Article pubs.acs.org/cm
Anisotropic Thermal and Chemical Expansion in Sr-Substituted LaMnO3+δ: Implications for Chemical Strain Relaxation Tor Grande,* Julian R. Tolchard, and Sverre M. Selbach Department of Materials Science and Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway ABSTRACT: The anisotropic thermal and chemical expansion of rhombohedral (R3c̅ ) La1−xSrxMnO3+δ (x = 0.2, 0.3) was investigated by in situ high temperature X-ray diffraction of submicrometer size powders in pure oxygen and nitrogen (inert) atmospheres. The thermal expansion of the long axis c was found to be close to twice as high as the thermal expansion of the short axis a. The large thermal expansion of the c-axis is caused by rectification of the antiferrodistortive tilting and decompression of the MnO6/2 octahedra. The unit cell parameters were shown to be strongly dependent on the partial pressure of oxygen, which was attributed to chemical expansion/ contraction due to reduction/oxidation of Mn. Anisotropic chemical expansion/ contraction was more pronounced for the unit cell parameter a than for c, and the chemical expansion/contraction was inferred to reflect the size of the MnO6/2 octahedra. The onset of chemical expansion in nitrogen and contraction in oxygen atmosphere during heating was discussed in terms of a gradual transformation from a nonequilibrium to an equilibrium point defect population in La1−xSrxMnO3+δ. Possible implications of slow relaxation of chemically induced stresses at the nanoscale and in epitaxial thin films are addressed. Finally, a second order phase transition from the ferroelastic (R3̅c) to paraelastic state (Pm3̅m) is reported for La0.7Sr0.3MnO3+δ at 850 ± 25 °C. The temperature of the phase transition decreases with increasing Sr content in La1−xSrxMnO3+δ. KEYWORDS: perovskite, point defects, thermal expansion, chemical expansion, chemical strain relaxation
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ent on the Sr content and the partial pressure of oxygen.12−17 The cation deficiency at low Sr-substitution level is due to the strong thermodynamic driving force to oxidize manganese from Mn3+ to Mn4+.18,19 Under oxidizing conditions LaMnO3+δ is therefore more correctly described as La1−δ/3Mn1−δ/3O3.16 An interesting consequence of the cation deficiency is the considerable mobility of the cations20−22 relative to oxygen anions.23,24 Cation mobility in oxides has usually been neglected, but there is an increasing awareness that the operational lifetimes of high temperature electrochemical systems may be limited by processes involving cation transport.25,26 The equilibration of point defects in oxygen hyperstoichiometric LSM has been reported to f reeze-in at a specific temperature,27,28 which means that the population of point defects is no longer in equilibrium. This may be regarded as broken ergodicity. LSM thin films are conventionally reported to be stoichiometric (δ = 0), although the actual oxygen stoichiometry is difficult to measure and is rarely reported. Recently though, it has been recognized that oxygen incorporation in LSM thin films is a matter of concern and may influence the properties of the LSM films (referred to as LSMO in thin film literature).29−34 Furthermore, cation segregation in LSM thin films have also been reported,30−32 which can be attributed to the creation of cation vacancies and possible cation demixing,25,26 which may in turn be driven by
INTRODUCTION The manganese perovskites R1−xAxMnO3±δ, where R is a rare earth and A is an alkaline earth metal, are among the most studied oxide materials in the last few decades due to demonstration of colossal magnetoresistance (CMR) and possible applications in spintronics.1 The magnetic and structural phase diagrams of La1−xSrxMnO3±δ (LSM) have been extensively investigated, showing that the magnetic and electronic structure of LSM is controlled by the Sr-substitution or implicitly by the oxidization state of Mn.2−6 La1−xSrxMnO3±δ (x ≤ 0.2) is semiconducting and antiferromagnetic, and an orthorhombic (Pbnm) crystal structure has been reported, but with two different unit cells due to changes in the oxidation state of Mn.3 In the composition region 0.2 < x < 0.45 the crystal structure is rhombohedral (R3̅c).2,3 An insulator to metal transition has been found close to the Pbnm-R3c̅ structural phase transition, and the metallic behavior is accompanied by ferromagnetic ordering.2 Above x = 0.45 the crystal structure becomes tetragonal (I4/mcm) and finally cubic (Pm3̅m) above x = 0.7.5,6 The low temperature magnetic structure for x > 0.5 is complex including charge ordering and structural phase transitions involving cooperative Jahn−Teller distortion.1 While Mn3+ is a Jahn−Teller ion, Mn4+ is not, and this opens up for possible charge ordering and structural distortions dependent on the valence state of Mn. LSM has been the prime cathode material in solid oxide fuel cells (SOFC)7 due to its high electronic conductivity8 and chemical9−11 and thermal compatibility7 with yttria stabilized zirconia. The defect chemistry of LSM is rather unique,12−16 possessing both cation deficiency and oxygen deficiency, depend© 2011 American Chemical Society
Received: October 11, 2011 Revised: December 1, 2011 Published: December 8, 2011 338
dx.doi.org/10.1021/cm2030608 | Chem. Mater. 2012, 24, 338−345
Chemistry of Materials
Article
The heating rate was 120 K/h. Scanning electron microscopy (SEM) was performed using a Hitatchi S-3400N instrument.
strain induced oxidation/reduction of Mn and lattice mismatch with the substrate. Gradients in the composition on the Sr site will induce a gradient in the valence state of Mn and change the defect chemistry from excess oxygen at low Sr content to oxygen deficiency at high Sr content.16,17 Similar effects of cation motion have also been speculated to occur in LSM cathodes due to cathodic overpotentials.35 Here, we demonstrate by in situ high temperature X-ray diffraction the onset of relaxation of chemically induced strain in LSM. Submicrometer powders of LSM, with a well-defined thermal history, were investigated during reheating in pure oxygen or inert atmosphere. The anisotropic thermal and chemical expansion of the materials are reported, and the crystallographic data are discussed in relation to the ferroelastic to paraelastic phase transition, previously described in detail for isostructural LaAlO336−38 and also reported for acceptor-doped LaCoO3.39,40 Finally, we discuss the importance of strain relaxation by chemical expansion/contraction,41 particularly at the nanoscale and at epitaxial interfaces.
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RESULTS The ambient crystal structure of La0.8Sr0.2MnO3+δ and La0.7Sr0.3MnO3+δ was indexed using the rhombohedral space group R3c̅ 44 in line with previous reports.2,3,17 The structural data for the two materials at ambient are summarized in Table 1. Table 1. Ambient Temperature Unit Cell Parameters (hexagonal setting) and the Displacement from the Cubic Position for O (e) of La0.8Sr0.2MnO3+δ and La0.7Sr0.3MnO3+δa Sr content
a [Å]
c [Å]
O displacement (e)
Rwp
0.2 0.3
5.5122(1) 5.5045(1)
13.3564(3) 13.3680(2)
0.0482(10) 0.0386(09)
5.49 4.70
a
The data was obtained by Rietveld refinement using the space group R3c̅ with the following atomic positions: La/Sr (0, 0, 1/4), Mn (0, 0, 0), and O (1/6 − e, 1/3, 1/12).
The unit cell parameters were in good agreement with literature.2,3,17 A SEM micrograph of the La0.8Sr0.2MnO3+δ powder is shown in Figure 1. The grain size of the two LSM powders was ∼200 nm with a relatively narrow particle size distribution.
EXPERIMENTAL SECTION
LaxSr1−xMnO3+δ (x = 0.2, 0.3) were prepared by spray pyrolysis of mixtures of nitrate solutions containing stoichiometric amounts of the cations. The cation content in the pure nitrate solutions before mixing was determined by thermogravimetrical analysis. The as-prepared powders were first calcined for 24 h at 800 °C to obtain single phase materials, and the materials were subsequently coarsened in air at 1050 °C for 12 h, with a cooling rate of 200 °C per h. The phase purity of the powders was confirmed by powder X-ray diffraction. 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 Physikalishe Geräte GmbH high temperature camera. Prior to heating the camera was evacuated and flushed three times with the appropriate sweep gas (O2 or N2), 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 (∼1 mm from the sample edge) was used for temperature determination. Calibration of the system against an Al2O3 standard gave an estimated temperature error < 15 °C. Patterns were collected from 100 to 1050 °C (every 25° above 300 °C), across an angular range 15−75° 2θ, which was the 2θ range possible using the radiant heater. A step size of 0.016° 2θ was used. 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 1 °C/s. Rietveld refinements were carried out with the Topas Academic software, v 4.1.42 The structure of the two phases was described using a rhombohedral model3 (R3̅c) at lower temperatures and cubic model (Pm3̅m) at higher temperatures. LaxSr1−xMnO3+δ (x ≤ 0.3) are oxygen hyper-stoichiometric, and cation vacancies in the materials are expected.16,17 Attempts to refine the data as cation nonstoichiometric did not significantly improve the fit though, and so a nominal stoichiometry was assumed for all refinements. 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 a five parameter Chebychev polynomial background function, lattice parameters, sample displacement, symmetry constrained atomic positions, and isotropic thermal displacement parameters. The surface roughness model of Suortti43 was also applied, with one variable refining. To increase the probability of finding the global minimum for refinements, 100+ cycles of convergence were calculated for every pattern, each cycle starting from a randomly different starting variable set. Thermogravimetric analysis (TGA) of the annealed La0.8Sr0.2MnO3+δ was performed using a Netzsch STA 449C TG-DSC in N2 atmosphere.
Figure 1. SEM micrograph of the La0.8Sr0.2MnO3+δ powder annealed for 12 h at 1050 °C.
The hexagonal unit cell parameters a and c of La0.8Sr0.2MnO3+δ as a function of temperature and atmosphere are shown in Figure 2a. Here, hexagonal c is parallel to the pseudocubic/rhombohedral 111-axis, about which the octahedra are rotated antiferrodistortively. A representative Rietveld refinement of the X-ray diffractogram for La0.8Sr0.2MnO3+δ at 550 °C in O2 is shown in Figure 3. Generally a good fit (Rwp
