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Solid State Ionics 178 (2

Experimental XRD and NMR, and molecular dynamics studyof Sr containing LaAlO3 perovskite

Enrique Lima a,⁎, María-Elena Villafuerte-Castrejón b, José M. Saniger c, Alejandro Ibarra-Palos b,Jorge E. Sánchez-Sánchez d, Luis Javier Álvarez d,e

a Universidad Autónoma Metropolitana, Iztapalapa, A. P. 55-532, Av. San Rafael Atlixco No. 186 Col. Vicentina, 09340 México D.F., Mexicob Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior, A. P. 70-360, 04510 México D.F., Mexico

c CCADET, Universidad Nacional Autónoma de México, Circuito Exterior, A. P. 70-188, 04510 México D.F., Mexicod Instituto de Matemáticas, Unidad Cuernavaca, Universidad Nacional Autónoma de México. Av. Universidad s/n. Col. Lomas de Chamilpa,

62220 Cuernavaca, Morelos, Mexicoe Departamento de Química Física, Facultad de Química, E-41012, Sevilla, Spain on sabbatical leave from Instituto de Matemáticas,

Unidad Cuernavaca, Mexico

Received 22 May 2007; received in revised form 15 January 2008; accepted 18 January 2008

Abstract

Strontium containing LaAlO3 perovskites, (La,Sr)AlO3, were prepared by co-precipitation method. 20% of lanthanum was replaced bystrontium into LaAlO3. Structural characterization of materials through XRD and NMR techniques suggests that Sr introduction causes thatunsaturated aluminum, AlIV and AlV appear in decrement of the amount of AlVI. This result was confirmed by molecular dynamics simulations anda mechanism of the formation of the unsaturated aluminum is given.© 2008 Elsevier B.V. All rights reserved.

Keywords: Aluminum; Perovskites; Strontium; Lanthanum; Molecular dynamics simulations

1. Introduction

Perovskites are a large family of crystalline ceramics thatderive their name from a specific mineral known as perovskite.The characteristic chemical formula of a perovskite ceramic isABO3, with A ions exhibiting charge of 2+ or 3+ whereas Bions are either 4+ or 3+. The structure is such that there are six-and twelve-coordinated cation sites. Indeed, the structure can bedescribed by octahedrons linked together by sharing corners toform a three dimensional framework. A ions occupy twelve-coordinated cavities and B ions occupy the octahedral ones[1,2]. Complex perovskite structures [3] can be manipulated in

⁎ Corresponding author. Departamento de Química, Universidad AutónomaMetropolitana, Iztapalapa, Av. San Rafael Atlixco No. 186 Col. Vicentina,09340, México D.F., Mexico. Tel.: +525 55804 4667; fax: +525 55804 4666.

E-mail address: lima@xanum.uam.mx (E. Lima).

0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2008.01.036

different ways such as charge doping, building layered crystalstructures based on a perovskite template and containing planarnetworks of other compounds or elements such as copper andoxygen, etc., to get new electronic, magnetic and supercon-ducting properties. Particularly, LaAlO3 perovskites are suitableto be doped with different cations to exhibit useful properties inorder to be applied to electrodes in fuel cells [4–6]. A widerange of magnetic and electric properties are also to be expected[7–10]. For instance the ionic conductivity of strontium–lanthanum–aluminum perovskites is 2–3 orders of magnitudehigher than that of lanthanum–strontium–chromium [11–13].

Alterations of perovskite-like structure caused by the in-corporation of divalent cations in LaAlO3 perovskites have beenseldom explained. Nuclear magnetic resonance (NMR) hasbeen successfully used to reveal ordering in a wide range ofmaterials and has proved to be a sensitive tool to describestructural short order modifications [14,15]. In this work weshow how the introduction of Sr2+ causes modifications in the

Fig 1. X-ray diffraction patterns of perovskite samples. LaAlO3, La0.95Sr0.05-AlO3 − δ, La0.90Sr0.10AlO3 − δ and La0.8Sr0.20AlO3 − δ, (a), (b), (c) and(d) respectively. The peaks were indexed with Miller index of JCPDS card31-0022 (LaAlO3 perovskite).

Fig. 2. 27Al MAS NMR spectra of lanthanum–strontium perovskites at variousSr content. LaAlO3, La0.95Sr0.05AlO3− δ, La0.90Sr0.10AlO3− δ and La0.8Sr0.20-AlO3− δ, (a), (b), (c) and (d) respectively. ⁎ indicates spinning side bands(samples spun at 10 kHz).

1945E. Lima et al. / Solid State Ionics 178 (2008) 1944–1949

structure of LaAlO3 perovskite in such way that the experi-mental data based on XRD and NMR techniques seldom detectthem but can be elucidated and explained based on moleculardynamics (MD) simulations.

Starting from a LaAlO3 perovskite a nominal substitution ofLa3+ by Sr2+ cations leads to a non-stoichiometric structure withan excess of oxygen atoms. On the other hand, if the excessoxygen atoms are removed there arise under-coordinated cat-ions of all three species, La, Sr and Al. Molecular dynamicssimulations are used to elucidate the structural changes pro-duced by Sr introduction in the structure in both cases, stoi-chiometric and non-stoichiometric structures.

2. Experimental

2.1. Materials

A series of La1− xSrxAlO3− δ perovskites were synthesizedby the co-precipitation method. Aqueous solutions of La(NO3)2•6H2O, Al(NO3)3•6H2O and Sr(NO3)2•9H2O with thedesired La/Sr ratio were mixed and precipitated with NH4OH.The precipitated solid was filtered under vacuum, washed withdeionized water and dried at 383 K. Finally, the solid wasthermally treated twice, the first time at 973 K and the secondone at 1673 K.

The Sr containing perovskites were prepared with the followingnominal composition: La0.95Sr0.05AlO3−δ, La0.90Sr0.10AlO3−δand La0.8Sr0.20AlO3−δ. As a reference, the LaAlO3 sample wasalso prepared.

2.2. Characterization

X-ray diffraction (XRD) patterns were collected on a D8Advance Bruker axs diffractometer using copper Kα radiation.Radial distribution functions, G(r), were calculated from

the full diffraction patterns as shown by Magini and Cabrini[16]. A molybdenum anode X-ray tube was used to reach therequired high values of the angular parameter h=(4πsinθ)/λ andthe X-ray pattern was measured by step scanning at angularintervals of 0.08° in a Siemens D5000 diffractometer.

Solid-state 27Al MAS NMR spectra were obtained underMAS conditions using an ASX 300 Bruker spectrometer with amagnetic field strength of 7.05 T, corresponding to a 27AlLarmor frequency of 78.3 MHz. A single pulse method wasused. π/2 pulse was calibrated at 2 μs. A recycle time of 0.5 swas used. The 27Al chemical shift was referenced using anaqueous solution of Al(NO3)3 as external standard.

The 139La resonance was located by searching in relativelycoarse steps (ca. 100 kHz) from the resonance frequency for thediamagnetic ion, using a t1–τ–t3 spin echo sequence withcomplex phase cycling [17]. The associated π/2 pulse lengthwas 10 μs. The acquisition parameters were: t1= t3=2.5 μs. Thechemical shifts were referenced to an external standard ofsaturated aqueous lanthanum chloride. The frequency was thenincremented in small steps and the line shape traced out bymeasuring the height of the Fourier transformed spin echo at thecurrent operating frequency [18,19].

3. Results and discussion

3.1. Experimental

The X-ray powder diffraction patterns displayed in Fig. 1confirm that all samples included in the series La1− xSrxAlO3− δ

maintain the perovskite-like structure as all they were identifiedwith the JCPDS file 31 0022. From Fig. 1 it is derived that thecell volume is increasing with increasing Sr content. This resultagrees with results reported by Hall et al. [20]. No strontiumcompounds such as strontium oxides were observed. No

Fig. 3. 139La NMR spectrum transition of LaAlO3 and La0.8Sr0.2O3− δ.

Fig. 4. Radial distribution functions of La0.80Sr0.20AlO3− δ samples asdetermined by dynamic molecular simulated (a) and from XRD experimentalpattern (b).

1946 E. Lima et al. / Solid State Ionics 178 (2008) 1944–1949

significant differences were observed between the XRDpatterns of samples containing strontium and the free strontiumsample.

Fig. 2 displays the 27Al MAS NMR spectra of varioussamples. The spectrum corresponding to strontium free sampleconsists of a narrow single isotropic resonance line close to10 ppm, which is due to octahedral aluminum [15,21]. Thepartial substitution of La by Sr promotes the formation of a verysmall amount of tetrahedral aluminum as revealed by the re-sonance ca. 60 ppm. The line width and the position of the peakclose to 10 ppm were not altered by the strontium incorporationin the perovskite structure. The resonance of Al(V) species, ifany, appears in the range from 20 to 30 ppm [22]. In the spectraof Fig. 2 the base of the peaks due to resonance Al(VI) ends at30 ppm, and therefore, it is difficult to say whether or not Al(V)cations are present in the Sr containing perovskite samples. Thiscoordination will be discussed in terms of the molecular dy-namics simulations.

A comparison of the 139La NMR spectra for samples LaAlO3

and La0.8Sr0.2AlO3− δ is shown in Fig. 3. The central Zeemantransition (−1/2,1/2) was clearly resolved for the strontium freesample. In this undoped material, the quadrupolar interaction isvery weak. From measurements of quadrupolar line shapes inthe LaAlO3 perovskite it is estimated to be ca. 0.6 MHz. Incontrast, the 139La resonance for sample La0.8Sr0.2AlO3− δ wascomposed of a very broad line. The quadrupolar contribution tothe linewidth is a consequence, apparently, of the distribution ofSr2+ and La3+ in the oxide network.

In summary, these results show that strontium cations canreplace lanthanum cations in the parent LaAlO3 perovskite witha random distribution in the structure. This feature does notproduce changes in the crystalline phases detected by XRD.

Table 1Parameters used for Pauling interaction potential for molecular dynamicssimulations

Atom q, e σ, Å

Al 1.92 0.53Sr 2.00 1.58La 2.23 1.40O −1.42 1.20

However, the 27Al NMR results revealed that local changesoccur. Thus, a small amount of octahedral sites turn out toacquire lower coordination. From these results it is not possibleto propose a mechanism of formation of such unsaturatedaluminum cations. In order to elucidate what this mechanismmight be, we carried out some molecular dynamics simulationsdescribed below.

3.2. Simulations

Molecular dynamics simulations were performed on twodifferent perovskite systems. The first one, as a reference, was apure LaAlO3 perovskite composed of 2835 particles of which567 represented La3+ cations, 567 Al3+ cations and 1701 oxy-gen anions. It has been already reported that symmetry ismaintained if Sr2+ in LaAlO3 is around 20%, therefore thesecond simulated sample was composed of 2778 particles rep-resenting 453 La3+ cations, 114 Sr2+ cations, 567 Al3+ cationsand 1644 oxygen anions. These last numbers are due to the factthat the simple nominal substitution of La3+ cations by Sr2+

cations yields a non-stoichiometric structure. In order to obtaina stoichiometric configuration one has to remove the excess ofoxygen atoms which amounted to 57 due to the Sr valence.

In order to explore the possibility of the existence of a realnon-stoichiometric structure a simulation was performed on asystem of 2835 particles in which a simple random substitutionof La3+ by Sr2+ cations was carried out. This simulated struc-ture does not allow for the formation of under-coordinatedaluminum ions and therefore its existence is precluded. Thissuggested the existence of oxygen vacancies in the real worldsamples.

Pauling interaction potentials were parameterised followingthe arguments given by Álvarez et al. [23]. The parameters werere-adjusted after substituting La3+ ions by Sr2+ ions, and re-moving the excess oxygen ions. Their final values are shown inTable 1.

Fig. 5. Partial radial distribution functions obtained from molecular dynamics simulations.

1947E. Lima et al. / Solid State Ionics 178 (2008) 1944–1949

In order to mimic the experimental procedure in the samplepreparation, and to allow the system to reorganize after thesubstitution of La3+ by Sr2+ cations and the concomitant intro-duction of oxygen vacancies, the system was subjected to aprocess of heating and cooling in eight sequential moleculardynamics simulation runs in the microcanonical ensemble. Thetemperatures at which the system was equilibrated and relaxedin the heating schedule were, starting at 300 K, every 500 K, upto 2300 K, totaling 171 ps in the process. The cooling schedulewas simply 2300, 1300 and 300 K justified by the lack ofchanges in the configurations. In all runs variations of totalenergy were of less than 0.01% around the mean value, andtemperature variations were within the expected limits.

Radial distribution functions were calculated in order toassure a match with the experimental corresponding functions.Fig. 4 shows the simulated radial distribution function G(r) forthe Sr containing perovskite structure, La0.8Sr0.2AlO2.9 sample,and the corresponding experimental G(r). Both G(r), experi-mental and simulated are very similar except, as expected,for some peaks which are not resolved experimentally becausethere are distances which are very similar for different atompairs. Fig. 5 shows the simulated partial radial distribution

Table 2La and Sr coordination distributions

Coordinationnumber

Number of cations Percentage

La Sr La Sr

9 2 1 0.44 0.8810 25 11 5.52 9.6411 126 35 27.82 30.7012 300 67 66.22 58.78

functions g(r) for the Sr containing perovskite, La0.8Sr0.2AlO2.9.The assignment of peaks of the experimental total G(r) wascarried out based on these pair-wise functions.

27Al MAS NMR experimental results leave open the ques-tion of whether or not five-fold aluminum ions are present in thestructure. On the other hand, as has been already mentioned,the construction of Sr containing perovskite for moleculardynamics simulations implies an excess of oxygen that have tobe removed in order to obtain a stoichiometric structure. Whenremoving oxygen atoms some under-coordinated aluminum andlanthanum cations necessarily appear and therefore also under-coordinated Sr atoms. For the final simulated configuration,Table 2 shows the coordination distribution of La and Sr andTable 3 the coordination distribution of Al. As it can be seenfrom Table 2, the coordination number distribution for Sr isessentially the same as that for La which, given that the gener-ation of oxygen vacancies was randomly generated, implies thatthe driving force for the distribution of O atoms, and thereforealso of O vacancies, is a combined effect of high temperatureand minimization of enthalpy. Note that the preparation of thesimulated sample was performed through a heating and coolingschedule. It is also important to remark that the synthesis of thematerial involves co-precipitation and sintering which may berather different from the process by which the simulated sample

Table 3Al coordination distribution

Coordination Number of cations Percentage

4 13 2.295 100 17.646 454 80.07

1948 E. Lima et al. / Solid State Ionics 178 (2008) 1944–1949

was prepared. In spite of these differences in the preparation ofthe real and simulated samples, the final result may be quitesimilar, given the coincidences of structural results.

For a strictly ordered system, in which all O vacancieswould be shared by two Sr neighbours, the average coordina-tion numbers of La and Sr should be 12.0 and 11.0, respectively.According to Table 2, they are 11.60 and 11.47, respec-tively. This suggests that the vacancy distribution is highlyuncorrelated with the Sr distribution. Nevertheless, the average

Fig. 6. Sequence of snapshots of the Al env

coordination numbers of La and Sr are not exactly equal,therefore there is a slight tendency of Sr to be associated withvacancies.

Molecular dynamics simulations allow for a detailed descrip-tion of the processes that take place after removing oxygenatoms. These processes are not necessarily the ones the realsystem follow, but allows us to give a very plausible explanationfor the existence of five and four coordinated Al cations. This,in turn, allows for a more accurate interpretation of 27Al MAS

ironment in La0.8Sr0.2AlO2.9 perovskite.

1949E. Lima et al. / Solid State Ionics 178 (2008) 1944–1949

NMR results. Indeed, the formation of a very small amount ofAl(IV) as a consequence of Sr incorporation was evidenced byNMR, and confirmed by the MD simulations. In the perovskitelattice O binds to two aluminums. Thus, there will be one under-coordinated aluminum per Sr substituent. In a sample ofcomposition Sr0.2La0.8AlO2.9 we thus expect 20% of the Alatoms to be five-coordinate, which is close to 17.64% shown inTable 3. Four coordinated aluminums appear at the expense offive coordinated ones which in turn are produced by theremoval of oxygen. From the theoretical 20% we have 17.64 Al(V) which, along with 2.29 Al(IV), yields 19.93%.

On the other hand, the MD also predicts that Al(V) should beformed. This signal is not resolved experimentally in the 27AlNMR spectra. It is worth mentioning that the spectra included inFig. 2 where obtained at a low magnetic field (7.05 T), at thisfield the second order effects are expected to be present as thealuminum is a quadrupolar nucleus (I=5/2). Probably, theelectric field gradient is stronger in the Al(V) than the Al(IV)and Al(VI) and as a consequence the NMR signal of Al(V)is not observed. In this way, note that radial distribution func-tions show that second neighbours change and this, of course,induces a change in the electric field gradient of aluminum.Fig. 6 shows six snapshots obtained from the early stages ofthe MD simulation to illustrate the process through which thesystem stabilizes. We centered our attention on a sixcoordinated Al atom, labeled Al1 in Fig. 6 (a). This Al1 is atthe centre of a cube formed by La3+ and Sr2+ ions. At the centreof the cube above, there is an under-coordinated Al atomproduced by the removal of oxygen atoms. This is labeled Al2.Because of the under-coordination of Al2 its mobility allows itto have long displacements from the equilibrium position andthis, in turn, produces an electrostatic attraction of oxygenlabeled O1 in the figure. This can be observed from panel (b).This departure of O1 from the original octahedron is alsopromoted by the presence of a third Al atom labeled Al3 asshown in panel (c). Once the original octahedron centered onAl3 is broken, panel (d), another oxygen atom, labeled O2 inthe figure, starts migrating through the same path as O1, panel(e), leaving Al1 four coordinated as is clearly shown in panel(f). The end situation is that there is a nearly perfect tetrahedronwhich forms by loosing two oxygen atoms that find a morestable position at the faces of the neighbouring cube above,where they stay associated to another Al atom whose coor-dination oscillates between 4 and 5.

4. Conclusion

Experimental and simulation approaches have been combinedto elucidate three important structural aspects of La1− xSrxAlO3−δperovskites, namely the coordination of Al and La ions whichturned out to have a distribution rather than a fixed number, whichexplains the existence of AlIV resonances in the NMR spectra;second,MDsimulations have allowed for a detailed description of

a plausible mechanism of formation of under-coordinated La andAl ions and further stabilization of the structure; finally this workstrongly suggests that the weak constitution of La1− xSrxAlO3− δperovskites as compared to LaAlO3 is due to the fact that theformer has less oxygen in order to be stoichiometric upon thesubstitution of La3+ by Sr2+.

Acknowledgements

Thanks are due to Dr. Claudio Zicovich and V. Lara for thevery insightful discussions and technical help, respectively. Allcalculations were carried out on an IBM SP3 supercomputerkindly donated to Laboratorio de Simulación of Instituto deMatemáticas, Unidad Cuernavaca, UNAM by Fundación ClínicaMédica Sur, A. C. from Mexico City. LJA gratefully acknowl-edgesMinisterio de Educación y Ciencia from Spain for financialsupport for a sabbatical stay at the University of Seville.

This work received financial support of the DGAPA, UNAM,PAPIIT No. IN103603, and Conacyt-SEP-2004-CO1-47541.

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