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Solid State Ionics 179

Formation of La2Zr2O7 on yttria-stabilized ZrO2(110) single crystals duringvapour–solid reaction

M.A. Schubert ⁎, S. Senz, D. Hesse

Max-Planck-Institut fur Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany

Received 4 December 2007; received in revised form 5 March 2008; accepted 7 March 2008

Abstract

La2Zr2O7 (LZO) islands were grown on yttria-stabilized ZrO2 (YSZ) (110) single crystals by reaction between La2O3-vapour and the heatedYSZ single crystal. The samples were investigated by AFM, XRD and TEM. The pole figure of the LZO(440) plane shows that LZO islands onYSZ(110) are slightly tilted around [11̄0] with maxima at tilt angles of ±0.6° and by ±0.9° around [001]. We explain this behaviour by interfacialdislocations with a Burgers vector component perpendicular to the interface, which causes the tilt, and a component parallel to the interface whichaccommodates the LZO/YSZ lattice misfit of +5%. The shape of the LZO islands can be explained by the assumption that {111} planes areenergetically preferred.© 2008 Elsevier B.V. All rights reserved.

Keywords: Solid state reaction; Transmission electron microscopy; YSZ; Pyrochlore

1. Introduction

High temperature solid oxide fuel cells (SOFC) are a subjectof intensive investigations. The reason is the possibility toachieve a high efficiency of approximately 70% for the co-generation of electricity and heat with a gas turbine powersystem [1,2]. Due to the high operation temperature of about1000 °C the frequently applied La1− xSrxMnO3 (LSM) cathodemay react with the yttria-stabilized ZrO2 (YSZ) electrolyte andform a La2Zr2O7 (LZO) layer with a pyrochlore structure at theinterface. This pyrochlore formation is most probably the mainreason for degradation of high temperature SOFCs [3]. To betterunderstand this LZO formation process solid state reactionbetween La2O3 and YSZ(001) single crystals is beinginvestigated [4,5]. During this process, LZO islands on YSZ(001) are formed. These islands have four domains tilted by 2.1°around b110N axes, and four other domains tilted by 0.9°

⁎ Corresponding author.E-mail address: aschub@mpi-halle.mpg.de (M.A. Schubert).

0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2008.03.017

around b100N axes. This effect was explained by interfacialmisfit dislocations which are present in a high density due to arelatively large misfit between LZO and YSZ. The latticeconstant of the LZO pyrochlore (a=1.079 nm) is nearly twice

Fig. 1. Part of anXRDpole figure recordedwith the LZO(440) reflection (Ψ rangesfrom 0° to 3°). The peak is split into four subpeaks. Intensity at Ψ=0° originatesfrom the YSZ substrate.

Fig. 2. Left: AFM image of LZO islands on YSZ(110); Right: TEM plan-view image of similar LZO islands. The contrast details in the LZO islands were mostprobably caused by a network of misfit dislocations.

454 M.A. Schubert et al. / Solid State Ionics 179 (2008) 453–457

that of YSZ (a=0.514 nm), so that the misfit is +5%. Theinterfacial misfit dislocations have Burgers vectors inclined by45° to the interface. The Burgers vector component parallel tothe interface accommodates the misfit of the pyrochlore islands,and the component perpendicular to the interface causes the tilt.This effect was already observed for spinel-forming reactions inthe case of a positive misfit of the spinel with respect to theMgO substrate [6]. So far this type of reactions was onlyinvestigated with a cubic substrate of (001) orientation. In thispaper we describe the results of the vapour–solid reactionbetween La2O3-vapour and a YSZ single crystal with (110)orientation.

2. Experimental details

LZO islands were formed on Y2O3-stabilized ZrO2(110)single crystals (from CrysTec GmbH, Berlin) by a topotaxialvapour–solid reaction of the latter with La2O3 vapour atelevated temperature. Pieces of La2O3 were evaporated in a highvacuum system using electron-beam evaporation. Pure oxygen

Fig. 3. Left: High-resolution TEM image of a LZO island on YSZ(110). Right: FFTofstructure. Indices are for YSZ.

was supplied into the chamber and a background pressure of1.3×10−2 Pa was established, to avoid oxygen losses. Duringdeposition the substrate temperature was kept at about 1200 °Cand the deposition rate was measured with a quartz micro-balance to be about 0.45 nm min−1. The nominal thickness ofdeposited La2O3 was 4 nm. Directly after deposition the sub-strate furnace was turned off. In the first minute the cooling rateis about 50 K/min. At lower temperatures the cooling ratedecreases and it takes around 2 h until the temperature is about300 °C and at this temperature the gas flow was stopped. Thetopography and morphology of the LZO islands wereinvestigated by atomic force microscopy (AFM) (modelDimension 5000, Digital Instruments, Inc.). The relativeorientation of the LZO lattice with respect to the YSZ latticewas determined by X-ray diffraction (XRD) (Philips X'PertMRD, Cu–Kα radiation). The LZO/YSZ reaction front wasinvestigated by transmission electron microscopy (TEM). TEMinvestigations were performed in a Philips CM20 Twin at200 kV and high-resolution TEM (HRTEM) images were ob-tained by a Jeol 4010 microscope at 400 kV. For TEM

the high-resolution TEM image with superstructure reflections of the pyrochlore

Fig. 4. Fourier-filtered image of the HRTEM image (Fig. 3) using YSZ(002) and LZO(004) reflections. The inset at the upper left is part of the Fourier-filtered imageusing YSZ(220) and LZO(440) reflections, note that this cut-out is magnified by a factor 1.5.

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observations thin plan-view and cross-sectional specimens wereprepared by mechanical polishing and then argon-ion milling.

3. Results

X-ray diffractometry and TEM investigations showed thatLZO islands have nearly the following orientation:

110ð ÞLZO jj 110ð ÞYSZ; 001½ �LZO jj 001½ �YSZ: ð1Þ

However, LZO islands have not exactly this orientation. Fig. 1shows a XRD pole figure recorded with the LZO(440) reflection:The peak is split into four subpeaks. Two subpeaks correspond toa tilt around the [1̄10] axis by ±0.6°, whereas the other two resultfrom a tilt around the [001] axis by ±0.9°. The intensity at the tiltangle of ψ=0° was caused mainly by a shoulder of the substratepeak. The LZO islands have a long edge along [11̄0] and a shortedge along [001] (Fig. 2). TEM plan-view images show contrastdetails which are caused by a network of misfit dislocations withspacings of about 9 nm.

High-resolution TEM (HRTEM) images of LZO islands showthat the surface of the pyrochlore islands is terminated by {111}planes (Fig. 3). In the fast Fourier transform (FFT) of this HRTEM

Fig. 5. Left: Stereographic projections for a cubic structure with (001)-orientation andYSZ(110).

image, superstructure reflections of the pyrochlore structure arevisible. The brighter spots are split into two reflections,whereby theinner spot is caused by theLZO island and the outer one by theYSZsubstrate. Correspondingly the lattice parameter of the LZO islandis clearly larger than that of the substrate, and misfit dislocationsshould be present at the interface. To visualize these misfit dislo-cations, Fourier-filtering has been carried out to reduce the noise intheHRTEM images. In theYSZ(002) Fourier-filtered image,misfitdislocations at the interface are clearly visible (Fig. 4). In the inset atthe upper left a YSZ(220) Fourier-filtered image of part of theimage around the left dislocation is shown. This dislocation showsone additional horizontal lattice plane on the right side.

4. Discussion

LZO islands were formed on YSZ(001) and YSZ(110) after avapour–solid reaction between La2O3 vapour and the YSZsingle crystal. In both cases the islands consisted of several tilteddomains. On YSZ(001), they consisted of four to eight tilted andone non-tilted domain [4,5]. The four main domains were tiltedby 2.1° around b110N axes. Sometimes four additional domainswith a tilt of 0.9° around b100N axes were found. Here we havefound that LZO islands on YSZ(110) single crystals are tilted

(110)-orientation; Right: schematic drawing of a dislocation with Yb jj 10P1

� �on

Table 1Calculated and measured tilt angles and directions for YSZ(001) and YSZ(110)assuming {101} slip planes

Substrate Type of Burgers vectors Yb Tilt around γmax γexp

YSZ(001) a /2[101] [010] 2.8° 0.9°[4]YSZ(001) a /2[011] and a / 2[101] 1

P10

� �4.0° 2.1°[5]

YSZ(110) a /2[101] 1P1P1

� �1.6° –

YSZ(110) a=2 01P1

� �and a=2 011½ � [001] 2.8° 0.9°

YSZ(110) a=2 01P1

� �and a=2 10

P1

� �1P10

� �2.0° 0.6°

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around theP110½ � axis with a maximum at a tilt angle of ±0.6°,

and additionally around the [001] axis by ±0.9°.Tilted domains on YSZ(001) and YSZ(110) can be explained

by misfit dislocations with a Burgers vector component Yb8perpendicular to the interface which causes the tilt, and a pa-rallel component YbO which accommodates the misfit betweenLZO and YSZ. If the misfit f and the Burgers vector are known,it is possible to calculate the maximum tilt angle γmax.

gmax ¼Yb8YbO

: f ð2Þ

We suppose that mainly {101} slip planes are active, i.e. withBurgers vectors in these slip planes. Fig. 5 shows stereographicprojections of a cubic material with the possible slip planesand a schematic drawing of a dislocation with Yb jj 10

P1½ �

on YSZ(110). For both orientations four different out-of-plane{101} slip planes with Burgers vectors Yb jj b101N are pos-sible. But the case of a slip plane parallel to the interface isdifferent: On YSZ(001) four Burgers vectors Yb jj b110N are

Fig. 6. Schematic drawing of an island on YSZ(001) with 8 tilted and one not tilted domodel islands; (b) model of dislocations with different b101N Burgers vectors; (c) t

possible, but on YSZ(110) only two Burgers vectors of typeYb jj b110N are possible that lie parallel to the interface.

In the case of only out-of-plane Burgers vectors of typeYb ¼a=2b101N tilt axes and the tilt angle can be calculated. Table 1shows calculated and measured tilt angles for LZO islands onYSZ(001) and YSZ(110). Tilt directions could be explained bythis model, but in both cases (for a YSZ(001) and for a YSZ(110) substrate) the measured tilt angles are clearly lower thanthe calculated values. The ratio of the tilt angle on a YSZ(110)substrate around [001] to the tilt angle around 1

P10½ � is similar to

the calculated ratio offfiffiffi2

p. Possible reasons for lower tilt angles

are the interaction of two mirror-symmetric slip planes anddislocations with a slip plane parallel to the interface. Due to thedifferent crystallography for a substrate with (110)-orientationthe tilt angles are lower than for the (001)-orientation. We didnot observe a tilt around b111N axes, corresponding to dis-locations with only one type of Burgers vector.

In experimental HRTEM images of LZO islands misfit dislo-cations were observed at the interface (Fig. 3). To analyse thesedislocations Fourier-filtering with YSZ(002) and YSZ(220)reflections was performed (Fig. 4). The left-most dislocationin Fig. 4 is characterized by two extra half planes, viz. one alongYSZ(002) and the other along YSZ(220). This can be explainedby a dislocation with a Burgers vector Yb ¼ a=2 101½ � or Yb ¼a=2 011½ �. The Burgers vector Yb ¼ a=2 101½ � has a componentYb ¼ a=2 011½ � with jYb2j=2.57 Å corresponding to the latticeplane distance d002 for YSZ. The second component of thisBurgers vector is Yb ¼ a=2 100½ � with jYb2j=2.57 Å, lying pa-rallel to the [100] axis at an angle of 45° to the [110] axis. In theHRTEM image we can only see a projection of this component

mains and an island on YSZ(110) with 4 tilted domains; (a) surface planes of thisilted domains of this islands.

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on the [110] axis with a value of jYb2j=ffiffiffi2

p=1.82 Å which is

identical to d220 for YSZ.LZO islands on YSZ(001) have a square shape with edges

along [110] [4], but on YSZ(110) they have a rectangular shapewith a long edge along 1

P10½ � and a short edge along [001]. This

can be explained by different surface energies. One can assumethat the ratio of surface energies for different surfaces is similarfor LZO and YSZ. For YSZ the {111} planes have a calculatedsurface energy of 65 meV/Å2 which is clearly lower than109 meV/Å2 for {100} or 90 meV/Å2 for {110} planes [7]. Weknow from HRTEM images that the surface of the LZO islandsis mainly limited by {111} planes. On a (110) substrate two{111} planes with an angle of 35.3° to the surface and four 111planes with an angle of 90° are possible. A lower angle to thesubstrate is favourable, because in this case the surface area issmaller. Islands on YSZ(110) are most probably limited by large(111) and 11

P1ð Þ planes and small (100) and (010) planes. LZO

islands on YSZ(001) are limited by four {111} planes withan angle of 54.7° to the interface and the (001) plane. In theliterature it was also observed that {111} planes are preferredsurfaces for LZO [8].

Fig. 6 shows schematic models of an island on YSZ(001)and on YSZ(110), showing (a) a model of the surfaces, (b) thenetwork of dislocations, and (c) the resulting tilted domains. Fordomain numbers 1–4 two sets of dislocations induce the tilt. OnYSZ(001) we observe four additional domains at the boundariesof domain numbers 1–4. This can be explained by dislocationreactions. The following example is for the border of domainsnumber 1 and number 2.

a=2 01P1½ � þ a=2 011½ � ¼ a 010½ �

In this case only one set of dislocations with Yb ¼ a=2 01P1½ �

induces the tilt of the domain, because the other dislocations withYb ¼ a 010½ � are parallel to the interface. On YSZ(110) we did not

observe an analogous effect, but in principle it should also bepossible. If additional domains would appear they would be smalland difficult to find.

5. Conclusions

The vapour–solid reaction of La2O3 vapour with a YSZ(110)single crystal was studied. LZO islands of rectangular shape wereformed with a long and a short edge, these islands are slightlytilted in relation to the substrate. We found that the effect of tiltedislands can be explained by interfacial misfit dislocations withdifferent slip planes and a Burgers vector component perpendi-cular to the interface. Compared to YSZ(001), the tilt angle islower; this is a consequence of the crystallographic differencesbetween (110) and (001) orientations. The shape of the islands onYSZ(001) and YSZ(110) is mainly an effect of the lower surfaceenergy for a {111}-plane than for a {100} or {110} plane.

Acknowledgement

Thisworkwas supported byDeutsche Forschungsgemeinschaftvia Sonderforschungsbereich 418.

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