006) 1307–1315www.elsevier.com/locate/ssi

Solid State Ionics 177 (2

Interstitial oxide positions in oxygen-excess oxy-apatites

Laura León-Reina, J. Manuel Porras-Vázquez, Enrique R. Losilla, Miguel A.G. Aranda ⁎

Departamento de Química Inorgánica, Cristalografía y Mineralogía, Universidad de Málaga, 29071-Málaga, Spain

Received 27 February 2006; received in revised form 10 May 2006; accepted 24 May 2006

Abstract

Several oxy-apatites materials, La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9Sr1(Si5.5Al0.5O24)O2.25, La9.67□0.33(Si5.5Al0.5O24)O2.25, La8.5Sr1.5(Si5.5Al0.5O24)O2 and La9.5□0.5(Si5.5Al0.5O24)O2 have been prepared as crystalline phases. The impedance study showed that allsamples are oxide ion conductors and their conductivities are similar to those previously reported for related oxy-apatites. A thorough study on theoxygen sublattices for oxygen excess samples has been carried out using neutron powder diffraction data by the Rietveld method. The structuralstudy shows the presence of interstitial oxide anions close to the periphery apatite channels. However, the interstitial oxide position in cationstoichiometric silicates, f. i. La8.65Sr1.35(Si6O24)O2.32, is different from that in germanates, f. i. La8.65Sr1.35(Ge6O24)O2.32. This is likely due to thedifferent structural flexibility of the two tetrahedral groups and it explains the previously reported evidence of higher oxygen contents forgermanates oxy-apatites. The structural characteristics of these oxide anion conductors are discussed.© 2006 Elsevier B.V. All rights reserved.

Keywords: SOFC; Oxide ion conductor; Apatite; Synchrotron; Neutron diffraction

1. Introduction

Solid oxide fuel cells (SOFCs) are all-solid-state electrochem-ical devices that allow the direct conversion of chemical to elec-trical energy with low emission of pollutants, low noise and highenergy-conversion efficiency [1,2]. The three basic components ina SOFC unit are: a porous cathode, a porous anode and a denseelectrolyte. The electrolyte must have high oxide conductivity,negligible electronic conductivity, good chemical compatibilitywith the electrodematerials and low thermal expansion coefficientsimilar to those of the electrodes [3]. Traditionally, the SOFCelectrolyte in commercial systems is a self-supported (≈ 10 μmthick) yttria stabilized zirconia (YSZ) film. This materialexhibits high oxide ion conductivity at elevated temperatures(1173–1273 K) and good chemical stability under reducing andoxidizing atmospheres. However, this high operating temperaturemay cause problems such as difficulties in cell sealing whichenforces the use of expensive materials and low lifetime of thecomponents. Hence, there is a huge research effort in the devel-opment of new oxide ion conductors to reduce the operating

⁎ Corresponding author. Tel.: +34 952131874; fax: +34 952132000.E-mail address: g_aranda@uma.es (M.A.G. Aranda).

0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2006.05.034

temperature to 873–1073 K [4]. With the aim of realizing theintermediate temperature SOFCs (IT-SOFCs), ionic conductors thathave high conductivities at lower temperatures are strongly desired.Therefore, other electrolytes with higher oxide conductivities arebeing intensively studied, for instance: Ce0.8Gd0.2O1.9 (GDC) [5];(Bi2O3)0.75–(RE2O3)0.25 [6]; perovskite-type oxides such as La0.9-Sr0.1Ga0.8Mg0.2O3 [7] and BIMEVOX (Bi2V0.86Ni0.14O5.29) [8].Rare earth oxy-apatites are also attracting considerable interest dueto their high oxide ion conductivities and low activation energies[9]. Research interest in this area has grown following the excitingreports by Nakayama et al. [10,11] about the high oxideconductivities of rare earth silicates, Ln10−x□x(Si6O24)O3−1.5x.

A key aspect of oxy-apatite materials is the wide range ofsubstitutional possibilities [9]. The general formula for an apatite-type material can be expressed as A10(T6O24)X2, where A isgenerally a large cation (alkali, alkaline earth, rare earth metals[RE]), TO4 is a tetrahedral group (T=P,V,As, Si, Ge, etc.) andX isan anion like O, OH, F, Cl, Br, I. In the terms of oxide ionconductivity, the best properties have been observed in materialswhere T=Si, Ge andA corresponds to rare earth and alkaline earthcations. The initials works on oxy-apatite conductorswere focusedon RE10−x□x(Si6O24)O3−1.5x systems, these phases have beenprepared as polycrystalline powders (by standard solid state

1308 L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315

techniques [12] and sol–gel synthesis [13]) and as single crystalsby the floating zone method [14]. There have also been severalstudies with Ge [15], and the La10−x□x(Ge6O24)O3−1.5x series hasbeen studied by neutron and synchrotronX-ray powder diffractionand impedance spectroscopy [16]. Mixed Ge–Si oxy-apatites [17]and derivatives [18] are known. There are also other compoundswith the same framework, but with atomic substitutions at the Aand T sites [9,11], some including transition metals [19]. Forexample, the conductivity of Ln10−x□x(T6O24)O3−1.5x (T=Si andGe) has been enhanced by partial substitution of Si/Ge by Al[20,21]. A particularly interesting feature of the oxy-apatitematerials is the effect of cation vacancies on the conductivity. Sr-doped oxygen stoichiometric systems, La9.33−xSr3x/2□0.67−x/2(T6O24)O2 (T=Si and Ge) [22–25] have shown that as the Srcontent increased and hence the number of cation vacanciesdecreased, the conductivity decreased.

Some important experimental contributions to the knowledge ofthe relationship between structure and oxide conductivity in theapatite-type compounds are those works using neutron powderdiffraction [16,23,25–28]. Furthermore, a recent work has beenpublished where a correlation between the silicon environment,studied by 29Si NMR spectroscopy, and the oxide conductivityvalues has been found [22]. We have carried out a systematic workto study the presence of interstitial oxygen in highly crystallinesingle phases of oxygen-stoichiometric apatites includingLa9.33□0.67(Si6O24)O2, La9.5□0.5(Ge5.5Al0.5O24)O2 and La8Sr2(Si6O24)O2 [23,26]. These oxygen stoichiometric samples hadinterstitial oxygens only if they also contained cation vacancies. Forexample, La8Sr2(Si6O24)O2 which don't have vacancies in thelanthanum sublattice, don't have interstitial oxygens. We alsostudied the position and population of interstitial oxygen in oxygen-excess apatites with cation vacancies, f.i. La9.55□0.45(Si6O24)O2.32

and La9.60□0.40(Ge6O24)O2.40 [23]. The position of the interstitialoxide anions in all these cation-deficient samples is placed verynear to the location earlier predicted by atomistic simulations forLa9.33□0.67(Si6O24)O2 [29,30]. This position for the interstitialoxide anion is very close (∼ 1.0 Å) to the average position of oneoxygen atom of a silicate/germanate tetrahedron but the atomisticcalculations showed that the lattice could accommodate the in-terstitial oxygen through a local lattice relaxation. This local relax-ation is the displacement and tilting of the tetrahedral group towardsthe cation vacancy of the lanthanum sublattice [29,30].

The goal of the present work is to study the presence, locationand population of interstitial oxygens in oxygen-excess apatiteswithout cation vacancies. To do so, three samples, La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32 and La9Sr1(Si5.5Al0.5O24)O2.25 have been structurally characterized byjoint refinement of neutron and X-ray powder diffraction data.Oxide conductivities for these, and other related samples, are alsogiven and discussed.

2. Experimental

2.1. Synthesis

Six compounds, La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9Sr1(Si5.5Al0.5O24)O2.25, La9.67□0.33

(Si5.5Al0.5O24)O2.25, La8.5Sr1.5(Si5.5Al0.5O24)O2 and La9.5□0.5

(Si5.5Al0.5O24)O2 have been prepared by the ceramicmethod in Ptcrucibles using high purity oxides: La2O3 (Alfa, 99.999%), GeO2

(Aldrich, 99.998%), SiO2 (ABCR, quartz powder 99.31%), γ-Al2O3 (Alfa, 99.997%) and SrCO3 (Alfa 99.99%). Lanthanumoxide was precalcined at 1273 K for 2 h in order to achievedecarbonation.

The synthesis of La8.65Sr1.35(Si6O24)O2.32 and La8.65Sr1.35(Ge6O24)O2.32 was carried out as previously reported [23]. Forthe other samples the oxides were ground using an agate mortarfor 20 min, pelletized (200 MPa, ca. 20 mm diameter and ca.3 mm thickness), and heated at 1773 K for 24 h. The resultingpowders were ground for 40 min using a Fritsch ball mill(model Pulverisette 7, 45 cm3 agate vessel containing sevenagate balls each with a diameter of 15 mm) at 200 rpm withreverse rotation every 5 min. A second thermal treatment wascarried out at 1773 K for 48 h. Finally, the samples were re-ground, pelletized and heated at the same temperature for 36 h.

2.2. Powder diffraction

All samples were characterized by laboratory X-ray powderdiffraction (LXRPD) at room temperature to check for possibleimpurity phases. The powder patterns were collected on aSiemens D5000 automated diffractometer using graphite-mono-chromated CuKα1,2 radiation. To carry out Rietveld studies, thecompounds were scanned between 15 and 110° (2θ) in 0.03°steps, counting 18 s per step.

Room temperature neutron powder diffraction patterns (RT-NPD) were collected on HRPT diffractometer [31] [SINQneutron source at Paul Scherrer Institut, Villigen, Switzerland]for La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9Sr1(Si5.5Al0.5O24)O2.25 and La9.5□0.5(Si5.5Al0.5O24)O2 with thesamples loaded in a vanadium can. High temperature neutronpowder diffraction patterns (HT-NPD) data were also collectedfor La9Sr1(Si5.5Al0.5O24)O2.25 at 1073 K and for La9.5□0.5

(Si5.5Al0.5O24)O2 at 773 and 1173 K. The neutron wavelength,∼ 1.886 Å, was selected by the (511) reflection of the verticallyfocusing Ge monochromator. The overall measuring time was≈ 6 h per pattern to have good statistics over the 2θ angularrange of 5–165° [21−0.95 Å] with 0.05° step size.

High resolution synchrotron X-ray powder diffraction patternsat room temperature (RT-SXRPD) were collected on ID31 dif-fractometer [European Synchrotron Radiation Facility (ESRF),Grenoble, France] for La8.65Sr1.35(Si6O24)O2.32 and La8.65Sr1.35(Ge6O24)O2.32. The samples were loaded in a borosilicate glasscapillary (ϕ=1 mm) and rotated during data collection. A pene-trating wavelength, λ=0.40084(5) Å (30.93 keV), was selectedwith a double-crystal Si (111)monochromator and calibratedwithSi standard from NIST (a=5.431195 Å). The overall measuringtime was≈ 1 h to have very good statistics over the angular range2–30° (in 2θ) [11.5–0.77 Å]. The data from the multi-analyzer Si(111) stage were normalized and summed up to 0.003° step sizewith local software to produce the final raw data.

RT full structural characterization was carried out forLa8.65Sr1.35(Si6O24)O2.32 and La8.65Sr1.35(Ge6O24)O2.32 from ajoint Rietveld analysis [32] of NPD and SXRPD data. The RT

Table 1Pellets compaction percentages, bulk and total conductivities (S cm−1) at 773 Kand bulk and total activation energies (eV)

Composition %C σT σbulk Ea(T) Ea(bulk)

La8.65Sr1.35(Si6O24)O2.32 76 1.2(1)10−4

– 1.05(2) – –

La8.65Sr1.35(Ge6O24)O2.32 74 1.8(1)10−4

– 1.05(2) – –

La9Sr1(Si5.5Al0.5O24)O2.25 65 4.2(3)10−5

9.5(3)10−5

0.70(1) 0.81(2)

La9.67□0.33(Si5.5Al0.5O24)O2.25 79 3.4(1)10−4

3.3(6)10−3

0.66(1) 0.82(4)

La8.5Sr1.5(Si5.5Al0.5O24)O2 76 8.2(2)10−6

4.1(4)10−5

0.72(1) 0.88(2)

La9.50□0.5(Si5.5Al0.5O24)O2 73 1.7(3)10−4

1.7(2)10−3

0.64(1) 0.82(1)

1309L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315

structures for La9.5□0.5(Si5.5Al0.5O24)O2 and La9Sr1(Si5.5Al0.5O24)O2.25 have been obtained from joint Rietveldanalysis [32] of NPD and LXRPD data. HT structures have beenderived only from NPD data. The GSAS suite of programs [33]was used for all calculations.

2.3. Impedance studies

Green pellets (∼ 10mm diameter and∼ 1mm thickness) wereobtained by pressing the fine powder at 400MPa for 2 min. Thesepellets were sintered in order to have dense materials with highmechanical strength: La8.65Sr1.35(Si6O24)O2.32 was heated at1673 K for 6 h; La8.65Sr1.35(Ge6O24)O2.32 at 1573 K for 6 h andLa9Sr1(Si5.5Al0.5O24)O2.25, La9.67□0.33(Si5.5Al0.5O24)O2.25,La8.5Sr1.5(Si5.5Al0.5O24)O2 and La9.5□0.5(Si5.5Al0.5O24)O2 at1823 K for 6 h. No weight losses were detected in these sinteringsteps. Electrodes were made by coating opposite pellet faces with

Fig. 1. Complex impedance plane plot for La9Sr1(Si5.5Al0.5O24)O2.25 at 673 K(873 K in the inset) as square points. The full line is the data fit using theequivalent circuit described in the text. Selected frequency and capacitancepoints are highlighted.

METALOR® 6082 platinum paste and gradually heating to1073 K at a rate of 10 K·min−1 in air to decompose the paste andharden the Pt residue. Successive treatments were made toachieve an electrical resistance of both pellets faces lower than1Ω. The impedance data were collected using a Hewlett-Packard4284A impedance analyzer over the frequency range 20 Hz to1 MHz from 473 to 1273 K. Measurement processes wereelectronically controlled by the winDETA package of programs[34]. The pellets were mounted in a home made alumina con-ductivity jig, with four Pt wires shielded in two alumina tubes,which were placed in a tubular furnace. Electrical data were takenevery 50 K. A delay time of 60 min at each temperature wasselected to ensure thermal equilibrium. Temperatures were repro-ducible to ±1 K.

3. Results and discussion

3.1. Synthesis and single phase existence

Six materials have been prepared as highly crystalline phases.Four silicates were single phases. La8.65Sr1.35(Ge6O24)O2.32 con-tained 1.2(1) wt.% of La2GeO5 and La9.5□0.5(Si5.5Al0.5O24)O2

contained 1.2(1)% of LaAlO3 (derived from the Rietveld analysesof the powder patterns). The unit cell volumes from LXRPD datafor La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9Sr1(Si5.5Al0.5O24)O2.25, La9.67□0.33(Si5.5Al0.5O24)O2.25, La8.5Sr1.5(Si5.5Al0.5O24)O2, and La9.5□0.5(Si5.5Al0.5O24)O2 were 589.24(2), 622.00(3), 590.48(2), 590.71(1), 591.68(2) and 590.16(2)Å3,respectively. As expected, germanates have higher volumes thansilicates. The unit cell volume variation is complex due to theinterplay of three parameters, i) the amount of cation vacancies, ii)

Fig. 2. Arrhenius plots of logσT for La8.65Sr1.35(Si6O24)O2.32 (+), La8.65Sr1.35(Ge6O24)O2.32 (⁎), La9Sr1(Si5.5Al0.5O24)O2.25 (Δ), La9.67□0.33(Si5.5Al0.5O24)O2.25 (○), La8.5Sr1.5(Si5.5Al0.5O24)O2 (∇) and La9.5□0.5(Si5.5Al0.5O24)O2 (□),and of logσbulk for La9Sr1(Si5.5Al0.5O24)O2.25 (▴), La9.67□0.33(Si5.5Al0.5O24)O2.25 (●), La8.5Sr1.5(Si5.5Al0.5O24)O2 (▾) and La9.5□0.5(Si5.5Al0.5O24)O2 (n).

Table 2Refined unit cell values, Rietveld disagreement factors, atomic positional parameters and occupation factors, at different temperatures for La8.65Sr1.35(Si6O24)O2.32,La8.65Sr1.35(Ge6O24)O2.32, La9Sr1(Si5.5Al0.5O24)O2.25 and La9.5□0.5(Si5.5Al0.5O24)O2 in P63/m space group

La8.65Sr1.35(Si6O24)O2.32 La8.65Sr1.35(Ge6O24)O2.32 La9Sr1(Si5.5Al0.5O24)O2.25 La9.5□0.5(Si5.5Al0.5O24)O2

RTa RTa RTb 1073 Kc RTb 773 Kc 1173 Kc

a (Å) 9.7100(1) 9.9120(1) 9.7111(2) 9.7870(3) 9.7260(1) 9.7665(3) 9.8100(4)c (Å) 7.2254(1) 7.3236(1) 7.2290(1) 7.2715(2) 7.2002(1) 7.2227(3) 7.2482(3)V(Å3) 589.973(2) 623.130(6) 590.40(2) 603.19(5) 589.85(2) 596.63(5) 604.08(6)RWPN /RWP

X (%) 3.57/8.17 2.86/7.98 3.57/9.19 3.22/– 3.59/11.63 4.08/– 3.66/–RPN/RP

X (%) 2.78/5.56 2.23/5.74 2.77/7.07 2.52/– 2.77/8.36 2.97/– 2.63/–RFN/RF

X (%) 0.83/1.50 0.97/1.95 0.86/2.17 1.47/– 1.30/2.55 1.52/– 1.67/–La(1), 6h, (x y 1/4)d

x 0.2289(1) 0.2305(1) 0.2299(1) 0.2285(3) 0.2293(1) 0.2274(3) 0.2275(3)y −0.0125(1) −0.0112(1) −0.0120(1) −0.0125(3) −0.0105(2) −0.0113(4) −0.0109(4)Uiso×100 (Å2) 0.53 0.85 0.39 1.59 0.58 1.27 1.98

La/Sr(2), 4f, (1/3 2/3 z)d

z −0.0007(1) 0.0008(1) −0.0009(2) 0.0001(4) −0.0014(2) 0.0002(5) 0.0008(6)Uiso×100 (Å2) 0.74 1.41 0.54 1.95 0.69 1.04 1.92

Ge/Si/Al, 6h, (x y 1/4)e

x 0.4020(1) 0.4008(1) 0.4020(2) 0.4015(5) 0.4016(2) 0.4019(5) 0.4014(6)y 0.3718(1) 0.3723(1) 0.3720(2) 0.3727(5) 0.3723(2) 0.3725(5) 0.3725(5)Uiso×100 (Å2) 0.24 0.54 0.05 1.34 0.45 0.77 1.36

O(1), 6h, (x y 1/4)x 0.3224(1) 0.3117(1) 0.3238(2) 0.3240(4) 0.3253(2) 0.3260(5) 0.3273(5)y 0.4847(1) 0.4866(1) 0.4863(2) 0.4867(4) 0.4866(2) 0.4878(5) 0.4885(5)Uiso×100 (Å2) 1.20 1.90 0.90 3.07 2.22 3.04 4.50

O(2), 6h, (x y 1/4)x 0.5956(1) 0.6029(1) 0.5972(2) 0.5946(4) 0.5965(2) 0.5946(4) 0.5937(5)y 0.4731(1) 0.4755(1) 0.4730(2) 0.4712(4) 0.4722(2) 0.4717(5) 0.4717(5)Uiso×100 (Å2) 0.64 1.46 0.53 2.55 0.90 1.91 3.13

O(3), 12i, (x y z)x 0.3443(1) 0.3408(1) 0.3446(1) 0.3430(3) 0.3469(1) 0.3460(4) 0.3450(4)y 0.2549(1) 0.2478(1) 0.2550(1) 0.2565(3) 0.2564(1) 0.2577(3) 0.2589(3)z 0.0701(1) 0.0615(1) 0.0691(1) 0.0703(3) 0.0684(1) 0.0690(3) 0.0701(3)Uiso×100 (Å2) 1.52 3.24 1.32 3.84 2.30 3.24 4.73

O(4), 2a, (0 0 1/4)Uiso×100 (Å2) 5.34 2.83 3.95 6.33 5.76 8.28 10.26Occ. factor. 1.00(– –) 0.929(4) 1.00(– –) 1.00(– –) 0.927(8) 0.94(2) 0.96(2)

O(5), 12i, (x y z)f

x 0.014(5) 0.009(2) 0.015(18) 0.051(12) −0.009(18) 0.01(5) 0.01(7)y 0.071(3) 0.213(2) 0.064(8) 0.083(10) 0.221(17) 0.23(4) 0.23(6)z 0.555(4) 0.598(1) 0.537(9) 0.556(12) 0.570(13) 0.57(3) 0.58(5)Occ. factor 0.023(1) 0.045(1) 0.020(2) 0.023(4) 0.012(1) 0.010(3) 0.006(3)

aJoint RT-SXRPD and RT-NPD refinements. bLXRPD and RT-NPD refinements. cHT-NPD refinements. dAll the Sr was located in the position of La(2), and theoccupation factors of La and Sr were fixed to the nominal values. eThe occupation factors for Ge and Al were fixed to the nominal ones. fTheUiso for O(5) was fixed to0.03 Â2 at all the temperatures.

1310 L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315

the amount of oxygen excess, and iii) the size of the element at thecenter of the tetrahedral group.

3.2. Impedance study

The sintering conditions for the pellets led to specimens notfully dense with compactions ranging between 70 and 80% of thetheoretical value (see Table 1). No weight losses were detectedduring the sintering process. Representative impedance data forLa9Sr1(Si5.5Al0.5O24)O2.25 at two temperatures are shown asimpedance complex plane plots in Fig. 1. Similar plots wereobtained for the remaining compositions. At low temperatures,i.e. 673 K, a set of overlapping semicircles can be observed. Thisset of semicircles indicates that several effects are involved in theelectrical response of the pellets, including bulk and grain bound-ary contributions. At intermediate temperatures, 873 K, a well-

developed spike can be observed with an associated capacitanceof 0.1 μF cm−1 (20Hz). Since it is inclined to the Z′ axis atroughly 45°, it indicates a partial-blocking electrode response thatallows limited diffusion. At higher temperatures the spikecollapses to a semicircular arc (not shown), indicating that oxy-gen molecules are able to diffuse through the entire thickness ofthe electrode. In summary, the conducting species appear to beoxide ions as for many other oxy-apatites. Impedance data forLa8.65Sr1.35(Si6O24)O2.32 and La8.65Sr1.35(Ge6O24)O2.32 werepreviously reported [23]. Bulk conductivities for these materialsmust be close to the total values given in Table 1, as there is analmost non-deformed semicircle in the impedance complex planeplots. Bulk results obtained by least squares equivalent circuitfittings are not given because the associated errors were high.

Total pellet conductivities (σT) are obtained from the inter-cept of the spikes and/or the arcs (low frequency end) on the Z′

Table 3Refined anisotropic thermal parameters at room temperature for selected samples

Atoms U11×100 U22×100 U33×100 U12×100 U13×100 U23×100

La/Sr(1), 6h, (x y 1/4)La8.65Sr1.35(Si6O24)O2.32 0.7(1) 0.4(1) 0.5(1) 0.2(1) 0 0La8.65Sr1.35(Ge6O24)O2.32 1.2(1) 0.4(1) 0.9(1) 0.4(1) 0 0La9Sr1(Si5.5Al0.5O24)O2.25 0.4(1) 0.4(1) 0.3(1) 0.3(1) 0 0La9.5□0.5(Si5.5Al0.5O24)O2 0.8(1) 0.6(1) 0.3(1) 0.3(1) 0 0La/Sr(2), 4f, (1/3 2/3 z)La8.65Sr1.35(Si6O24)O2.32 0.7(1) 0.7(1) 0.7(1) 0.4(1) 0 0La8.65Sr1.35(Ge6O24)O2.32 1.9(1) 1.9(1) 0.5(1) 0.9(1) 0 0La9Sr1(Si5.5Al0.5O24)O2.25 0.5(1) 0.5(1) 0.6(1) 0.2(1) 0 0La9.5□0.5(Si5.5Al0.5O24)O2 0.3(1) 0.3(1) 1.4(1) 0.2(1) 0 0Ge/Si/Al, 6h, (x y 1/4)La8.65Sr1.35(Si6O24)O2.32 0.1(1) 0.3(1) 0.3(1) 0.2(1) 0 0La8.65Sr1.35(Ge6O24)O2.32 0.6(1) 0.4(1) 0.6(1) 0.3(1) 0 0La9Sr1(Si5.5Al0.5O24)O2.25 −0.3(1) 0.2(1) 0.2(1) 0.1(1) 0 0La9.5□0.5(Si5.5Al0.5O24)O2 0.9(1) 0.0(1) 0.5(1) 0.9(1) 0 0O(1), 6h, (x y 1/4)La8.65Sr1.35(Si6O24)O2.32 1.3(1) 1.4(1) 0.9(1) 1.2(1) 0 0La8.65Sr1.35(Ge6O24)O2.32 2.4(1) 1.8(1) 1.5(1) 2.0(1) 0 0La9Sr1(Si5.5Al0.5O24)O2.25 0.9(1) 1.3(1) 0.5(1) 1.0(1) 0 0La9.5□0.5(Si5.5Al0.5O24)O2 2.5(1) 2.8(1) 1.3(1) 2.6(1) 0 0O(2), 6h, (x y 1/4)La8.65Sr1.35(Si6O24)O2.32 0.6(1) 0.4(1) 0.9(1) 0.2(1) 0 0La8.65Sr1.35(Ge6O24)O2.32 0.6(1) 0.5(1) 3.3(1) 0.4(1) 0 0La9Sr1(Si5.5Al0.5O24)O2.25 0.4(1) 0.4(1) 0.8(1) 0.1(1) 0 0La9.5□0.5(Si5.5Al0.5O24)O2 0.9(1) −0.1(1) 1.9(1) 0.2(1) 0 0O(3), 12i, (x y z)La8.65Sr1.35(Si6O24)O2.32 2.7(1) 1.2(1) 0.7(1) 1.3(1) −0.9(1) −0.5(1)La8.65Sr1.35(Ge6O24)O2.32 5.2(1) 2.5(1) 2.0(1) 2.7(1) −2.2(1) −1.1(1)La9Sr1(Si5.5Al0.5O24)O2.25 2.4(1) 1.1(1) 0.4(1) 1.2(1) −0.9(1) −0.4(1)La9.5□0.5(Si5.5Al0.5O24)O2 4.3(1) 1.6(1) 1.0(1) 1.5(1) −1.4(1) −0.7(1)O(4), 2a, (0 0 1/4)La8.65Sr1.35(Si6O24)O2.32 0.7(1) 0.7(1) 14.6(2) 0.3(1) 0 0La8.65Sr1.35(Ge6O24)O2.32 −0.6(1) −0.6(1) 9.7(1) −0.3(1) 0 0La9Sr1(Si5.5Al0.5O24)O2.25 0.4(1) 0.4(1) 11.0(3) 0.2(1) 0 0La9.5□0.5(Si5.5Al0.5O24)O2 0.1(1) 0.1(1) 17.1(5) 0.1(1) 0 0

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axis, and are given in Fig. 2, in the traditional Arrhenius format.In order to quantitatively estimate the bulk and grain boundaryconductivities of these samples, complex impedance spectrawere analyzed by nonlinear least squares fittings of equivalentcircuits using the program Zview [35]. The pellets behavior hasbeen modeled using an equivalent circuit formed with the seriesassociation of two parallel circuits: one corresponding tothe bulk response (a resistor and an ideal capacitor), and theother to the grain boundary region (a resistor and a constantphase element). These parameters have been determined in the473–673 K temperature range for all compositions. The cal-culated spectra (solid line) for La9Sr1(Si5.5Al0.5O24)O2.25 at673 K is shown in Fig. 1 as an example of this type of fits. Thesecalculations gave the bulk conductivities, which are alsoplotted against the inverse of temperature in Fig. 2. This figurealso includes the total conductivities for La8.65Sr1.35(Si6O24)O2.32 and La8.65Sr1.35(Ge6O24)O2.32. Arrhenius plots for thetotal and bulk conductivities fall on a set of approximatelyparallel lines, showing a maximum in conductivity forLa9.67□0.33(Si5.5Al0.5O24)O2.25. From these plots, the totaland bulk conductivities at 773 K and intrinsic activationenergies have been determined, Table 1.

As it can be seen in Fig. 2 and Table 1, La8.5Sr1.5(Si5.5Al0.5O24)O2 has the lowest conductivity of reported compounds.La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9.67□0.33

(Si5.5Al0.5O24)O2.25 and La9Sr1(Si5.5Al0.5O24)O2.25 have higherconductivities because of their excess-oxygen content. HoweverLa9.5□0.5(Si5.5Al0.5O24)O2 has the same overall oxygen contentthan La8.5Sr1.5(Si5.5Al0.5O24)O2 but much higher conductivitywhich must be related to the presence of vacancies in the lan-thanum sublattice. This behavior was already reported for othersoxy-apatite compounds [22,23,25]. The cation vacancies areimportant as they allow the local lattice relaxation around themigrating interstitial oxide. However La8.65Sr1.35(Ge6O24)O2.32,La8.65Sr1.35(Si6O24)O2.32 and La9Sr1(Si5.5Al0.5O24)O2.25 do nothave vacancies in the La sublattice, so aNPD study is important tounderstand the conductivity behavior of these stoichiometries andfor the comparison of their crystal structures with those of otheroxy-apatites (with cation vacancies).

3.3. Crystal structures

The main objective of this work is to study the presence,location and population of interstitial oxygens in oxygen-excess

Table 4Selected bond distances (Å) for La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9Sr1(Si5.5Al0.5O24)O2.25 and La9.5□sub 0.5(Si5.5Al0.5O24)O2 at differenttemperatures

La8.65Sr1.35(Si6O24)O2.32 La8.65Sr1.35(Ge6O24)O2.32 La9Sr1(Si5.5Al0.5O24)O2.25 La9.5□0.5 (Si5.5Al0.5O24)O2

RTa RTa RTb 1073 Kc RTb 773 Kc 1173 Kc

A(1)–O(1) 2.744(1) 2.745(1) 2.759(2) 2.783(5) 2.790(3) 2.809(6) 2.835(7)A(1)–O(2) 2.510(1) 2.525(1) 2.502(2) 2.549(4) 2.518(2) 2.550(5) 2.561(5)A(1)–O(3)×2 2.482(1) 2.455(1) 2.476(2) 2.495(2) 2.477(1) 2.487(3) 2.643(4)A(1)–O(3)×2 2.603(1) 2.624(1) 2.605(2) 2.635(3) 2.606(2) 2.629(4) 2.450(3)A(1)–O(4) 2.285(1) 2.342(1) 2.293(1) 2.300(2) 2.283(1) 2.278(2) 2.287(2)<A(1)–O> 2.53 2.54 2.53 2.55 2.53 2.55 2.55A(2)–O(1)×3 2.495(1) 2.486(1) 2.491(1) 2.500(3) 2.492(2) 2.488(4) 2.493(4)A(2)–O(2)×3 2.550(1) 2.578(1) 2.540(1) 2.565(3) 2.533(2) 2.557(4) 2.575(5)A(2)–O(3)×3 2.871(1) 2.937(1) 2.868(1) 2.912(2) 2.853(2) 2.879(3) 2.907(4)<A(2)–O> 2.64 2.67 2.63 2.66 2.63 2.64 2.66T–O(1) 1.626(1) 1.751(1) 1.628(3) 1.633(5) 1.617(2) 1.628(5) 1.628(6)T–O(2) 1.629(1) 1.735(1) 1.642(2) 1.637(5) 1.642(2) 1.630(6) 1.634(6)T–O(3)×2 1.630(1) 1.746(1) 1.637(2) 1.636(3) 1.631(2) 1.629(4) 1.622(4)<T–O> 1.63 1.74 1.63 1.63 1.63 1.63 1.63O(5)···O(3) 2.37(2) 1.15(2) 2.46(5) 2.39(9) 1.13(15) 1.0(4) 1.0(6)O(5)···O(4) 1.54(3) 2.33(1) 1.64(7) 1.58(9) 2.54(12) 2.6(3) 2.5(4)aJoint RT-SXRPD and RT-NPD refinement. bJoint LXRPD and NPD refinement. cHT-NPD refinement.A = La, Sr and T = Si, Ge, Al.

1312 L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315

apatites without cation vacancies. The interstitial oxygen hasalready been experimentally found in oxygen-excess and oxygen-stoichiometric samples with vacancies at the La site [23,26].Recently Tolchard et al. [28] have also reported the presence ofinterstitial oxygen in an oxygen-excess sample, La10(Si5CoO24)O2.5, but the results were not conclusive due to the high disorderimposed by the presence of the cobalt cations. Here, structuraldescriptions are only reported for the materials where NPD datahave been collected: La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9Sr1(Si5.5Al0.5O24)O2.25 and La9.5□0.5

(Si5.5Al0.5O24)O2.The startingmodel for the structure refinements was that earlier

reported [26] for La9.55□0.45(Si6O24)O2.32 in P63/m space group.The occupation factors for lanthanum and silicon sites were con-venientlymodified to describe the stoichiometries. For La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32 and La9Sr1(Si5.5Al0.5O24)O2.25, the strontium cations have been found to belocated only at the La(2) site, in full agreement with a very recentreport for La8Sr2(Si6O24)O2 [36]. For La9.5□0.5(Si5.5Al0.5O24)O2,the La vacancies are completely located at La(2) site, in fullagreement with many previous structural studies [9]. The nominalaluminum content was placed at the tetrahedral site. The usualparameters: histogram scale factors, background coefficients, unitcell parameters, zero error and peak shape coefficientswere varied.

The positional atomic parameters for the pairs Sr/La(2) andSi/Al were constrained to be the same. Atomic displacementparameters (ADP's) for all atoms except interstitial oxygenshave been refined anisotropically. Final unit cell parameters,Rietveld disagreement factors, atomic positional parameters andoccupation factors are given in Table 2. The anisotropic roomtemperature displacement factors are given in Table 3. Selectedbond distances are given in Table 4. As an example of theexcellent fits, the Rietveld plots of RT-SXRPD and RT-NPD forLa8.65Sr1.35(Si6O24)O2.32 are given in Fig. 3.

The refinement of the loosely bonded atoms (oxide anions)in these materials is vital to understand the oxide conductionproperties and it has been studied very carefully. The jointrefinements for La8.65Sr1.35(Si6O24)O2.32 indicated that does notpresent oxide vacancies at the center of the channels and the freerefinement of the occupation factor for the interstitial oxygensconverged to 0.023(1), see Table 2. This value yields thefollowing refined stoichiometry, La8.65Sr1.35(Si6O24)O2.28(1),which is very close to the expected one. Analogous refinementsfor La9Sr1(Si5.5Al0.5O24)O2.25 gave no oxide vacancies and anoccupation factor for O(5) of 0.020(1), which results in a refinedstructural stoichiometry of La9Sr1(Si5.5Al0.5O24)O2.24(2). How-ever, similar joint refinements for La8.65Sr1.35(Ge6O24)O2.32

showed the presence of oxide vacancies at the center of thechannels and the free refinement of the occupation factors for O(4) and O(5) converged to 0.928(4) and 0.045(1), see Table 2.These values yield La8.65Sr1.35(Ge6O24)O2.40(2) as refinedstructural stoichiometry which is not far from the expected one.

The joint refinements converged to a structural descriptionwith interstitial oxygens at (0.009(2) 0.213(2) 0.598(1)) and(−0.009(18) 0.221(17) 0.570(13)) for La8.65Sr1.35(Ge6O24)O2.32 and La9.5□0.5(Si5.5Al0.5O24)O2, see Table 2. This positionis the same than that predicted theoretically [29,30] andexperimentally found from NPD [23,26]. This position is veryclose to the average position of O(3) (∼ 1.1 Å) but the atomisticcalculations showed that the lattice can accommodate theinterstitial oxygen through a local relaxation. The interstitialoxygens converged to (0.014(5) 0.071(3) 0.555(4)) and (0.015(18) 0.064(8) 0.537(9)) for La8.65Sr1.35(Si6O24)O2.32 and La9Sr1(Si5.5Al0.5O24)O2.25, respectively, see Table 2.

It must be underlined that the positions of the interstitialoxide anions in La8.65Sr1.35(Si6O24)O2.32 and La8.65Sr1.35(Ge6O24)O2.32 are different and, to the best of our knowledge,it has not been reported so far. For the silicate apatites, without

Fig. 3. Observed (crosses), calculated (full line) and difference curve (bottom) of a) RT-SXRPD (λ=0.40 Å) and b) RT-NPD patterns for La8.65Sr1.35(Si6O24)O2.32

(λ=1.89 Å). The inset shows a selected RT-SXRPD region (20–30°/2θ) for the same sample.

1313L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315

cation vacancies, the interstitial site is much closer to the centerof the oxide channels and far away from O(3). The average (notreal) O(5)···O(4) and O(5)···O(3) distances for La8.65Sr1.35(Si6O24)O2.32 are 1.54(3) and 2.37(2) Å, respectively. Theaverage O(5)···O(4) and O(5)···O(3) distances for La8.65Sr1.35(Ge6O24)O2.32 are 2.33(1) and 1.15(2) Å, respectively. Twoprojections of the crystal structures of La8.65Sr1.35(Si6O24)O2.32

and La8.65Sr1.35(Ge6O24)O2.32 with the interstitial oxygen atomshighlighted are shown in Figs. 4 and 5. As it can be seen in Fig. 4,the interstitial oxides are closer to the center of the channel forLa8.65Sr1.35(Si6O24)O2.32 and far away for La8.65Sr1.35(Ge6O24)O2.32. Fig. 5 highlights the existing difference at the interstitialoxide positions for the silicate and germanate apatites. Further-more, the c-axis path for the O(4) and O(5) migration is alsovisible in Fig. 5.

Formal inclusion of the interstitial oxygens at the channelperiphery in diffraction data analysis is difficult because thestable interstitial site lies too close to other atoms (either anoxygen of the silicate substructure in La8.65Sr1.35(Ge6O24)O2.32

or the loosely bounded oxide of the center of the channels inLa8.65Sr1.35(Si6O24)O2.32). This closeness (1.1–1.5 Å) requireslocal relaxation of the structure around the site, which is notpossible to model accurately with diffraction methods. How-ever, some clues about the local relaxation can be indirectlyextracted from the anisotropic ADP's values when studying aseries in a consistent way as previously underlined [23,26].

The main anisotropic ADP to detect the local relaxation of thesilicate groups is U11 of O(3) which is very large (close to0.05 Å2) for those compositions with the interstitial oxygenposition located very close to the average position of the tetra-hedral groups. As it can be seen in Table 3,U11 values of O(3) aresignificantly larger for La8.65Sr1.35(Ge6O24)O2.32 (0.052(1) Å2)andLa9.5□0.5(Si5.5Al0.5O24)O2 (0.043(1) Å

2) than for La8.65Sr1.35(Si6O24)O2.32 (0.027(1) Å2) and La9Sr1(Si5.5Al0.5O24)O2.25

(0.024(1) Å2). This finding indirectly supports the structuralresult where the oxygen interstitials are closer to the silicategroups in the two former apatites, and hence, they must be moredisplaced (relaxed), i.e. higher ADP values. These results are in

Fig. 5. a-axis view of the crystal structure of a) La8.65Sr1.35(Ge6O24)O2.32 and b)La8.65Sr1.35(Si6O24)O2.32, with the atoms displayed as in Fig. 4.

Fig. 4. c-axis view of the crystal structure of a) La8.65Sr1.35(Ge6O24)O2.32 and b)La8.65Sr1.35(Si6O24)O2.32, showing the TO4 groups as tetrahedra and La(1), La/Sr(2) and O(4) atom as balls. The cation vacancies are not shown. Interstitial oxygen,O(5), is highlighted (white-grated balls) and shown at 16%of occupancy for clarity.

1314 L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315

agreement with many previous reports. For instance, U11 valuesof O(3) for La9.33□0.67(Si6O24)O2 and Nd9.33□0.67(Si6O24)O2,were 0.048(2) and 0.063(2) Å2, respectively and for La8Sr2(Si6O24)O2 and Nd8Sr2(Si6O24)O2, were 0.023(1) and 0.021(1) Å2, respectively [36]. These oxy-apatites with cationvacancies have high oxide conductivities due to oxygen inter-stitials close to the silicate tetrahedral that must be locally relaxedleading to very high U11 values for O(3).

Finally, we must underline the different behavior betweenthe oxy-germanates and oxy-silicates. La8.65Sr1.35(Ge6O24)O2.32 has oxide vacancies at the center of the oxide channelsand the interstitial oxygen position is far away from the center ofthe channels close to the germanate sublattice. Conversely,La8.65Sr1.35(Si6O24)O2.32 do not have oxide vacancies at thecenter of the oxide channels and the interstitial oxygen positionis very close to these oxide anions. This is an important differ-ence that it justifies their different crystalchemistry properties.

Hence, La10−(1−x)2/3□(1−x)2/3(Ge6O24)O2+x series is single phasein the compositional range (0.18≤×≤0.62) being hexagonal(space group s.g. P63/m) for 0.18≤×≤0.33 and triclinic (s.g. P-1)for 0.33≤x≤0.62 [16]. On the other hand, La10−(1−x)2/3□ (1−x)2/3(Si6O24)O2+x is single phase in the compositional range(0≤x≤0.40) and the symmetry is hexagonal for all compositions[26]. Oxy-germanates may incorporate higher amount of oxygensin the structure because germanate tetrahedra are more flexibleand they undergoes the local relaxation easier, see above. Whenthe amount of oxygen interstitial reaches a limit, 0.35 mol ofoxide per chemical formula in the former series, the symmetrychanges to triclinic at room temperature. Many other oxy-apatitesseries have been prepared with different aliovalent substitution[9] but the maximum oxide content for germanates is alwayslarger than for silicates.

HT-NPD data showed that there is no phase transition onheating. The Rietveld refinements were carried out as pre-viously described using only neutron data. The final results aregiven in Tables 2 and 4. The bond distances slightly increase onheating due to the thermal expansion and the thermal vibrationparameters increase, as expected.

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4. Conclusions

The reported NPD study have shown an important structuraldifference for La8.65Sr1.35(T6O24)O2.32 (T=Si, Ge) materials.The framework of these compounds is the same but the inter-stitial oxygen positions are different. La8.65Sr1.35(Ge6O24)O2.32

has vacancies at the center of the oxide channels and a higheramount of interstitial oxygens which are close to the locallyrelaxed germanate sublattice. La8.65Sr1.35(Si6O24)O2.32 do nothave such oxide vacancies and the interstitial oxygen position isvery close to oxide channels which prevents the stabilization ofmaterials with high oxygen excess.

Acknowledgments

Financial support from the MAT2003-7483-C2-1 researchgrants is acknowledged. LLR thanks the Junta de Andalucía fora studentship. This work was partially performed at the spal-lation neutron source SINQ, Paul Scherrer Institut, Villigen,Switzerland. ESRF is thanked for the provision of X-ray syn-chrotron facilities at ID31. We thank Dr. Sheptyakov for hisassistance with the NPD data collection.

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