Crystalline Intermediate Phases in the Sol–Gel-Based Synthesisof La2NiO41d

Konstantin Efimov,w Mirko Arnold, Julia Martynczuk, and Armin Feldhoff

Institute of Physical Chemistry and Electrochemistry, Leibniz Universitat Hannover, D-30167 Hannover, Germany

X-ray diffraction and transmission electron microscopy wereapplied to investigate a sol–gel synthetic process for the mixedoxygen ion and electron conductor La2NiO41d with a K2NiF4structure type. The development of the La2NiO41d is elucidatedconsidering the influence of calcination temperatures and dwelltimes. Following the thermal decomposition of nitrate and or-ganic precursors in an intermediate step, the lanthanum nickeloxide is obtained after a short dwell time above 7501C. Thisoccurs by the transformation of an ultrafinely dispersed powderconsisting of lanthanum oxycarbonate, lanthanum oxide, andnickel oxide. The pure La2NiO41d phase was obtained by sim-ilar solid-state reactions between nanocrystalline powder parti-cles at just 9501C.

I. Introduction

HIGH oxygen permeation fluxes with infinite selectivity ofmixed ionic–electronic conductors (MIECs) find use as

membranes of interest for various industrial processes. Manyapplications of MIECs have already been reported, e.g., as cath-ode material in solid-oxide fuel cells (SOFCs), in the productionof oxygen-enriched air and in the conversion of hydrocarbons tosynthesis gas.1–4

Indeed, many alkaline-earth perovskites are known that have ahigher oxygen permeation rate than La2NiO41d, but they tend toform carbonates in the presence of CO2 at relevant temperatures(5001–9001C), sometimes even in air.5,6 This property excludesthem from use in SOFC operated in the intermediate temperature(IT) range, i.e., 5001–7501C. It is expected that La2NiO41d and itsderivates may reveal a better stability against CO2, because theformation of lanthanum carbonates or oxycarbonates is not aspronounced as in the case of alkaline-earth metals.

The complex oxide La2NiO41d, with oxygen hyper-stoichiometry made of an alternate sequence of La2O2 bilayersand NiO monolayers (K2NiF4 structure type

7) is the n5 1 mem-ber of the Ruddlesden–Popper structure series with the generalformula LaO(LaNiO3)n (n5 1, 2, 3, N).8 The oxygen transportin La2NiO41d occurs predominately by interstitial oxygenthrough La2O2 bilayers and can be considered independent ofthe vacancy migration mechanism in the perovskite layers.9–11

By possible intergrowth of Ruddlesden–Popper phases (n41)with La2NiO41d,

12 the layer stacking changes at the cost ofLa2O2 bilayers and the oxygen conductivity may be impaired.Therefore, the development of Ruddlesden–Popper phases(n41) must be avoided during the production of La2NiO41d.

It is difficult to produce pure La2NiO41d by a standard ce-ramic technique, because it involves high calcination tempera-tures, long dwell times, and several intermediate grindingsteps.13 In contrast to this, sol–gel methods offer the clear

advantage of high phase purity at comparably low synthesistemperatures and short dwell times.

To our knowledge, just four reports on sol–gel synthetic pro-cesses of La2NiO41d are available.

14–17 Fontaine and colleaguesinvestigated the formation of La2NiO41d through a variety ofsol–gel methods by X-ray diffraction (XRD), infrared spectros-copy, and thermal analysis to a large extent. The examined pro-cesses involved the complexing of nitrate precrusors withhexamethylenetetraamine (HMTA) or HMTA, acetic acid, andacetylacetone or citric acid and ethylene glycol as complexingagents. It was found that a Ruddlesden–Popper phase formsabove 6001C, and pure La2NiO41dwas obtained after calcinationat 10001C. The attribution of intermediate phases was not given.Nevertheless, it was concluded that intermediate lanthanum oxy-carbonate formation may occur after thermal decomposition of acomplex precursor. Huang and colleagues and Guo and col-leagues reported analogous results by employing nitrate precur-sors and citric acid or lanthanum hydroxide, nickel carbonate,and diethylenetriaminepentaacetic acid as complexing agents.

We prepared La2NiO41d using nitrate precursors as well asethylenediaminetetraacetic acid (EDTA) and citric acid as com-plexing and gelating agents. For the related synthesis of complexperovskites, it has been recently shown that the product isformed by solid-state reactions between nanocrystalline inter-mediates.18–20 Regarding this, we studied the reaction sequencesin very fine detail on partly reacted and quenched powders thatwere synthesized at different temperatures and dwell times,respectively. The intermediates and products were examinedby XRD and (scanning) transmission electron microscopy(STEM, TEM) combined with high-resolution TEM (HRTEM)and energy-dispersive X-ray spectroscopy (EDXS).

II. Experimental Procedure

The synthetic process for La2NiO41d was conducted via a sol–gel route employing EDTA and citric acid as organic ligands.Given amounts of La(NO3)3 and Ni(NO3)2 were dissolved inwater followed by the addition of EDTA and citric acid. Themolar ratio of La(NO3)3:Ni(NO3)2:EDTA:citric acid was equalto 2:1:1:1.5. The pH value of the solution was adjusted to therange of 7–9 with NH3 �H2O. The transparent reaction solutionwas then heated at 1501C under constant stirring for severalhours to obtain a blue-colored gel. The gel was precalcined inthe temperature range of 3001–4001C. After this, the precalcinedamorphous powders were ground and fired at different temper-atures (5001–9501C) and dwell times (2–20 h). The product andthe crystalline intermediate phases were examined by severaltechniques that are described below.

XRD was performed in a y/2y geometry on a Philips X’pert-MPD instrument (Philips Analytical, Almelo, the Netherlands)using monochromated CuKa radiation at 40 kV and 40 mA,with a receiving slit of 0.10 mm and count times of 5 s/step. Datawere collected in a step–scan mode in the range of 201–601 withintervals of 0.021. Data for interpretation were taken from theICDD PDF-2 database. For the Rietveld analysis, XRD data ofproduct and intermediate phases were collected in a step-scanmode in the range from 151 to 1201 with intervals of 0.021 and

L. Levinson—contributing editor

This work was financially supported by DFG grant number FE 928 1-1.wAuthor to whom correspondence should be addressed. e-mail: konstantin.efimov@

pci.uni-hannover.de

Manuscript No. 25251. Received September 23, 2008; approved December 12, 2008.

Journal

J. Am. Ceram. Soc., 92 [4] 876–880 (2009)

DOI: 10.1111/j.1551-2916.2009.02943.x

r 2009 The American Ceramic Society

876

count times of 8 s/step. The structure refinement was carried outusing Topas V4.1 (Coehlo Software, Brisbane, Australia).

STEM and TEMwere conducted at 200 kV on a JEOL JEM-2100F-UHR field-emission instrument (Jeol Ltd., Tokyo, Ja-pan) equipped with a light-element EDXS detector (INCA 200TEM, Oxford Instruments pls, Oxfordshire, UK). EDX spectrawere quantified by the Cliff—Lorimer technique. TEM speci-mens were prepared by epoxy gluing them between two pieces ofsilicon wafer. Rectangular pieces of 1 mm� 1.5 mm� 3 mmwere cut and polished on polymer-embedded diamond lappingfilms to approximately 0.01 mm� 1� 2.5 mm (Allied High TechProducts Inc., Rancho Dominguez, CA). Finally, Ar1 ion sput-tering was used at 3 kV (Gatan, model 691 PIPS, precision ionpolishing system, Gatan Inc., Plesanton, CA) under shallow in-cident angles of 61 until electron transparency was achieved.

La2O2CO3(s) as a phase mixture of monoclinic, tetragonal,and hexagonal variants was synthesized by the EDTA/citrateroute. La2(CO3) � xH2O was obtained from a commercial source(purity 99.9%, ABCRGmbH& Co. KG, Karlsruhe, Germany).Quartz glass equipment flanged to an oil diffusion pump servedto determine the CO2 equilibrium pressure over both carbonatesas a function of temperature. After sealing and evacuating, thepressure was measured with a capacity manometer at increasingtemperatures after equilibration times of approximately 45 min.

III. Results and Discussion

Figure 1 shows XRD patterns taken from powders quenched atdifferent temperatures after 2 h. The first crystalline intermedi-ates occurred at 5001C (Fig. 1(a)) and have been identified asLa2O2CO3 and NiO. The oxycarbonate La2O2CO3 is present asa mixture of monoclinic, tetragonal, and hexagonal phases.Synthesis at 6001–7001C for 2 h (Figs. 1(b) and (c)) leads toLa2O2CO3 and NiO. However, the oxycarbonates exhibit differ-ent mass ratios between monoclinic, tetragonal, and hexagonalvariants than after calcination at 5001C. This is visible, e.g., byintensities in the 2y ranges of 251–281 and 301–321 in Figs. 1(b)and (c), if compared with Fig. 1(a). The desired La2NiO41d

product formed at 7501C (Fig. 1(d)), and was accompanied byhexagonal La2O2CO3, trigonal La2O3 and cubic NiO. The com-position of this powder has been determined by quantitativeRietveld-analysis based on structure data from Rabenau et al.7

for La2NiO41d, from Olafsen et al.21 for La2O2CO3, and fromthe ICSD 2008.1 (FIZ–Karlsruhe) database for NiO and La2O3.The sample contains 40 wt% La2NiO41d, 35 wt% La2O2CO3,10 wt% NiO, 5 wt% La2O3 and o10 wt% of an unidentifiedphase. The powder calcined at 8001C (Fig. 1(e)) consists of 80wt% La2NiO41d, 5 wt% La2O3, 5 wt% NiO, and o10 wt% ofan unidentified phase. Pure La2NiO41d forms after calcinationfor 2 h at 9501C (Fig. 1(f)). Monitoring of phase purity andstructure refinement was carried out via Rietveld-analysis. Theobtained La2NiO41d had a tetragonal lattice (a5 3.860 A,c5 12.693 A) with symmetry I4/mmm (139), which is in accor-dance with Rabenau et al.7

The powders calcined for 2 h at 7501 and 8001C have beeninvestigated in detail by (S)TEM, because the product phase andintermediates were obtained simultaneously. Annular dark-fieldSTEM (Figs. 2(a), (d), and (i)) shows that, after calcination at7501C, grains are below 100 nm in size. Elemental maps byEDXS (Figs. 2(b) and (c)) reveal a homogeneous distribution oflanthanum and nickel. Inhomogeneity of the cation distributionis revealed at magnifications that allow the resolution of indi-vidual nanoscale grains (Figs. 2(e), (f), (j), and (k)). This ultra-fine intermixing is considered to be characteristic and a majoradvantage of sol–gel synthesis.6,18–20

The bright-field STEM micrograph in Fig. 3(a) shows a sam-ple area of powder calcined at 8001C. Elemental distributionsacquired by EDXS clearly indicate the presence of two differentphases (Figs. 3(b) and (c)). The first phase contains lanthanumand nickel, and the second phase lacks lanthanum. EDXS com-bined with Cliff–Lorimer quantification demonstrated that aNiO intermediate grain is surrounded by La2NiO41d productgrains.

The HRTEM micrographs in Fig. 4 display contact zonesbetween two grains of a sample calcined at 7501C. One of them(Fig. 4(a)) is the contact zone between a La2O2CO3 andLa2NiO41d grains. The La2O2CO3 grain exhibits lattice fringesthat arise from (002) lattice planes separated by a distance of 8A. Structure determination of La2NiO41d was conducted viatwo-dimensional fast Fourier transformation (FFT) of the cor-responding area (Fig. 4(c)). The La2NiO41d grain is orientedalong the [3,1,1] zone axis and shows up with (0-11) and (10-3)lattice fringes. Figure 4(b) shows a phase boundary betweenNiO and La2NiO41d grains. The La2NiO41d lattice fringes arisefrom (101) lattice plane with a separation distance of 3.7 A. Theorientation of the NiO grain has been found (with support ofFFT) to be along the [2,1,1] zone axis (Fig. 4(d)). In both ex-amples, the grains are in intimate contact. Hence, the contactzone between the product and intermediates can be consideredas a reaction front of the solid-state transformation.

The XRD and TEM investigations suggest that the purecomplex oxide, La2NiO41d, is formed after a short dwell time(2 h) at 9501C via a solid-state reaction between nanocrystal-line NiO and La2O2CO3 or La2O3. Carbon dioxide is evolvedwhile some oxygen from air is incorporated into the reaction

La2O2CO3ðsÞ þNiOðsÞ �!þO2La2NiO4þdðsÞ þ CO2ðgÞ

or

La2O3ðsÞ þNiOðsÞ �!þO2La2NiO4þdðsÞ

Because lanthanum oxycarbonates were found in intermedi-ate steps of the synthetic process of La2NiO41d, their stability atapplied temperatures has been examined by the determinationof an Ellingham diagram (Fig. 5). The equilibrium pressure ofCO2 has been measured above the La2O2CO3(s) mixture ofmonoclinic, tetragonal, and hexagonal phases, which was syn-thesized by the EDTA/citrate route with crystal sizes below 100

Fig. 1. (a–f ) X-ray powder diffraction taken from the synthesis ofLa2NiO41d after 2 h (a) at 5001C; (b) at 6001C; (c) at 7001C; (d) at7501C; (e) at 8001C; (f) at 9501C. (I–VI) Bragg positions for differentphases. (I) tri-La2O3 [5–602]; (II) c-NiO [4–1159]; (III) h-La2O2CO3 [37–804]; (IV) t-La2O2CO3 [23–320]; (V) m-La2O2CO3 [23–322]; (VI)t-La2NiO41d [11–557]. PDF numbers are given in parentheses.

April 2009 Crystalline Intermediate Phases in the Sol–Gel-Based Synthesis of La2NiO41d 877

nm, and La2(CO3)3(s) with crystal sizes in the range of few mi-crometers. Subsequently, the CO2 chemical potential duringdecomposition of the carbonates was calculated with respectto standard conditions and plotted in Fig. 5. The decompositionof carbonates can take place only if it forces a CO2 potential thatis higher than the one in the surrounding atmosphere. This sit-uation arises if the curve for a decomposition reaction lies higherin the Ellingham diagram than the curve for CO2 for its relevantpartial pressure. Thus, the thermodynamic decomposition pointwas determined to be approximately 4501C for the La2O2CO3(s)and approximately 1901C for the La2(CO3)3(s) in ambient airp(CO2)5 30 Pa. Dashed lines marked by single and double as-terisk present m(CO2) plots above coarse crystalline La2O2-

CO3(s) calculated with data reported by Shirsat et al.22 andWatanabe et al.23 The CO2 chemical potential above nanocrys-talline La2O2CO3 is approximately 6 kJ/mol higher than abovecoarse crystalline oxycarbonates in air. Correspondingly, thedecomposition temperature of nanocrystalline oxycarbonateslies approximately 301C lower than that of the coarse crystals.This result is in agreement with the Thomson–Freundlich equa-tion,24 which predicts a higher chemical potential of carbondioxide above nanocrystalline powders compared with coarsecrystalline carbonates. Thus, the existence of lanthanum(oxy)carbonates is thermodynamically unfavorable in air aboveapproximately 4801C. The presence of La2O2CO3 in powders

calcined at 5001–7501C (see Figs. 1 and 4) can be explained withreaction kinetics. This conclusion is supported by the fact thatno lanthanum oxycarbonates have been found in powders ca-lcined at 6001–8001C for 20 h (Fig. 6). The m(CO2) plot forNiCO3 calculated with data from Barin et al.25 and included inFig. 5 illustrates that the formation of this carbonate does notplay any role in the solid-state reaction. Furthermore, by meansof the Ellingham diagram, it is clear that lanthanum carbonatesare less stable than barium and strontium carbonates. Therefore,a good stability of La2NiO41d against CO2 in contrast to alka-line-earth perovskites is confirmed.

The effect of dwell times on the formation of pure La2NiO41dhas been elucidated by the synthesis of powders at 6001–8001Cwith dwell times of 3 h up to 20 h (not all data are presentedhere). Figure 6 displays XRD taken from the powders after ca-lcination at different temperatures for 20 h. None of the syn-theses lead to pure La2NiO41d. The powders are composed ofLa2NiO41d, and new phases with Bragg refletions having similarlocations to those of La2NiO41d. Comparing reflection intensi-ties in the 2y range of 311–341 with Bragg positions of differentrelated oxides leads to this conclusion (Fig. 6). The 103 reflectionof La2NiO41d has a higher intensity than the 110 reflection, andno other reflections are located between the 103 and 110 posi-tions (see Fig 1(f) ). The intensity of corresponding reflections inFig. 6, however, is inverted, and distinct reflection intensities lie

Fig. 3. Product and intermediates formed after 2 h at 8001C. (a) Energy-dispersive X-ray spectroscopy, bright-field; (b, c) elemental distributions byenergy-dispersive X-ray spectroscopy.

Fig. 2. Product and intermediates formed after 2 h at 7501C. (a, d, and i) energy-dispersive X-ray spectroscopy, annular dark-field; (b, c, e, f, j, and k)elemental distributions by energy-dispersive X-ray spectroscopy.

878 Journal of the American Ceramic Society—Efimov et al. Vol. 92, No. 4

between them. Because a considerable amount of La2O3 (seeFig. 6(I)) was not reacted, and NiO (see Fig. 1(II)) was com-pletely removed, the new phase must be Ni enriched. The newNienriched phase can be described as a Ruddlesden–Popper phase(n41) with an aperiodic stacking of perovskite and rocksalt lay-ers with the formula LaO(LaNiO3)n. This is aided by consider-ation of the XRD pattern of the Ruddlesden-Popper phasesLa4Ni3O10 (LaO(LaNiO3)3, n5 3, Fig. 6(II)), La3Ni2O7 (LaO(LaNiO3)2, n5 2, Fig. 6(III)) and LaNiO3 (n5p, Fig. 6(IV)),whose Bragg reflections have similar locations compared withLa2NiO41d.

Figure 7 shows the investigated reaction pathways in brief.After thermal decomposition of nitrate and an organic precur-sor (Fig. 7(a)), ultrafinely dispersed nanocrystalline intermedi-ates of La2O2CO3 (phase mixture) and NiO (cubic) occurredafter calcination for 2 h (Fig. 7(b)). Subsequent to this, twodifferent reaction pathways are possible, where only one leads tothe desired La2NiO41d as a pure product:

(1) The tetragonal La2NiO41d forms during solid-state re-actions between cubic NiO and hexagonal La2O2CO3 or trig-onal La2O3, if the calcination temperature is raised to 7501C forshort dwell time (Fig. 7(c)). The pure product is obtained after2 h of calcination at 9501C (Fig. 7(e)).

(2) Raising the dwell times above 2 h at temperatures from6001 to 8001C does not lead to pure La2NiO41d. Instead, La2O2-depleted Ruddlesden–Popper phases LaO(LaNiO3)n are formed(Fig 7(d)).

Fig. 4. (a) High-resolution transmission electron microscopy (HRTEM) micrograph of the contact zone of La2O2O3 and La2NiO41d grains.(b) HRTEM micrograph of the contact zone of NiO and La2NiO41d grains. (c–d) Diffraction data acquired via fast Fourier transformation relatedto the marked positions in (a–b).

Fig. 5. Ellingham diagram for the decomposition of carbonates underdifferent partial pressures. Chemical potential of CO2 above NiCO3,SrCO3, and BaCO3 have been calculated with thermodynamic data.25

The dashed lines represent the chemical potential of CO2 in the sur-rounding atmosphere for different partial pressures. �Calculations havebeen made with thermodynamic data.22 ��Calculation has been madewith thermodynamic data.23 p1(CO2)5 101.3 kPa refers to standardconditions.

April 2009 Crystalline Intermediate Phases in the Sol–Gel-Based Synthesis of La2NiO41d 879

Because the oxygen transport in La2NiO41d occurs throughLa2O2 bilayers, the formation of Ruddlesden–Popper phases(n41) must be prevented through the preparation of pureLa2NiO41d ceramics. This is realized if the reaction system isbrought to a reaction temperature of 9501C quickly and withoutintermediate steps.

IV. Conclusions

The synthetic process of La2NiO41d via the commonly appliedsol–gel method has been studied in considerable detail. First,crystalline intermediates have been identified as La2O2CO3

(phase mixture) and NiO (cubic). The La2NiO41d product ispartly formed via a solid-state reaction between nanocrystallineNiO and La2O2CO3 or La2O3 after a short dwell time (2 h) at7501–8001C. Raising the dwell time above 2 h at 6001–8001C

does not lead to pure La2NiO41d. Instead, La2O2-depletedRuddlesden–Popper phases LaO(LaNiO3)n are formed. PureLa2NiO41d is obtained only if the reaction temperature is in-creased to 9501C.

The stability of lanthanum (oxy)carbonates has been investi-gated with the help of an Ellingham diagram. Lanthanum(oxy)carbonates are thermodynamically unstable above approx-imately 4801C in air. The presence of La2O2CO3 in powderscalcined at 5001–7501C can be explained with reaction kinetics.

Acknowledgment

The authors greatly acknowledge fruitful discussion with Prof. J. Caro.

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Fig. 6. X-ray diffraction taken from the powders calcined for 20 h.(a) At 6001C; (b) at 7001C; (c) at 7501C; (d) at 8001C. (I–V) Bragg po-sitions for different phases. (I) tri-La2O3 [5–602]; (II) La4Ni3O10 [35–1242]; (III) La3Ni2O7 [35–1243]; (IV) LaNiO3 [34–1077]; (V) La2NiO41d[11–557]. PDF numbers are given in parentheses.

Fig. 7. Scheme of La2NiO41d formation by the sol–gel route.

880 Journal of the American Ceramic Society—Efimov et al. Vol. 92, No. 4