0.3, 0.4) in This Studya
a
x
temperature [oC]
0.3 0.3 0.3 0.4 0.4 0.4
725 800 875 725 800 875
δ in N2 (5 × 10−5 atm) 0.093 0.131 0.170 0.145 0.185 0.226
± ± ± ± ± ±
0.005 0.005 0.005 0.005 0.005 0.005
δ in O2 (1 atm)
ΔV [Å3]
± ± ± ± ± ±
0.467 0.662 0.864 0.554 0.680 0.879
0.008 0.020 0.035 0.040 0.060 0.090
0.002 0.002 0.002 0.002 0.002 0.002
αc = 1/3[ΔV/(V0Δδ)] 0.031 0.034 0.036 0.030 0.031 0.036
± ± ± ± ± ±
0.003 0.002 0.002 0.002 0.002 0.002
αc by dilatometry
0.043 (600−900 °C)
Corresponding chemical expansivity measured by dilatometry22 is given for comparison.
pressure of oxygen needs to be calculated. The expansion εa and εc changing from O2 to N2 atmosphere as a function of temperature below the phase transition are shown in Figure 4.
equilibrate the oxygen content in the material. Contraction of the lattice due to oxidation of the materials in O2 could not be observed, which support that the materials were essentially stoichiometric after cooling slowly in air. La0.7Sr0.3CoO3−δ has previously been reported to be stoichiometric at ambient, while La0.6Sr0.4CoO3−δ has been reported to be slightly nonstoichiometric.6,42 However, here the thermal history and the particle size or dimensions of bulk materials may give variation in the oxygen stoichiometry due to a f reeze-in of the point defect equilibria during cooling.23 Isotropic Chemical Expansion. The isotropic chemical expansion29 was estimated from the measured difference in the unit cell volume (ΔV/V) by going from O2 to N2 atmosphere at constant temperature and corresponding data for the change in the oxygen stoichiometry (Δδ) calculated by eq 4.22,33−35 The oxygen partial pressure surrounding the sample is most likely higher than that in the N2 sweep gas (10−5 atm) due to the considerable amount of oxygen released from the material. We have therefore estimated the oxygen partial pressure in N2 to correspond to 5 × 10−5 atm. The average isotropic chemical expansion coefficients at four different temperatures, calculated as 1/3[ΔV/(VΔδ)], is summarized in Table 4. The chemical expansion, particularly at high temperature, where the error is less, is in good agreement with previous data reported by Lein et al. and Chen et al.22,45 The chemical expansion calculated here based on the unit cell lattice parameters are slightly lower than the chemical expansivity measured by dilatometry.22 One possible reason for the discrepancy is that the oxygen partial pressure in N2 is underestimated in our experiments, which means that the change in the oxygen nonstoichiometry is overestimated. The chemical expansion reported here are typical values for the chemical expansion of perovskite materials reported previously.29,45−50 The increase in chemical expansivity with increasing temperature simply reflects the fact that the chemical expansion is nonlinear.22 The significant chemical expansion of LSC is one of the main challenges using this material in oxygen-permeable membranes.21 The partial pressure of oxygen on the permeate side and feed side of the membrane is significantly different, which induce a permanent gradient in the chemical potential of oxygen. The potential gradient results in a chemically induced stress due to the chemical expansion. The stress may lead to mechanical failure or chemically-induced creep. Anisotropic Chemical Expansion. At low temperature no detectable chemical expansion could be measured, and at elevated temperature especially above the phase transition temperature the crystal structure becomes isotropic and the chemical expansion is also isotropic. Thus, anisotropic chemical expansion could only be measured in a narrow intermediate temperature interval between To and Tc. To determine the anisotropic chemical expansion, the relative change in the lattice parameter ah and ch due to a change in the partial
Figure 4. Expansion of the unit cell of La1−xSrxCoO3−δ (x = 0.3, 0.4) as a function of temperature in pure O2 and N2 atmospheres relative to the unit cell at ambient temperature. The dotted lines are linear fit to the thermal expansion in the temperature interval from ambient temperature to the phase transition temperature in N2.
For anisotropic materials in the absence of external mechanical stress, the anisotropic expansion can be defined as a − ah0 Δεa(T , δ) = hi = αa TΔT + αa cΔδ ah0 (5) Δεc(T , δ) =
chi − ch0 = αc TΔT + αc cΔδ ch0
(6)
where αaT, αcT, and αac, αcc are thermal and chemical expansivity along ah and ch axes, respectively. Δεa and Δεc are the expansions relative to the lattice parameters measured at ambient temperature. If ΔT = 0, only chemical expansion 931
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and Co3+,51 corresponding to αac ≈ 12(rCo3+ − rCo4+)/ao.22 However, the calculated chemical expansion estimated by this model gives higher values than the measured values. A possible explanation for the overestimation by this model could be related to the neglected effect of the oxygen vacancies and the much stronger expansion along the c-axis. The chemical expansion along the c-axis seems not to follow a linear trend with the relative change in oxygen nonstoichiometry. The antiferrodistortive tilting (major) and compression (minor) of the CoO6/2 octahedra due to the ferroelastic transition gives a compression along ch,6 and the chemical expansion rapidly reduces the compression, leading to a pronounced increase in cpc approaching apc (Figure 2). The point defect equilibrium 2 leads to both the introduction of oxygen vacancies and the increasing size of the CoO6/2 octahedra due to the change in the oxidation state of Co. The more pronounced expansion along ch demonstrates a strong coupling between the octahedral tilt angle (see Supporting Information) and the concentration of oxygen vacancies. This is also evident from the strong depression of the ferroelastic to paraelastic phase transition temperature with decreasing partial pressure of oxygen. Chen et al.22 discussed in detail a possible explanation for the nonlinear relationship between chemical expansion and oxygen vacancy concentration without drawing any conclusion regarding possible influence of strain effects due to size effects and vacancy ordering or segregation. The data here clearly show that anisotropic chemical expansion is more pronounced for the unit cell parameter ch than for ah. Compared with oxygen excess (or more precisely cation deficient) perovskite materials such as La1−xSrxMnO3+δ23(LSM) isostructural to LSC, the thermal expansion of both LSM and LSC is larger for the unit cell parameter ch than for ah, while for LSM the chemical expansion is larger for ah than for ch, which is totally different from the anisotropic chemical expansion properties of LSC. This shows that oxygen vacancies and cation vacancies have a different effect on the tilting (or compression) of the octahedra. Ferroelastic to Paraelastic Phase Transition. The evolution of the rhombohedral angle as a function of temperature and composition is illustrated in Figure 6. At the phase transition temperature the angle becomes 60°. The
contributes to the total expansion, and then eqs 5 and 6 can be simplified by Δεa c = αa cΔδ
(7)
Δεc c = αc cΔδ
(8)
where Δεac and Δεcc are chemical expansions along ah and ch axes, respectively, at constant temperature. Thus, by measurement of the lattice expansion as a function of the change in the oxygen nonstoichiometry (Δδ) at constant temperature, it is possible to determine the anisotropic chemical expansivity (αac and αcc). The calculated chemical expansion of the two LSC materials as a function of the change in oxygen nonstoichiometry (Δδ) measured at constant temperature is shown in Figure 5. The chemical expansion displays strong anisotropy and is significantly lower along the ah-axis relative to the ch-axis. The linear chemical expansivity along the a-axis extracted from the data shown in Figure 5 and shown as dotted lines is 0.020 ± 0.001 and 0.017 ± 0.001 for LSC-73 and LSC-64, respectively. The close to linear chemical expansion along the a-axis can be rationalized by a function of the ionic radii of Co4+
Figure 5. Anisotropic chemical expansion for La1−xSrxCoO3−δ (x = 0.3, 0.4) as a function of Δδ at constant temperature. The red symbols denote the chemical expansion along the c-axis, and the black symbols denote the chemical expansion along the a-axis. The dotted line is the linear fit to the chemical expansion along the a-axis.
Figure 6. Rhombohedral angle of La1−xSrxCoO3−δ (x = 0.3, 0.4) as a function of temperature in O2 and N2 atmospheres. 932
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rhombohedral angle decreases almost linearly with temperature in O2 atmosphere, while nonlinear behavior in N2 is evident due to the onset of chemical expansion. The ferroelastic properties of LaCoO3-based perovskites are strongly dependent on the dopant substitution level which significantly influences the crystal structure and causes a shift of the phase transition temperature.52,53 The structure phase diagram of La1−xSrxCoO3−δ (x = 0.3, 0.4) is shown in Figure 7.
Article
ASSOCIATED CONTENT
S Supporting Information *
This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This publication has been produced with support from the BIGCCS Centre, performed under the Norwegian research program Centres for Environment-friendly Energy Research (FME). The authors acknowledge the following partners for their contributions: Aker Solutions, ConocoPhillips, Det Norske Veritas, Gassco, Hydro, Shell, Statoil, TOTAL, GDF SUEZ, and the Research Council of Norway (193816/S60).
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Figure 7. Phase transition diagram of the system La1−xSrxCoO3−δ showing the rhombohedral and cubic stability fields as a function of atmosphere and Sr content. The dotted lines are guides to the eye.
The second-order phase transition from the rhombohedral phase to the cubic phase is obviously reduced with increasing the Sr substitution. The Goldschmidt tolerance factor increases toward unity as the Sr content increases and it is therefore expected that the deviation from cubic symmetry becomes less with increasing Sr substitution.6 The ferroelastic phase transition temperature also reduces with lowering oxygen partial pressure at constant Sr content. The transition temperature shifts downward by about 100 °C from oxidizing to inert atmosphere for the two materials investigated here. The ferroelastic properties of LSC give a nonlinear stress− strain behavior due to reorientation of ferroelastic domain.52,53 Under the mechanical load less energically favorable domains start to rearrange, forming reorientation of domains to conform the new energy field. Previous in situ synchrotron X-ray diffraction investigation has confirmed the domain reorientation during compression.54 Such energy dissipation mechanism does result in toughening of ceramics.55−57
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REFERENCES
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CONCLUSIONS
The crystal structure and thermal and chemical expansion of Srsubstituted LaCoO3 are reported based on an in situ HTXRD study in O2 and N2 atmospheres. A strong anisotropy of the chemical expansion of the materials is demonstrated below the ferroelastic to paraelastic phase transition temperature analogous to the strong anisotropy of the thermal expansion. The rhombohedral to cubic phase transition temperature for these materials is shown to be strongly depressed by increasing Sr substitution and reducing partial pressure of oxygen. 933
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Chemistry of Materials
Article
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