Cation Self-Diffusion in LaCoO3 and La2

J. Phys. Chem. B 2007, 111, 2299-2308

2299

Cation Self-Diffusion in LaCoO3 and La2CoO4 Studied by Diffusion Couple Experiments Maria´ n Palcut, Kjell Wiik, and Tor Grande* Department of Materials Science and Engineering, Norwegian UniVersity of Science and Technology, 7491 Trondheim, Norway ReceiVed: December 5, 2006

Reaction kinetics between dense, polycrystalline pellets of La2O3 and CoO were investigated at temperatures of 1370-1673 K and oxygen partial pressures of 40 Pa - 50 kPa. At high oxygen partial pressures, single phase LaCoO3 was formed. The growth of the LaCoO3 phase followed the parabolic rate law. The location of Pt markers demonstrated that diffusion of Co3+ cations in LaCoO3 dominated over diffusion of La3+. The diffusion coefficient of Co3+ was determined from the parabolic rate constant, and an activation energy of (250 ( 10) kJ mol-1 was found. The diffusion coefficient of Co3+ in LaCoO3 decreased with decreasing oxygen partial pressure. At the lowest oxygen partial pressure investigated, two product phases, LaCoO3 and La2CoO4, were observed. The diffusion coefficient of Co cations in La2CoO4 was estimated. Results were discussed in relation to cation diffusion in other LnBO3 oxides (B ) Cr3+, Mn3+, Fe3+). A correlation between diffusion of the B cation and the melting point was found for LnBO3 materials.

Introduction Materials based on perovskite-type lanthanum cobaltite are promising for oxygen separation membranes,1-3 electrodes,4,5 and other electrochemical devices.6-8 The appropriate substitution with alkali earth metals and 3d elements enhances their transport properties to a considerable extent. La0.6A0.4Co0.8Fe0.2O3-δ (A ) Ca, Sr) and La0.6Sr0.4Co0.8B0.2O3-δ (B ) Ni, Cu) oxides, in addition to high electronic conductivities, have an ionic conductivity comparable to or higher than yttriastabilized zirconia (YSZ).9 SrCo1-xFexO3-δ (x ) 0.2-0.35) and La1-xSrxCoO3-δ (x ) 0.65-0.75) materials10 possess a valuable combination of high oxide ionic conductivity and electronic conductivity, resulting in a superior oxygen permeability. La1-xSrxCoO3-δ (LSC) perovskites have been tested as cathodes in solid oxide fuel cells (SOFC).11 LSC is an ideal cathode material in combination with ceria-based electrolytes, and its performance is even superior to that of La1-xSrxMnO3-δ (LSM).12 However, the use of LSC in the YSZ electrolyte failed due to the formation of insulating phases resulting from cation interdiffusion.13 Reactivity between Ca- or Sr-substituted LaCoO3 and calcium silicate sealing materials has also been reported.14 These studies demonstrate that cation mobility greatly reduces the performance and durability of high-temperature devices. Cation diffusion, although very low in perovskites, may cause a significant degradation of materials exposed to thermodynamic gradients.15,16 Cation diffusion also determines the rate of sintering17 or creep.18 LaCoO3 has a rhombohedrally distorted perovskite lattice.19 Oxygen vacancies are the major point defects in LaCoO3,20-27 and the oxygen deficiency can be given as LaCoO3-δ with δ ) .20 The stability region of the LaCoO3 phase is narrow in kpO-0.5 2 comparison with LaMnO3 or LaFeO3. Two other ternary phases, La2CoO4 and La4Co3O10, are stable at lower oxygen partial pressures.28-32 The crystal structures of these phases are of the Ruddlesden-Popper type. The La2CoO4 phase consists of * Corresponding author. E-mail: [email protected].

alternating layers of LaCoO3 perovskite and LaO rock-salt structures along the c axis.33 The structure of La4Co3O10 consists of stacking of two two-dimensional perovskite sheets separated by a single rock-salt LaO layer.34 Cation diffusion in perovskite-type oxides has been investigated by three independent techniques: tracer annealings,35-41 interdiffusion,42,43 and following the diffusion-controlled solidstate reaction between two binary oxides.44-48 Cation diffusion of radiotracers Co* and Ln* in pure LnCoO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) was studied by the radiotracer technique in air at 1273-1523 K.49 The same authors also reported solid-phase interactions of powder samples of Ln2O3 (Ln ) Sm, Eu, Gd, Dy, Ho) with CoO,50 and a kinetic study between powders of CoO and La2O3 has also been recently repoted.51 Since the complexity of the kinetic problem in powder reactions is high, there is a general agreement that fundamental studies with dense samples, preferably single crystals, with a proper definition of boundary conditions are needed.52 To the best of our knowledge, the kinetic behavior between dense, polycrystalline pellets of CoO and La2O3 was investigated only in the work of A. N. Petrov et al.53 The authors followed the reaction in a narrow temperature interval in air and found that DCo > DLa. The moving phase boundary in the reaction was not marked, and the cation diffusion was only estimated. More detailed information about the reaction mechanism and cation defect chemistry of LaCoO3 is, therefore, lacking. To the best of our knowledge, cation diffusion in La2CoO4 and La4Co3O10 phases has not been reported yet. In the present paper, we report kinetic studies on the formation of lanthanum cobaltite following the solid-state reaction kinetics between dense polycrystalline bodies of La2O3 and CoO in close contact at temperatures of 1370-1673 K and oxygen partial pressures of 40 Pa - 50 kPa. A wide range of temperatures and oxygen partial pressures, together with separate marker experiments, has enabled us to provide a comprehensive insight into the reaction mechanism and cation diffusion in LaCoO3 and La2CoO4.

10.1021/jp068343s CCC: $37.00 © 2007 American Chemical Society Published on Web 02/13/2007

2300 J. Phys. Chem. B, Vol. 111, No. 9, 2007

Figure 1. Cross-sectional views of diffusion couples heated at 1673 K for 10 h (a) and 30 h (b) in air. Phases from the left: La2O3, LaCoO3, CoO. Platinum markers are located close to the LaCoO3-CoO reaction interface (bright spots). Samples were mounted in epoxy resin before analysis.

Experimental Powders of La2O3 (99.99% purity, KEBOLab) and cobalt(II) hydroxide carbonate (99.9% purity, Riedel-de Haen AG Seelze-Hannover), were calcined for 5 h at 1073 and 893 K, respectively, prior to use. Fine ball-milled powders (with added 0.7 wt % ethylcellulose) were then uniaxially pressed (50 MPa) into cylindrical pellets (15-mm diameter, 5-7-mm high). La2O3 pellets were sintered to 94-96% relative density in air at 1873 K for 5 h. CoO pellets with 91-92% relative density were prepared by heat treatment in air at 1723 K for 2 h. Higher densities of CoO pellets could not be obtained due to severe grain growth. The pellets were ground and polished using 1-µm diamond abrasive. The diffusion couples were made by facing the polished surfaces of La2O3 and CoO between two alumina plates and pressed together by a load of 0.5 kg. The initial interface was marked by a thin layer of colloidal platinum paint. Platinum paint was placed on isolated positions of the sample only, since a complete layer could possibly hinder the perpendicular transport process. The diffusion couples were heat-treated in dry and CO2-free air in a vertical tube furnace controlled at 1370-1673 K for 10-100 h. Heating and cooling rates were both 200 K/hour. Samples were also kept at 1573 K for 10-60 h in three different gas mixtures of oxygen and nitrogen. The oxygen partial pressure was monitored by a type-DS oxygen probe with an SIRO2 oxygen sensor (Australian Oxytrol systems) installed inside the furnace near the reactants. After each annealing, samples were mounted in the epoxy resin, cut perpendicular to the reaction interface, and polished by 1-µm diamond abrasive in the final step. The product phase was

Palcut et al. characterized by X-ray diffraction (Siemens D5005, Siemens Germany) with Cu KR radiation (λ ) 1.5418 Å). The microstructure was investigated by low-vacuum scanning electron microscope (Hitachi S3500N) with a solid-state backscattered detector and an EDS (energy dispersive spectroscopy) detector. Accelerating voltage for the electron beam was 15 kV. The element distribution in the product phase was studied by wave-dispersive spectroscopy (JEOL JXA-8900). Three to four line scans of different positions in the sample were obtained by applying the accelerating voltage of 15 kV for the electron beam. Line scanning was kept perpendicular to the reaction interfaces. Step size was 1 µm, and the scanning started in the bulk of one of the reacting phases. The standard for the electron microprobe analysis (EMPA) was lanthanum cobaltite prepared by spray pyrolysis. The precursor was prepared from nitrate solutions of Co-EDTA and La-EDTA complexes (EDTA ) ethylenediamine tetraacetic acid) mixed in the exact molar ratio of 1:1. The powder from pyrolysis was then calcined at 900 °C for 24 h in air. Dense pellets of LaCoO3 were prepared by conventional sintering at 1400 °C for 2 h in air (93% relative density). Results 1. The LaCoO3 Product Layer. Typical cross-sectional views of diffusion couples in air are shown in Figure 1. Images were recorded by a scanning electron microscope working in backscatter mode to provide the element contrast. A single product layer is evident between the two reactants. The thickness of the product layer is in the range of 10-100 µm, depending on the annealing conditions. The product layer is homogeneous and uniform across the whole region (Figure 1a). Signs of nucleation or early stage growth were not detected at any experimental temperature. The reactants were in immediate contact during the reaction. Platinum markers were always found at the CoO-product interface (Figure 1b). The adjacent threephase system fractured along the phase boundaries. Epoxy resin, introduced before the SEM analysis, is evident between the separated phases. The diffusion couples fractured along the phase boundaries during cooling, probably due to thermal mismatch between the materials. The X-ray diffraction pattern taken from a separated reaction interface is given in Figure 2. The single product phase was rhombohedral (hexagonal) LaCoO3. The composition of the layer was confirmed by EDS.

Figure 2. Hexagonal (rhombohedral) diffraction pattern of LaCoO3 formed at 1573 K and p(O2) ) 20 kPa. Due to the certain penetration depth of the X-ray beam, there are also signals of La2O3 (2).

Cation Self-Diffusion in LaCoO3 and La2CoO4

J. Phys. Chem. B, Vol. 111, No. 9, 2007 2301

Figure 3. Chemical composition of the LaCoO3 product phase. The starting oxides are on the left (La2O3) and on the right (CoO) side of the diffusion couple. The trend line is meant to guide the eye. The sample taken for the analysis reacted at p(O2) ) 50 kPa and T ) 1573 K for 60 h.

The chemical composition of the product phase was further investigated by wave-dispersive spectroscopy. Line scans measured across the product layer are given in Figure 3. Measurements started in the bulk of La2O3, continued across the layer, and ended in the CoO phase. Scanning was kept perpendicular to the interface. Limits at L ) 0 and L ) L0 reflect the composition of the LaCoO3 phase in equilibrium with the La2O3 and CoO phases, respectively. Three independent line scans from different vertical positions of the sample are given. The La/Co molar ratio remains close to 1 in most of the layer. The results for the O/Co molar ratio are more scattered, especially in the right half of the layer. This is due to the lower accuracy of EMPA in the analysis of light elements, such as oxygen. Despite this inaccuracy, the O/Co molar ratio increases toward the Co-rich phase boundary. This may reflect a change in the valence state of cobalt across the layer. Growth kinetics of the LaCoO3 layer in air is shown in Figure 4. Thickness of the product, x, measured at 10-15 different vertical positions in the sample, follows the simple parabolic rate law

x2 ) 2kpt + C

(1)

where kp is the parabolic rate constant, t is the dwelling time, and C is an integration constant. This result indicates the diffusion-controlled character of the reaction. Since all lines cross the origin, C is equal to 0. The layer growth during heating or cooling is negligible. An Arrhenius plot of the parabolic rate constant is displayed in Figure 5. The activation energy of the transport process is EA ) (250 ( 10) kJ mol-1. Growth kinetics of the product layer was also monitored with respect to oxygen partial pressures. The result is given in Figure 6. The parabolic rate law is again valid. The rate constant of the diffusion-controlled process increases with increasing oxygen partial pressure (Figure 7). 2. The La2CoO4 Product Layer. Results obtained in a reducing atmosphere (p(O2) ) 40 Pa) were not included in the previous section, since the main reaction product was different. The X-ray diffraction pattern taken from the interface after the

Figure 4. Parabolic plot of the thickness of the LaCoO3 phase versus the dwelling time of thermal anneal at 1370 (b), 1478 (2), 1573 (9), and 1673 K ([) in air.

Figure 5. Arrhenius plot of parabolic rate constant in air.

reaction at 1573 K and p(O2) ) 40 Pa is given in Figure 8. In addition to weak Bragg reflections due to LaCoO3, there are also reflections showing the presence of La2CoO4. Subsequent SEM/EDS analysis of the samples confirmed that the major phase was La2CoO4, which formed a complete homogeneous layer (Figure 9). Islands of LaCoO3 were isolated, and they were present only at the phase boundary with CoO (Figure 9b). The reaction system fractured along the phase boundaries. The chemical composition of the La2CoO4 phase was further confirmed by EMPA (Figure 10). Three line scans from different vertical positions of the sample are given, and they overlap each other. Growth kinetics of the La2CoO4 layer is shown in Figure 11. The growth obeys a simple parabolic rate law (1) fairly well. Small deviations can be reasonably explained from Figure 9c. The La2CoO4 layer in contact with LaCoO3 (line 1) is apparently thinner than the La2CoO4 layer measured in position 2.


Palcut et al.

Figure 8. Diffraction patterns of La2CoO4 (b) and LaCoO3 (9) phases formed after the reaction at 1573 K and p(O2) ) 40 Pa for 60 h. There are also signals of La2O3 (2) and platinum (1). Figure 6. Parabolic plot of the LaCoO3 product thickness versus the annealing time at 1573 K in atmospheres with p(O2) ) 1 (1), 20 (b), and 50 kPa (9).

Figure 7. Logarithmic plot of the parabolic rate constant versus the oxygen partial pressure at 1573 K. The trendline is meant to be a guide for the eye.

Discussion 1. Cation Diffusion in LaCoO3. The overall chemical reaction of the studied process is

1 1-δ CoO(s) + La2O3(s) + O2(g) ) LaCoO3-δ(s) 2 2

(2)

The oxygen deficiency (δ) is relatively small and neglected for simplicity in the following. Gaseous oxygen plays an important role in reaction 2. The O2(g) supply in our experiments was always significantly higher than the amount needed according to eq 2. O2(g) is transported from the sample edges to the reaction site. This process takes place probably along grain boundaries, since there is no open porosity in the product phase (Figure 1). We did not register any significant differences in thickness between the edges and the center of the sample. This confirms that the entire layer was equally saturated with gaseous oxygen during the reaction. It is therefore reasonable to conclude that the transport of molecular oxygen did not control the kinetics of reaction 2.

Figure 9. Cross-sectional view of diffusion couples heated at 1573 K for 60 h at p(O2) ) 40 Pa in a backscatter mode. Magnification of the rectangular area is given in Figure 9b. Overview is schematically given in Figure 9c. Product phases: 1 ) La2CoO4, 2 ) LaCoO3. Platinum markers are located at the interface with CoO phase (bright spots). Differences in thickness of La2CoO4 layer measured perpendicularly to the La2O3-La2CoO4 interface are indicated by dashed lines. Samples were mounted in epoxy resin (dark phase) before the analysis.

The diffusion-controlled growth of ternary compounds from binary oxides was first quantitatively described by C. Wagner.52 Oxide growth may occur by counterdiffusion of cations or by unidirectional diffusion of anions and cations. In the case of mixed conductors, the cation flux may also be compensated by the flux of electrons or holes. The parabolic rate constant is related to the diffusion coefficients of ions taking part in the reaction and to the standard Gibbs energy of formation of the ternary oxide. The oxide grows between two other compounds in close and immediate contact. The phase boundary between the materials moves as the reaction proceeds. The location of the marker after the reaction reflects the cation transport during the reaction. The presence of platinum markers at the LaCoO3/ CoO interface show that the Co3+ diffusion in LaCoO3



Figure 10. Chemical composition of the La2CoO4 phase. The starting oxides are on the left (La2O3) and on the right (CoO/Co3O4) sides of the sample. Stoichiometric composition is illustrated by the dashed line. The sample taken for the analysis reacted at p(O2) ) 40 Pa and T ) 1573 K for 60 h.

Figure 12. Principle of the marker experiment illustrated on the sample heated at 1673 K for 30 h at p(O2) ) 20 × 103 Pa. Platinum markers indicate the position of the initial interface after the reaction, and they thus reflect the direction of mass transport. Phases: 1 ) La2O3, 2 ) CoO, 3 ) LaCoO3.

cations, and µCoXCo(La) and µCoXCo(Co) are the chemical potentials of Co cations at the interfaces. The actual formation of LaCoO3 takes place at the La2O3/ LaCoO3 phase boundary. The dissolution of La2O3 in LaCoO3 may be written as

1 3 X 3 •• La O ) LaXLa + V/// Co + OO + VO 2 2 3 2 2 Figure 11. Parabolic plot of the La2CoO4 layer thickness versus dwelling time of thermal anneal at 1573 K and p(O2) ) 40 Pa.

dominates over La3+ diffusion, that is, DCo . DLa. Cation flux is schematically given in Figure 12. The diffusivity of O2- at 1273 K is ∼2-3 orders of magnitude higher than the parabolic rate constant of the present reaction at 1370-1673 K.54,55 The chemical diffusion coefficient of O2- is p(O2)-dependent and decreases with increasing oxygen partial pressure as p(O2)-0.46 or p(O2)-0.31.55 The parabolic rate constant, on the other hand, increases with p(O2) (Figure 7). These arguments lead us to conclude that the O2- diffusion does not limit the parabolic oxide growth, i.e., DO . DCo. The parabolic rate constant is therefore related to the diffusion of Co3+ ions, DCoXCo,

Vm kp ) RT

∫µ

µCoX Co(La) X CoCo(Co)

The vacancies at the Co cation sublattice are formed. The oxygen and cobalt ions diffuse from the CoO/LaCoO3 interface and disappear with the corresponding vacancies. The reaction taking place at the CoO/LaCoO3 interface may be given as

1 3 3 CoO + O2(g) ) CoXCo + OXO + V••O + V/// La 2 2 2

(5)

Molecular oxygen needed in eq 5 diffuses along the CoO/ LaCoO3 interface. The sum of eqs 4 and 5 gives eq 2 with δ ) 0, taking into account the Schottky equilibrium, eq 6. The cobalt diffusion may occur by the vacancy mechanism. The dominant point defect in LaCoO3 is the oxygen vacancies, as described previously.20 The vacancies in the cation sublattices are formed due to the Schottky equilibrium, /// nill ) 3V••O + V/// La + VCo

cCoXCoDCoXCo dµCoXCo

(4)

(6)

(3)

where Vm is the molar volume of reaction product formed by transport of Co, cCoXCo is the molar concentration of Co3+

The fluxes of the cobalt cations and vacancies are compensated,

cCoXCoDCoXCo ) [V/// /// Co]DVCo

(7)


Palcut et al.

TABLE 1: Experimental Parabolic Rate Constants for the Layer Growth of LaCoO3 in Air Compared to the Calculated Cobalt Diffusion Coefficients T/K

-∆G0r /(RT)a

kp cm2s-1

/ 2 -1 b DCo X cm s Co

1370 1478 1573 1673

3.50 2.83 2.38 1.96

(3.11 ( 0.69) × 10-12 (1.49 ( 0.14) × 10-11 (5.01 ( 0.16) × 10-11 (1.22 ( 0.22) × 10-10

3.02 × 10-12 1.40 × 10-11 4.55 × 10-11 1.05 × 10-10

a

Ref 31. b Calculated using formula 10.

where [V/// Co] is the concentration of cobalt vacancies and DV/// Co is the diffusion coefficient of cobalt vacancies. The cobalt diffusion coefficient may be evaluated if the equilibrium 4 is valid for the system. The equilibrium constant of reaction 4 is

K4 )

•• 3/2 [LaXLa][OXO]3/2 [V/// Co][VO] 1/2 aLa 2O3

(8)

-1/2 26 Since we have [V•• the concentration of cobalt O] ∝ pO2 , vacancies and diffusion coefficient, correspondingly, can be expressed as

1/2 3/4 DCoXCo ∝ [V/// Co] ∝ aLa2O3 pO2

(9)

This expression explains the observed increase in the parabolic rate constant with oxygen partial pressure (Figure 7). Analysis of the problem according to eq 7 leads to the following expression,47,48

(

/ DCo X ) kp 1 - exp Co

( )) ∆G0r RT

-1

(10)

/ where DCo X is the cobalt diffusion coefficient at the La2O3/ Co LaCoO3 phase boundary, and ∆G0r is the Gibbs free energy of the following reaction at the experimental temperature.21,22

1 1 La O + CoO + O2 f LaCoO3 2 2 3 4

(11)

Values calculated according to eq 10 are given in Table 1. The self-diffusion coefficient of Co3+ in LaCoO3, DCo, may be finally given as

( )

DCo ) DoCopOn 2 exp -

EA RT

Figure 13. Parabolic rate constant of the formation of lanthanum cobaltite in air from the present study compared to the data from the literature. Symbols: b, present data; O, data reported by A. N. Petrov et al.;53 and 1, tracer diffusion coefficient of 60Co in LaCoO3.49

(12)

with n ) +(0.24 ( 0.14) and EA ) (250 ( 10) kJ mol-1, determined by diffusion couple experiments. DCo is equal to / DCo X at the La2O3/LaCoO3 phase boundary, where the activity Co of La2O3 is ∼1. At the CoO/LaCoO3 phase boundary or in the bulk of the LaCoO3 phase, the self-diffusion coefficient of Co3+ in LaCoO3 is lower, and it has to be adjusted for the activity of La2O3 in LaCoO3.47,48 The solid-state kinetics between pellets of La2O3 and CoO in air was previously studied by A. N. Petrov et al.53 Results are compared in Figure 13. Numerical values for the rate constants are in good agreement. The authors also determined that the faster-diffusing cation in LaCoO3 is Co3+. The activation energy found in the previous study,53 (550 ( 20) kJ mol-1, disagrees with the value reported here. The precision of this determination, however, is questionable, since they followed the reaction only in a 50 K interval. The activation energy from the present study, based on a 300 K temperature interval, represents a more reliable value.

Tracer diffusion of 60Co in polycrystalline LaCoO3 in air is also shown in Figure 13. The chemical and tracer diffusion coefficients of Co3+ in LaCoO3 lie in the same region. The activation energy for the tracer diffusion is (170 ( 15) kJ mol-1.49 The lower activation energy for the tracer diffusion in perovskites is generally known (see, for example, refs 41,47). This fact may well represent the principal differences in the mechanism between these two processes. Samples for tracer diffusion measurements are usually prepared as dense disks by the conventional ceramic procedure. The tracer is subsequently applied onto the sample surface and annealed at high temperature for a defined period of time. The preparation method of the oxide is a source of defects in terms of pores and grain boundaries. The oxide layer in diffusion couple experiments, on the other hand, is always formed in situ. The rate constant of the parabolic oxide growth therefore reflects the combination of two elemental processes at the same time: the formation of the oxide (with its equilibrium number of vacancies) and cation diffusion. The apparent activation energy from diffusion couples reflects both the activation energy for diffusion and the enthalpy of formation of cation vacancies. The tracer diffusion reflects only cation diffusion in the sample. The activation energy is thus lower than the activation energy from diffusion couple experiments. Tracer diffusion is also related to the thermal history of the sample. O. Schulz et al., for example, report that the activation energy for 138La, 84Sr, and 25Mg tracer diffusion in La Sr Ga Mg O 0.9 0.1 0.9 0.1 2.9 is lower at low temperatures.40 This is due to the “frozen-in” effect of vacancies at low temperatures. The correlation between the thermal history of the sample and tracer diffusion coefficients in oxide materials has been recently reviewed by W. D. Carlson in the case of garnet-type oxides.56 It is clearly shown that there exists a systematic dependence of diffusion rates on the host garnet microstructure and composition. The diffusion coefficient of La3+ cations in LaCoO3 has not yet been experimentally determined. Estimation of the La3+ diffusion, however, can be made from platinum marker experiments and also from the powder reaction kinetics, since the reaction between powders may be limited by the diffusion of the least mobile species. From the present study, we conclude that DCo . DLa because the platinum markers were not found



inside the layer. The study by B. Yan et al. provides the rate coefficients for reaction between powders of La2O3 and Co3O4.51 They are 2-3 orders of magnitude lower than Co3+ diffusion coefficients from the present study. The Ln* tracer diffusion was studied for PrCoO3, SmCoO3, and EuCoO3 only.49 The DCo*/DLn* ratios at 1300 K in air are 0.16, 1.7, and 3.3 respectively. The authors explained this trend by the transformation of the LnOm polyhedra in the crystal lattice with decreasing radius of the Ln cation. It is clear that LaCoO3 does not fit into this trend. Fast diffusion of Co3+ cations in LaCoO3 is also surprising with regard to the electrostatic considerations in the perovskite lattice.57,58 Tracer diffusion of 60Co was investigated in LnCoO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd).49 The diffusion coefficient of 60Co follows the sequence NdCoO3 < GdCoO3 < SmCoO3 < EuCoO3. This sequence does not correlate with the decreasing radius of the Ln cation. Explanations other than those resulting from the static crystallographic observations must be sought. In the following discussion, we want to emphasize that the diffusion experiments are often carried out at temperatures close to the melting point of the materials. It is therefore of interest to investigate the cation diffusion of LnCoO3 materials with respect to their fusion temperature. Temperature dependence of the diffusion coefficient (Arrhenius law) can be rewritten as

log D ) log A -

1 EA Tm 2.303R Tm T

(13)

where A is a pre-exponential factor, EA is the activation energy, Tm is the melting temperature, and other symbols have their usual meaning. The melting temperature of LnCoO3 materials in air is taken from ref 27. The plot of log D versus Tm/T for Co* and Ln* cations in LnCoO3 materials is given in Figure 14. There is a clear correlation between the melting point and the diffusion coefficient of the B cation. Tracer diffusion coefficients of 60Co in LnCoO3 fall into one line for all the materials. This is especially true as the temperature approaches the melting point. Data for Ln* diffusion coefficients, on the other hand, do not necessarily fall into one line. Data for NdCoO3 and EuCoO3 are close; tracer diffusion of 141Pr in PrCoO3 significantly differs. Two important conclusions can be found in Figure 14a. First, the diffusion coefficients extrapolated for the melting temperature reach a constant value of ∼10-9.5 cm2 s-1, regardless of the character of the Ln cation in all LnCoO3 samples. Second, from the slope of the linear behavior, one can conclude that the ratio EA/Tm is also constant. The value is (72 ( 9) J K-1 mol-1. The activation energy for the Co cation transport is proportional to the melting temperature. The Co3+ diffusion and melting process of the solid oxide are probably closely related in all LnCoO3 oxides. Cation diffusion in LnBO3 oxides (Ln ) La, Nd, Y; B ) Cr, Mn, Fe) has been studied by diffusion couple measurements previously.44-48 The collection of data is presented in Figure 15. In LnCrO3 oxides, the Ln cation diffusion dominates. Mn, Fe with respect to Co cation diffusion, on the other hand, is dominant in LaMnO3, LaFeO3, and LaCoO3. In light of the previous discussion we can compare the melting points of these materials in air. Melting points of chromites are LaCrO3, (2703 ( 30) K and YCrO3, (2583 ( 30) K.27 The melting point of NdCrO3 is known only in inert atmosphere, Tm ) 2603 - 2690 K.59 Melting points of these materials are relatively close. Nevertheless, the diffusion coefficients of La3+ and Y3+ cations

Figure 14. Tracer diffusion coefficients for Co (a) and Ln cations (b) in LnCoO349 for the reduced temperature scale.

in chromites differ substantially. The Ln cation diffusion coefficients in LnCrO3 do not fall into one line in the log D vs Tm/T plot. The values of the B cation diffusion coefficients (B ) Mn, Fe, Co) in LaMnO3, LaFeO3, and LaCoO3 lie in the same region (Figure 15a). LaFeO3 melts congruently at ∼2163 K in air.60 The melting point of LaMnO3 was reported at 2173 K.61 The chemical diffusion of the B cation and the melting of the crystal lattice are again related. The slow diffusion of Cr3+ in LnCrO3 materials35 may be now well explained by the high fusion temperatures of LnCrO3 materials in air. Since there is generally no correlation between the Ln cation diffusion and the melting point, we suggest that the fusion of the perovskite crystal structure is related to the properties of the B-O-B framework. The pO2-dependence of the cation diffusion coefficients in LaBO3 (B ) Fe, Co, Mn) is compared in Figure 15b. It is apparent that the actual transport paths, transport mechanisms, and defect chemistry of these oxides may differ. To fully explain these peculiarities, one needs to carry out more sophisticated experiments. The interdiffusion experiments between dense, polycrystalline LaCoO3 and LaMnO3 are in progress.62 These experiments will provide important information on Mn3+ diffusion in LaCoO3 and Co3+ diffusion in LaMnO3. Because the present technique does not provide cation penetration profiles, it is not possible to obtain bulk and grain boundary


Palcut et al.

Figure 16. Isothermal section of the La2O3-CoO phase diagram at 1573 K. Experimental oxygen partial pressure lies in the belt between two dashed lines.

Figure 15. Cation diffusion in perovskite oxides in air (a) and in atmospheres with different oxygen partial pressures (b): O, present data for LaCoO3; 4, LaMnO348; 0, LaFeO347; b, LaCrO344; 9, NdCrO345; and 2, YCrO346. The p(O2)-dependence for LaFeO3 was studied at 1423 K, and for LaCoO3 and LaMnO3, at 1573 K.

diffusion coefficients of Co3+ cations. Nevertheless, the accumulation of Mn3+ cations at grain boundaries in LaCoO3 is significant.62 The grain boundary diffusion could also be the possible transport path for Co3+ cations in this material. 2. Cation Diffusion in La2CoO4. The La2CoO4 phase can be, in principle, formed by the reaction of La2O3 with either CoO or LaCoO3. The overall reaction can be written as

La2O3(s) + CoO(s) ) La2CoO4(s)

(14)

La2O3(s) + LaCoO3(s) ) La2CoO4(s) + O2(g)

(15)

or

The presence of markers at the interface with the CoO phase reflects the position of the initial interface (Figure 9). The reaction takes place at the La2O3-product phase boundary. Co cations diffuse from the CoO phase through the product layer(s).

The LaCoO3 phase does not form a complete homogeneous layer. Islands of LaCoO3 are located at several positions along the La2CoO4-CoO phase boundary (Figure 9a). The islands are typical for the initial stage in the layer growth when the system is not yet equally saturated with its surroundings. The LaCoO3 phase did not grow into a uniform layer simply by increasing the annealing time in our experiments. The reason for this may be 2-fold. First, the LaCoO3 phase is either thermodynamically unstable under the experimental conditions, or the limit of its stability is very close. Second, the more stable La2CoO4 phase grew faster than the LaCoO3 phase and, thus, consumed it (see eq 15). Nevertheless, the LaCoO3 product phase was present in the reaction system. Reasons for this must be sought. It is of interest to compare the thermodynamic stabilities of two-phase regions in the pseudobinary system La2O3-CoO. To the best of our knowledge, the phase diagram of the pseudobinary system La2O3-CoO at 1573 K has not been reported yet. Important conclusions, however, can be drawn from thermodynamic data reported for lower temperatures. The LaCoO3 and La2O3 phases do not coexist at low oxygen partial pressures.28,32 Our observations are in agreement with this statement, since the LaCoO3 phase was located only between the CoO and La2CoO4 phases. The LaCoO3 and CoO phases may coexist down to p(O2) ≈ 10-5 bar at 1273 K.28 This limit shifts slowly up to p(O2) ≈ 10-4 bar at 1373 K.32 The experimental oxygen partial pressure in the present study was 40 Pa, that is, 10-3.4 bar at 1573 K. The La2CoO4 and CoO phases, on the other hand, may coexist only below p(O2) ≈ 10-4.5 at 1373 K.32 The LaCoO3 phase is needed to stabilize the La2CoO4-CoO interface. The two-phase regions at 1573 K from the present study are schematically given in Figure 16. According to our observations, the La2CoO4 and LaCoO3 phases coexist. The phase diagrams reported at 1373 and 1273 K, however, exclude the coexistence of La2CoO4 and LaCoO3. There should be a third phase, La4Co3O10, coexisting with both the La2CoO4 and LaCoO3 phases.32 We have never observed a third product phase in our reaction system. It does not necessarily mean that it is thermodynamically unstable at 1573 K. There may be a kinetic reason for its absence. The major phase, La2CoO4, could have grown fast enough to suppress the formation of other ternary phases. The growth of the La2CoO4 layer is diffusion-controlled (Figure 11). Small deviations may result from difficulties to accurately measure the thickness in systems with two product phases. The parabolic rate constant of the process at 1573 K and p(O2) ) 40 Pa is (4.0 ( 0.5) × 10-11 cm2 s-1. This value is comparable to the rate constant for the LaCoO3 growth at

Cation Self-Diffusion in LaCoO3 and La2CoO4 higher oxygen partial pressures (Figure 7). The similarity between Co diffusion in LaCoO3 and La2CoO4 is surprising, since the La2CoO4 phase has a layered structure where perovskite structures are altered with rock salt layers.61 One would expected the Co diffusion in La2CoO4 to be slower than in LaCoO3. In an ideal La2CoO4 phase, the Co diffusion would take place mostly in the ab plane. The Ruddlesden-Popper type phases, however, are often nonstoichiometric.33,63,64 A significant number of vacancies at both oxygen and lanthanum sites were reported for the La2CoO4 phase.33 Co diffusion in La2CoO4 can, in principle, take place also along the c axis, probably via vacant La sites. Apparently, more information is needed to clarify the Co transport paths in La2CoO4. Conclusion The main conclusions of the present work may be given as follows: 1. The reaction kinetics between dense and polycrystalline La2O3 and CoO is a diffusion-controlled process. A single homogeneous phase, LaCoO3, was formed at p(O2) ) 1-50 kPa and T ) 1370-1673 K. At the lowest oxygen partial pressure, p(O2) ) 40 Pa, two product phases, LaCoO3 and La2CoO4, were present. These observations are in line with expectations from the pseudobinary La2O3-CoO phase diagram. 2. Diffusion of Co cations in LaCoO3 and La2CoO4 dominates over La3+ diffusion. The diffusion rate constants in LaCoO3 and La2CoO4 were similar. The Co diffusion in La2CoO4 thus can, in principle, take place both in the ab plane and perpendicularly to the rock salt layers in the crystal structure. 3. Diffusion coefficients of B cations in LaBO3 oxides (B ) Fe, Co, Mn) significantly correlate with melting point of these materials. Since there is generally no correlation between the Ln cation diffusion and the melting point, we suggest that the fusion of the perovskite crystal structure is related to the stability of the B-O-B framework. Acknowledgment. Mrs. Eli Beate Jakobsen is thanked for help with the construction of the high-temperature furnace. Mr. Morten Peder Raanes is thanked for his help with the EMPA analysis. This work received financial support from the Research Council of Norway (Grant no. 158517/431, Functional oxides for energy technology), NTNU Trondheim, and UNIFOR Oslo. A preliminary version of this work was presented at the ninth International Conference on Inorganic Membranes, June 2529, 2006, at Lillehammer, Norway, and was published in the volume proceedings.65 We thank the participants for their comments and suggestions. References and Notes (1) Kharton, V. V.; Yaremchenko, A. A.; Kovalevsky, A. V.; Viskup, A. P.; Naumovich, E. N.; Kerko, P. F. J. Membr. Sci. 1999, 163, 307. (2) Figueiredo, F. M.; Marques, F. M. B.; Frade, J. R. Solid State Ionics 1998, 111, 273. (3) Liu, Y. Y.; Tan, X. Y.; Li, K. Catal. ReV. - Sci. Eng. 2006, 48, 145. (4) Meadowcroft, D. B. Nature 1970, 226, 847. (5) Libby, W. F. Science 1971, 171, 499. (6) Yamamoto, O.; Takeda, Y.; Kanno, R.; Noda, M. Solid State Ionics 1987, 22, 241. (7) Skinner, S. J. Int. J. Inorg. Mater. 2001, 3, 113. (8) Pen˜a, M. A.; Fierro, J. L. G. Chem. ReV. 2001, 101, 1981. (9) Teraoka, Y.; Nobunaga, T.; Okamoto, K.; Miura, N.; Yamazoe, N. Solid State Ionics 1991, 48, 207. (10) Kharton, V. V.; Naumovich, E. N.; Nikolaev, A. V. J. Membr. Sci. 1996, 111, 149. (11) Petric, A.; Huang, P.; Tietz, F. Solid State Ionics 2000, 135, 719. (12) Go¨dickemeier, M.; Sasaki, K.; Gauckler, L. J.; Riess, I. Solid State Ionics 1996, 86-88, 691.

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Cation Self-Diffusion in LaCoO3 and La2

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