Oxygen vacancy ordering in strontium doped rare earth cobaltate

perovskites Ln1�xSrxCoO3�d (Ln = La, Pr and Nd; x > 0.60)

M. James a,*, T. Tedesco a, D.J. Cassidy b, R.L. Withers c

a Bragg Institute, Australian Nuclear Science and Technology Organisation, Building 58,

ANSTO, PMB 1, Menai NSW 2234, Australiab Materials and Engineering Sciences, Australian Nuclear Science and Technology Organisation,

PMB 1, Menai NSW 2234, Australiac Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia

Received 9 December 2004; received in revised form 2 January 2005; accepted 28 February 2005

Abstract

A family of Sr-doped perovskite compounds Ln1�xSrxCoO3�d (Ln = La3+, Pr3+ and Nd3+; x > 0.60), were

prepared by sol–gel chemistry and reaction at 1100 8C under 1 atm of oxygen. This structural family has been

shown to be present only for rare earth ions larger than Sm3+ and an upper limit of Sr2+ solubility in these phases

was found to exist between x = 0.90 and 0.95. X-ray diffraction shows oxygen-deficient, simple cubic (Pm-3m)

perovskite crystal structures. The combination of electron and powder neutron diffraction reveals that oxygen

vacancy ordering occurs, leading to a tetragonal (P4/mmm) superstructure and a doubling of the basic perovskite

unit along the crystallographic c-axis. No additional Ln3+/Sr2+ cation ordering was observed.

# 2005 Elsevier Ltd. All rights reserved.

Keywords: A. Oxides; C. Electron diffraction; C. Thermogravimetric analysis (TGA); D. Crystal structure

1. Introduction

There has been substantial recent interest in strontium-doped rare earth perovskites (Ln1�xSrxCoO3�d)

as cathode materials for solid oxide fuels cells [1–4] and as high temperature ceramic membranes [5,6].

These phases also display novel magnetic behaviour, including glassiness [7–9] and room temperature

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Materials Research Bulletin 40 (2005) 990–1000

* Corresponding author. Tel.: +61 2 9717 9299; fax: +61 2 9717 3606.

E-mail address: mja@ansto.gov.au (M. James).

0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2005.02.020

ferromagnetism [10–15]. We have lately explored structural variation within the Ln1�xSrxCoO3�d

perovskite phase diagram [16,17]. For the largest rare earth elements (La3+–Sm3+) an extended range

of Sr-doping has previously been observed by a number of authors [3,18–21]. Prior to our own work, the

upper solubility limit for these phases has been shown to be x = 0.80 for La [3,18], and x = 0.50 for Nd

and Pr [19,20]. In this study we demonstrate that the upper solubility limits for each of these

Ln1�xSrxCoO3�d (Ln = La, Pr and Nd) phases are between x = 0.90 and 0.95 and in particular we show

oxygen vacancy ordering to be present for x � 0.67.

2. Experimental

2.1. Synthesis

Polycrystalline samples of Ln1�xSrxCoO3�d were prepared from spectroscopic grade powders of

SrCO3 (+98%), Co(NO3)2�6H2O (98%) and either Ln2O3 (Ln = La and Nd) (�99.9%) or Pr6O11 (99.9%).

The powders were dissolved in dilute nitric acid and an intimate mixture of the metal oxides was formed

via the decomposition of a citric acid–ethylene glycol sol–gel. The residues were pelleted and sintered in

a tube furnace at 1100 8C under flowing oxygen for up to 3 days with intermediate re-grinding and re-

pelleting until no further reaction was evident by powder X-ray diffraction. The samples were cooled

from 1100 8C to room temperature at a rate of 2 8C/min.

2.2. Thermogravimetry

Thermogravimetry of ca. 70 mg of each of the Ln1�xSrxCoO3�d samples were carried out with the use

of a SETARAM TAG24 Simultaneous Thermogravimetric and Differential Thermal Analyser. The

samples were reduced under a mixture of 3.5% hydrogen in nitrogen over a temperature range of 25–

950 8C at a heating rate of 58 C/min. Each of the samples studied decomposed under hydrogen reduction

to give the component oxides Ln2O3 and SrO as well as Co metal. The measured mass-loss was then used

to determine the number of oxygen vacancies present in phase, as well as the average oxidation state of

the cobalt ions.

2.3. Electron diffraction

Electron diffraction (ED) was carried out on a Philips EM 430 Transmission Electron Microscope

operating at 300 kV. Samples suitable for TEM work were prepared by the dispersion of finely ground

material onto a holey carbon film.

2.4. Powder diffraction measurements

Powder X-ray diffraction measurements were made on a Scintag Inc. XGEN 4000 X-ray diffract-

ometer at ambient temperature using Cu Ka radiation and a flat-plate sample holder. Data of sufficient

quality for structural refinement were collected, over 58 < 2u < 1058, in 0.028 steps, with 10 s

acquisition times per step. Powder neutron diffraction data were collected on the MRPD instrument

(l = 1.6661 A) at the HIFAR facility, Sydney, Australia. Structure refinements were carried out by the

M. James et al. / Materials Research Bulletin 40 (2005) 990–1000 991

Rietveld method [22] using the RIETICA program [23] with pseudo-Voigt peak shapes and refined

backgrounds.

Further details of the crystal structure investigations can be obtained from the Fachinformationszen-

trum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49 7247 808 66; email: crysda-

ta@fiz-karlsruhe.de), quoting the (CDS) depository number. These numbers are listed in Table 1 above.

3. Results and discussion

3.1. The Ln1�xSrxCoO3�d perovskite phase diagram

Our earlier work [17] has characterised the phase diagram for rare earth perovskite Ln1�xSrxCoO3�d

structures, the results of which we have summarised in Fig. 1. The largest rare earth elements (La3+–

Sm3+) show an extended range of solid solution with a number of different structure types, being trigonal,

orthorhombic and tetragonal and cubic. For rare earth elements smaller than Sm3+ the solid solution

range decreases (from 0.40 � x � 0.95 for Gd3+ to 0.90 � x � 0.95 for Yb3+) with decreasing ionic radii.

Compounds with compositions x � 0.60 form orthorhombic structures, while those with 0.60 < x � 0.90

crystallise with an I4/mmm (2ap � 2ap � 4ap) superstructure [16], and those for x = 0.95 crystallise with

a P4/mmm (ap � ap � 2ap) superstructure [17].

The structures of the un-doped end members LnCoO3 (Ln = La3+, Pr3+ and Nd3+) have previously

been studied in some detail. LaCoO3 forms a rhombohedrally-distorted structure (indicated by (^) in

M. James et al. / Materials Research Bulletin 40 (2005) 990–1000992

Table 1

Thermogravimetric and crystallographic data for Ln1�xSrxCoO3�d (x = 0.67 and 0.90) as determined from thermogravimetric

and X-ray diffraction data

La0.33Sr0.67

CoO3�d

La0.10Sr0.90

CoO3�d

Pr0.33Sr0.67

CoO3�d

Pr0.10Sr0.90

CoO3�d

Nd0.33Sr0.67

CoO3�d

Nd0.10Sr0.90

CoO3�d

CDS-number 415227 415228 415226 415225 415224 415223

Mass 208.022 196.610 208.970 196.938 210.262 197.063

d 0.21 (1) 0.19 (1) 0.19 (1) 0.18 (1) 0.18 (1) 0.20 (1)

Co3+/Co4+ 24 52 27 53 30 51

Space group Pm-3m Pm-3m Pm-3m Pm-3m Pm-3m Pm-3m

Z 1 1 1 1 1 1

a (A) 3.8380 (2) 3.8426 (1) 3.8207 (1) 3.8370 (1) 3.8193 (1) 3.8396 (2)

V (A3) 56.535 (6) 56.738 (1) 55.772 (2) 56.490 (2) 55.713 (1) 56.607 (4)

rcalc (g cm�3) 6.110 5.754 6.222 5.789 6.267 5.781

Co (Biso; A2 � 100) 1.4 (1) 1.8 (1) 0.9 (1) 2.1 (1) 2.2 (1) 1.8 (1)

Ln/Sr (Biso; A2 � 100) 2.1 (1) 2.1 (1) 1.4 (1) 2.4 (1) 2.5 (1) 2.1 (1)

O (Biso; A2 � 100) 3.0 (1) 3.3 (1) 2.7 (1) 3.1 (1) 3.2 (1) 3.1 (1)

O occupation 0.91 0.89 0.90 0.89 0.88 0.89

Co–O (A) 1.9190 (1) 1.9213 (1) 1.9103 (1) 1.9185 (1) 1.9097 (1) 1.9198 (1)

Ln/Sr–O (A) 2.7139 (2) 2.7171 (1) 2.7016 (1) 2.7132 (1) 2.7007 (1) 2.7150 (1)

RP (%) 6.6 5.2 7.6 5.9 4.8 5.4

RWP (%) 8.3 6.6 9.7 7.5 6.5 6.8

RB (%) 1.4 1.4 1.3 1.3 0.9 1.0

Fig. 1) [24]. The smaller rare earth ions Pr and Nd form orthorhombic x = 0 end members (indicated by

(~) in Fig. 1) [19,25]. The x = 1 end member ‘‘SrCoO3�d’’, when prepared via sol–gel methods and fired

at 1100 8C under flowing oxygen, actually forms the trigonal compound Sr6Co5O15 [26] along with a

small amount of Co3O4. A number of other authors have studied the structural variation of

La1�xSrxCoO3�d as a function of Sr-doping [3,18,27–30]. These studies show that La1�xSrxCoO3�d

forms rhombohedrally distorted cubic perovskite phases for x � 0.5 (^). Earlier studies of

La1�xSrxCoO3�d for x > 0.5 reported simple cubic (Pm-3m) perovskite structures (*) [28–30]. More

recently, van Doorn and Burggraaf have demonstrated using X-ray, neutron and electron diffraction that

distinct microdomians were present in La0.3Sr0.7CoO3�d with a tetragonal ap � ap � 2ap superstructure

as well as regions without such a superstructure (^). They determined that the superstructure could be

rationalised by removal of the oxygen atoms and formation of vacancies in every second (0 0 1) (La/Sr)O

plane [3]. Upper limits to the La1�xSrxCoO3�d solid solution have been reported to occur between 0.70

[29,30] and 0.80 [3,18], depending upon the preparation methods. In the case of our own study, X-ray

diffraction of samples produced via sol–gel methods under 1 atm of oxygen indicate a single phase

composition for x = 0.90.

Prior studies of Pr1�xSrxCoO3�d [19,20,31] and Nd1�xSrxCoO3�d [20,31,32] phases also show

transformations from orthorhombic structures (~) (for x � 0.33) [19,32] to rhombohedral crystal

symmetry (^) for x = 0.5 [20]. Brinks et al. by way of contrast reported a monoclinic structure for

Pr0.5Sr0.5CoO3�d [19]. As shown in Fig. 1, our earlier studies revealed simple cubic phases with upper

solid solution limits for Pr1�xSrxCoO3�d and Nd1�xSrxCoO3�d between x = 0.90 and 0.95 [16,17].

M. James et al. / Materials Research Bulletin 40 (2005) 990–1000 993

Fig. 1. The Ln1�xSrxCoO3�d perovskite phase diagram. Compositions having the metrically tetragonal (P4/mmm;

ap � ap � 2ap) superstructure are shown by crosses (+), while those having the metrically tetragonal (I4/mmm;

2ap � 2ap � 4ap) superstructure are indicated by (�). Compositions having rhombohedral structures are indicated by (^).

Simple cubic structures are indicated by (*). Orthorhombic structures are indicated by (~). Metrically cubic structures with

orientational twinning of a local tetragonal (ap � ap � 2ap) superstructure are indicated by (^).

3.2. Thermogravimetry

The overall oxygen content and average cobalt oxidation state for each single-phase sample

was determined using thermogravimetric analysis. Fig. 2 depicts a typical thermogravimetric mass-

loss trace as a function of temperature, that for La0.1Sr0.9CoO3�d. It was found that the amount of

oxygen vacancies (d) in these Ln1�xSrxCoO3�d phases varied between 0.18 and 0.21 and was

essentially constant as a function of x (Table 1). The sections of the trace that begin to plateau at

approximately 370 and 600 8C correspond to intermediate cobalt oxidation states of 3+ and 2+,

respectively.

The level of oxygen vacancies observed in this study are consistent with those observed in our earlier

studies of Ln1�xSrxCoO3�d (Ln = Y, Sm–Yb) phases for x > 0.60 [16,17,33]. These results confirm that

there appears to be no systematic variation in oxygen content as a function of Sr doping levels. The

average cobalt oxidation state on the other hand is heavily influenced by the amount of strontium present

in the structure. The average amount of Co4+ in these phases increases from 27% for Ln0.33Sr0.67CoO3�d

to 52% for Ln0.10Sr0.90CoO3�d (Ln = La, Pr, Nd); these levels being consistent with those previously

observed for these perovskite phases Ln1�xSrxCoO3�d containing the smaller rare earth ions (Ln = Y,

Sm–Yb).

3.3. Electron diffraction

Fig. 3 shows the h0 0 1ip zone axis selected area electron diffraction pattern (EDP) of

Pr0.10Sr0.95CoO3�d, which is typical of these larger rare earth ion containing perovskite compounds

for x > 0.6 and is of the same type shown by van Doorn and Burggraaf (Fig. 6 in ref. [3]). Note

the presence of weak G � 1=2h0 0 1ip type satellite reflections in addition to the strong Bragg

reflections of the underlying perovskite type average structure (labelled with the subscript p in

Fig. 3). Also of significance is the fact that the G � 1=2h0 0 1ip type satellite reflections are much

stronger along one direction than the other suggesting that doubling locally occurs along one direction

at a time.

M. James et al. / Materials Research Bulletin 40 (2005) 990–1000994

Fig. 2. Thermogravimetric mass-loss trace vs. temperature for La0.1Sr0.9CoO3�d.

3.4. X-ray diffraction

Powder X-ray diffraction profiles were collected for Ln1�xSrxCoO3�d (Ln = La, Pr and Nd) and

examined using the Rietveld method. In each case, they could be indexed using a simple cubic (Pm-3m;

a 3.83 A) unit cell. Careful examination of these profiles did not reveal any other superstructure peaks

or additional splitting of the main perovskite diffraction lines. In particular, there was no evidence of the

ap � ap � 2ap superstructure that was observed using electron diffraction.

The structure of each single-phase Ln1�xSrxCoO3�d composition was refined based on a simple cubic

perovskite cell. Each of the atoms were located at their typical high-symmetry positions, with Sr2+ and

Ln3+ disordered over the 1b (1/2, 1/2, 1/2) site. Refinement of the isotropic thermal parameters for each

compound gave a very high value for the 3d (1/2, 0, 0) oxygen site, suggesting substantial disorder

between oxygen and oxygen vacancies. In final stages of each structural determination, the occupancy of

the oxygen site was refined and high levels of oxygen vacancies (up to 12%) were found to be present

(Table 1). These refined oxygen occupancies were found to be consistently lower than that expected from

thermogravimetric analysis, however some caution must be taken in association with these refined

values. The relative strength of the X-ray scattering factors means that the scattering will be dominated by

the (rare earth) metal atoms, and perhaps more significantly, substantial correlations were found to be

present between the oxygen occupancies and thermal parameters.

The refined unit cell parameters and metal–oxygen bond lengths for the Ln1�xSrxCoO3�d phases are

listed in Table 1. Typical observed (+), calculated (—) and difference (—) X-ray diffraction profiles

(those for Pr0.33Sr0.67CoO3�d and Pr0.10Sr0.90CoO3�d) are shown in Fig. 4(a and b), respectively. The

refined unit lengths edges vary from 3.8193(1) A (for Nd0.33Sr0.67CoO3�d) to 3.8426(1) A (for

La0.10Sr0.90CoO3�d). As has been previously noted for other Ln1�xSrxCoO3�d compounds, the unit

cell volume increases with Sr content; this being despite oxidation of Co3+ to the smaller Co4+ [16].

The Co–O bond lengths for these phases range between 1.9097(1) A and 1.9213(1) A. Based on

tabulated values of ionic radii for Co3+ and Co4+ [34], our refined bond lengths suggest that the ionic radii

M. James et al. / Materials Research Bulletin 40 (2005) 990–1000 995

Fig. 3. A typical h0 0 1ip zone axis EDP of Pr0.10Sr0.90CoO3�d.

of both Co3+ and Co4+ are too short for them to be present in high-spin ground states. Unfortunately, our

data are not able to suggest whether these cobalt ions occupy intermediate or low spin state config-

urations. Comparison of the refined Ln/Sr–O bond lengths with literature values [34] reveals shorter

bonds than expected for 12-fold coordinate metal ions. By way of example one would expect a Nd/Sr–O

bond length of 2.823 A, whereas the refined value is 2.7150(1) A. These values are quite reasonable

however in the light of the reduced coordination number due to the presence of oxygen vacancies (in this

instance a coordination number of 10.6).

3.5. Neutron diffraction

Relative to powder X-ray diffraction, powder neutron diffraction is a powerful tool for the determina-

tion of cation and oxygen vacancy ordering within these phases. Precise determination of the location and

quantity of oxygen atoms and vacancies in the presence of heavy rare earth and transition metal ions is

difficult using X-rays given their weak contribution to the scattering. In contrast, the coherent neutron

scattering factor (b) for oxygen (5.803 fm) is comparable to that of La (8.24 fm) and Sr (7.02 fm) and

substantially larger than that of Co (2.49 fm).

M. James et al. / Materials Research Bulletin 40 (2005) 990–1000996

Fig. 4. The observed (+), calculated (—) and difference (—) X-ray diffraction profiles for (a) Pr0.33Sr0.67CoO3�d and (b)

Pr0.10Sr0.90CoO3�d with indexing based on Pm-3m simple cubic perovskite structures.

Careful examination of the powder neutron diffraction profile of La0.10Sr0.90CoO3�d reveals the

presence of several weak superstructure reflections based on the tetragonal (P4/mmm) ap � ap � 2ap

supercell. These data support our current electron diffraction results for these compounds. The positions

of each of the atoms were initially set at idealised values with Co1 on the 2h (1/2, 1/2, z) sites (z = 1/4) and

La3+ and Sr2+ ions disordered over the 1a (0, 0, 0) and 1b (0, 0, 1/2,) sites. All of the oxygen sites were set

as being fully occupied. Early stages of the refinement did not place calculated intensity in any of the

superstructure peaks. While most of the atoms occupied special sites within the structure, the Co and O2

4i (1/2, 0, z) atoms were allowed to move along the z-axis. The Co atom refined to a stable position at

z = 0.2578(12), while the O2 atoms did not appear to move substantially from z = 0.25 and so were set at

this position. Independent refinement of the oxygen thermal parameters led to a significantly higher value

at the O3 1d (1/2, 1/2, 1/2) site suggesting that significant oxygen vacancies were present in the z = 1/2

plane. In the final stages of the refinement, the oxygen thermal parameters were constrained to be the

same value, and the occupancy of the O3 site was allowed to vary. The refined O3 site occupancy of

0.65(1) therefore suggests an oxygen vacancy level (d) for La0.10Sr0.90CoO3�d of 0.18 and an overall

oxygen content of 2.82 which is in very good agreement with that determined using thermogravimetric

methods (Table 1). In those instances where an oxygen vacancy is present at the O3 site, the adjacent

cobalt atoms adopt a square pyramidal coordination with a single axial Co1–O1 bond and four Co1–O2

bonds in the xy plane.

The observed (+), calculated (—) and difference (—) neutron powder diffraction profiles for

La0.10Sr0.90CoO3�d are shown in Fig. 5; while the refined structure is shown in Fig. 6. The refined

structural parameters for La0.10Sr0.95CoO3�d are listed in Table 2, and the refined atomic positions,

thermal parameters and refined bond lengths are listed in Table 3.

A range of cobalt–oxygen bond lengths are present in this structure: a long, axial Co1–O1 bond

(1.982 A); four intermediate Co1–O2 bonds (1.923 A) in the xy plane and a single short axial bond

(1.863 A) between Co1 and the O3 site containing oxygen vacancies. Oxidation of Co3+ to Co4+ is

accompanied by a reduction in ionic radii from 0.61 to 0.53 A [34]. Given the average cobalt oxidation

state of 3.52+, one would expect a cobalt oxygen bond length of 1.97 A. The average refined cobalt-

oxygen bond length at 1.923 A was found to be slightly shorter, but still consistent with other compounds

containing mixtures of Co(III) and Co(IV).

M. James et al. / Materials Research Bulletin 40 (2005) 990–1000 997

Fig. 5. The observed (+), calculated (—) and difference (—) neutron diffraction profiles for La0.10Sr0.90CoO3�d.

A number of possible cobalt ground-state electronic configurations may contribute to the average

cobalt–oxygen bond lengths observed by powder neutron diffraction. Approximately two-thirds of the

cobalt atoms have an octahedral coordination to oxygen; while the remaining adopt a square pyramidal

coordination. The latter would be expected to raise the energy of the xy orbitals (relative to the xz and yz)

and the x2 � y2 orbitals (relative to the z2). In addition to the different coordination spheres, three possible

M. James et al. / Materials Research Bulletin 40 (2005) 990–1000998

Fig. 6. The refined structure of La0.10Sr0.90CoO3�d.

Table 2

Structural parameters for La0.10Sr0.90CoO3�d, refined from powder neutron diffraction data

Formula La0.10Sr0.90CoO3�d

Mass 196.610

Space group P4/mmm

Z 2

a (A) 3.8369 (4)

c (A) 7.6886 (9)

V (A3) 113.19 (2)

c/2a 1.0019

rcalc (g cm�3) 5.774

Number of reflections 75

RP; RWP; RB (%) 2.36; 3.06; 0.79

spin states can exist for each different cobalt oxidation state. While the precise nature of these spin states

is unclear from our powder diffraction data, the irregular nature of the cobalt–oxygen bonds supports the

notion of both Co(III) and Co(IV) having intermediate spin states that allow for Jahn–Teller distortions to

lift the degeneracy of the octahedral energy levels. Ultimately, single-crystal magnetism and structural

studies will be required to clarify the electronic ground states of these phases.

Little variation was observed for the different La1/Sr1–O bonds within this structure (Table 3). The

average refined La1/Sr1–O bond length of 2.715 A was found to be shorter than that expected for a

perovskite phase containing a 1:9 ratio of La3+ to Sr2+ (2.83 A) [34]. This may in part be a reflection due

to the lower coordination number about the A-site cations in this compound due to the presence of oxygen

vacancies within the structure. In contrast to the other perovskite phases (Ln1�xSrxCoO3�d) containing

smaller rare earth ions (Ln = Y, Sm–Yb) over a similar compositional range (0.33 � x � 0.10) [16,33],

we found no evidence from our neutron diffraction data to suggest that cation ordering was occurring

between La3+ and the slightly larger Sr2+ ions.

4. Conclusions

We have studied the stability of highly Sr-doped perovskite compounds Ln1�xSrxCoO3�d containing

the larger rare earth ions La3+, Pr3+ and Nd3+. We have found an upper limit of Sr2+ solubility in the

perovskite phase between x = 0.90 and 0.95. Above this phase boundary the trigonal phase Sr6Co5O15 is

produced. A combination of electron and powder neutron diffraction have revealed that oxygen vacancy

ordering takes place in these phases, leading to a doubling of the basic perovskite unit along the

crystallographic c-axis. No additional Ln3+/Sr2+ cation ordering was observed.

Acknowledgment

R.L.W. and M.J. acknowledge the Australian Research Council (ARC) for financial support in the

form of ARC Discovery Grants.

M. James et al. / Materials Research Bulletin 40 (2005) 990–1000 999

Table 3

Refined atomic positions and thermal parameters (Biso; A2 � 100) and bond lengths for La0.10Sr0.90CoO3�d

Atom Site X y z Biso Occupation

Co1 2h 1/2 1/2 0.2578 (12) 0.8 (1) 1

La1/Sr1 1a 0 0 0 0.7 (1) 0.10/0.90

La2/Sr2 1b 0 0 1/2 0.7 0.10/0.90

O1 1c 1/2 1/2 0 1.6 (1) 1

O2 4i 1/2 0 1/4 1.6 1

O3 1d 1/2 1/2 1/2 1.6 0.65

Co1–O1 (�1) 1.982 (9) La1/Sr1–O1 (�4) 2.713 (1)

Co1–O2 (�4) 1.923 (1) La1/Sr1–O2 (�8) 2.716 (1)

Co1–O3 (�1) 1.863 (9) La2/Sr2–O2 (�8) 2.716 (1)

Average Co–O 1.923 La2/Sr2–O3 (�4) 2.713 (1)

Average La/Sr–O 2.715

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