High Oxide Ion Conductivity in Al-Doped Germanium Oxyapatite

596

Chem. Mater. 2005, 17, 596-600

High Oxide Ion Conductivity in Al-Doped Germanium Oxyapatite Laura Leo´n-Reina,† Enrique R. Losilla,† Marı´a Martı´nez-Lara,† M. Carmen Martı´n-Seden˜o,† Sebastia´n Bruque,† Pedro Nu´n˜ez,‡ Denis V. Sheptyakov,§ and Miguel A. G. Aranda*,† Departamento Quı´mica Inorga´ nica, UniVersidad de Ma´ laga, 29071-Ma´ laga, Spain, Departamento Quı´mica Inorga´ nica, UniVersidad de la Laguna, 38200 La Laguna, Tenerife Spain, and Laboratory for Neutron Scattering, ETHZ & PSICH-5232 Villigen Paul Scherrer Institut, Switzerland ReceiVed September 20, 2004

The apatite La10-x0x(Ge5.5Al0.5O24)O2.75-1.5x (10 - x ) 9.80, 9.75, 9.67, 9.60, 9.50, and 9.40) series has been prepared and the single phase existence range has been established, 9.75 g 10 - x g 9.45. The hexagonal crystal structures of La9.500.5(Ge5.5Al0.5O24)O2 have been determined at room temperature, 500 °C, and 900 °C from neutron powder diffraction data using the Rietveld method. The room-temperature unit cell parameters were a ) 9.9206(4) Å, c ) 7.2893(3) Å, V ) 621.29(6) Å3, and Z ) 1, and this refinement converged to RWP ) 3.03 and RF ) 1.30%. The most important structural result is the presence of interstitial oxygen ion associated with vacancies at the apatite oxide anions channels. Oxide ion conductivities have been measured by impedance spectroscopy. La9.500.5(Ge5.5Al0.5O24)O2 shows very high oxide conductivity, 0.16(1) S‚cm-1 at 800 °C, with negligible electronic contribution. The ionic transport number, obtained by combination of impedance and ion-blocking data, is higher than 0.99 in the studied oxygen partial pressure range, 0.21 to 10-20 atm.

Introduction Solid oxide fuel cells, SOFCs, are all-solid-state electrochemical devices that convert the chemical energy of fuels directly to electricity with high efficiency, low emission of pollutants, and low noise. The three basic components in every SOFC unit are a porous cathode, a porous anode, and a dense electrolyte. The electrodes are separated by the electrolyte which must have high oxide conductivity, negligible electronic conductivity, good chemical compatibility with the cathode/anode materials, good mechanical stability, and low/compatible thermal expansion.1,2 The general electrolyte in commercial SOFCs is yttria-stabilized zirconia (YSZ) which exhibits high oxide ion conductivity but only at elevated temperatures (>900 °C). This high operating temperature may cause problems such as difficulties in cell sealing, which enforces the use of expensive materials, and low lifetime of the components. The operating temperature may be decreased by either reducing the thickness of the electrolyte or using alternative materials with higher conductivities. Therefore, other electrolytes with higher oxide conductivities are being intensively studied, for example, Ce0.8Gd0.2O1.9 (GDC);3 (Bi2O3)0.75-(RE2O3)0.25;4 perovskitetype oxides such as La0.9Sr0.1Ga0.8Mg0.2O35 and BIMEVOX (Bi2V0.86Ni0.14O5.29).6 Rare earth oxyapatites are also attracting * To whom correspondence should be addressed. E-mail: [email protected]. † Universidad de Ma ´ laga. ‡ Universidad de la Laguna. § Paul Scherrer Institut.

(1) Ormerod, R. M. Chem. Soc. ReV. 2003, 32, 17. (2) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. (3) Torrens, R. S.; Sammes, N. M.; Tompsett, G. A. Solid State Ionics 1998, 111, 9. (4) Sammes, N. M.; Tompsett, G. A.; Nafe, H.; Aldinger, F. J. Eur. Ceram. Soc. 1999, 29, 1801. (5) Ishihara, T.; Matsuda, H.; Takita, Y. J. Am. Chem. Soc. 1994, 116, 3801.

considerable interest due to their high oxide ion conductivities and low activation energies.7 The initial works on oxyapatite conductors were focused on the RE10(SiO4)6O3 systems.7-11 Other stoichiometries including cation defect, anion-excess, and germanate materials have been reported. Moreover, several atomic substitutions in oxyapatites have also been described at the rare earth and tetrahedral sites. For example, the conductivity of La9.3300.67(SiO4)6O2 was enhanced by partial substitution of Si by Al.8 Neutron powder diffraction (NPD) was used to gain better insight into the crystal structures of La9.3300.67(SiO4)6O2,12 La9.3300.67(GeO4)6O2,13 La10-x0x(GeO4)6O3-1.5x,14 and La10-x0x(SiO4)6O3-1.5x.15 The precise mechanistic fea(6) Krok, F.; Abrahama, I.; Bango, D.; Bogusz, W.; Nelstrop, J. A. G. Solid State Ionics 1998, 111, 37. (7) Nakayama, S.; Kageyama, T.; Aono, H.; Sadaoka, Y. J. Mater. Chem. 1995, 5, 1801. Arikawa, H.; Nishiguchi, H.; Ishihara, T.; Takita, Y. Solid State Ionics 2000, 136, 31. (8) Abram, E. J.; Sinclair, D. C.; West, A. R. J. Mater. Chem. 2001, 11, 1978. (9) Nakayama, S.; Sakamoto, M. J. Eur. Ceram. Soc. 1998, 18, 1413. (10) Tao, S.; Irvine, J. T. S. Mater. Res. Bull. 2001, 36, 1245. Higuchi, M.; Katase, H.; Kodaira, K.; Nakayama, S. J. Cryst. Growth 2000, 218, 282. Nakayama, S.; Sakamoto, M.; Higuchi, M.; Kodaira, K.; Sato, M.; Kakita, S.; Suzuki, T.; Itoh, K. J. Eur. Ceram. Soc. 1999, 19, 507. Higuchi, M.; Kodaira, K.; Nakayama, S. J. Cryst. Growth 2000, 216, 317. Higuchi, M.; Kodaira, K.; Nakayama, S. J. Cryst. Growth 1999, 207, 298. Kolitsch, U.; Seifert, H. J.; Aldinger, F. J. Solid State Chem. 1995, 120, 38. Tolchard, J. R.; Sansom, J. E. H.; Slater, P. R.; Islam, M. S. J. Solid State Electrochem. 2004, 8, 668. (11) Sansom, J. E. H.; Tolchard, J. R.; Slater, P. R.; Islam, M. S. Solid State Ionics 2004, 167, 17. (12) Sansom, J. E. H.; Richings, D.; Slater, P. R. Solid State Ionics 2001, 139, 205. (13) Berastegui, P.; Hull, S.; Garcı´a, F. J.; Grins, J. J. Solid State Chem. 2002, 168, 294. (14) Leo´n-Reina, L.; Martı´n-Seden˜o, M. C.; Losilla, E. R.; Cabeza, A.; Martı´nez-Lara, M.; Bruque, S.; Marques, F. M. B.; Sheptyakov, D. V.; Aranda, M. A. G. Chem. Mater. 2003, 15, 2099. (15) Leo´n-Reina, L.; Losilla, E. R.; Martı´nez-Lara, M.; Bruque, S.; Aranda, M. A. G. J. Mater. Chem. 2004, 14, 1142.

10.1021/cm048361r CCC: $30.25 © 2005 American Chemical Society Published on Web 01/14/2005

Al-Doped Germanium Oxyapatite

tures of oxygen ion transport in La9.3300.67(SiO4)6O2 and La8Sr2(SiO4)6O2 at the atomic level have been recently studied by Islam et al.16 using computer modeling techniques. The atomistic modeling proposed that the high ionic conductivity and low activation energy of La9.3300.67(SiO4)6O2 is due to an interstitial mechanism with a sinusoidal-like pathway along the c-axis. The merit of this approach is that it models the local lattice relaxation around the migrating oxygen ion. We have recently reported the relationship between structure and oxide ion conductivity in the La10-x0x(MO4)6O3-1.5x (M ) Si, Ge) series.15 The location and population of the interstitial oxygen ion in hexagonal La9.5500.45(SiO4)6O2.32 and La9.600.4(GeO4)6O2.40 have been determined from NPD data for three temperatures (RT, 500 °C, and 900 °C) at the position earlier predicted by the independent theoretical work.16 The population of interstitial oxygen ion in La9.5500.45(SiO4)6O2.32 did not change with temperature, and thus, there is only one ion conduction mechanism: interstitial, yielding a straight line in the Arrhenius plot. The population of interstitial oxygen ion in La9.600.4(GeO4)6O2.4 decreased with increasing temperature. This was likely due to the filling of the oxygen vacancies, and consequently, there is a change in the ion conduction mechanism: from joint vacancies and interstitial at low temperatures to only interstitial at high temperatures. This may explain the curvature in the Arrhenius plot toward lower activation energies at high temperatures.14 Here, we report aluminum doping in germanium oxyapatite in order to increase the oxide ion conductivity. Notably, La9.500.5(Ge5.5Al0.5O24)O2 shows pure very high oxide conductivity, 0.16(1) S‚cm-1 at 800 °C. Experimental La10-x0x(Ge5.5Al0.5O24)O2.75-1.5x (10 - x ) 9.80, 9.75, 9.67, 9.60, 9.50, and 9.40) series was prepared by the ceramic method using La2O3 (Alfa, 99.999%), γ-Al2O3 (Alfa, 99.997%), and GeO2 (Aldrich, 99.998%). Lanthanum oxide was previously heated at 1000 °C for 2 h. The precursors were mixed in the appropriate ratios (to prepare 8 g of sample), ground in an agate mortar for 15 min, pelletized, and heated at 1100 °C for 6 h in Pt crucibles. Next, the samples were ground for 3 h in a Fritsch ball mill. Then, the resulting powders were pelletized again and a second thermal treatment was carried out at 1250 °C for 12 h. Weight losses, linked to Ge volatilization, at these temperatures and times were negligible. Hereafter, this series is labeled as La10-xAl0.5 La10-xAl0.5 compounds have been analyzed by the Rietveld method17 using laboratory X-ray powder diffraction (LXRPD) data collected on a Siemens D5000 automated diffractometer using graphite-monochromated Cu KR1,2 radiation. All samples were scanned between 15 and 110° (2θ) in 0.03° steps, counting 20 s per step. NPD data were collected for La9.50Al0.5 on an HRPT diffractometer [SINQ neutron source at Paul Scherrer Institut, Villigen, Switzerland]. La9.50Al0.5 was initially loaded in a vanadium can to collect the room-temperature pattern. Then, the sample was transferred to a steel can to collect the 500 and 900 °C patterns. The wavelength was 1.8860 Å, the scanned angular range was 10160° (2θ), and the overall measuring time was ∼5 h per pattern. (16) Islam, M. S.; Tolchard, J. R.; Slater, P. R. Chem. Commun. 2003, 1486. Tolchard, J. R.; Islam, M. S.; Slater, P. R. J. Mater. Chem. 2003, 13, 1956. (17) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65.

Chem. Mater., Vol. 17, No. 3, 2005 597 Impedance data were collected on cylindrical pellets in the experimental conditions already reported.14,15 The pellets were sintered at 1300 °C for 6 h with compactions ranging between 87 and 94%. The compaction of La9.5Al0.5 was 94%. No weight losses were detected at 1300 °C. Similar pellets were prepared pressing YSZ-8 (batch TZ8Y from Tosoh) and sintering at 1400 °C for 48 h giving a compaction of 98% as previously reported.18 The n-type conductivity of La9.5Al0.5, and its variation with the oxygen partial pressure, was evaluated by the ion blocking Hebb-Wagner technique. The cell design and experimental procedure has been described elsewhere.19

Results and Discussion Single-Phase Existence Range. The La10-xAl0.5 (10 - x ) 9.80, 9.75, 9.67, 9.60, 9.50, and 9.40) series has been prepared as apatite-type crystalline phases but two compositions were not single phase. La9.40Al0.5 contained 1.4(2)% (w/w) of La2Ge2O7. La9.50Al0.5 and La9.60Al0.5 were hexagonal single phases and La9.67Al0.5 and La9.75Al0.5 were triclinic single phases.14 La9.80Al0.5 contained 2.2(1)% (w/w) of La2GeO5. Hence, the single-phase existence range is close to 9.45 e 10 - x e 9.75. The unit cell volumes were 621.61(3), 620.08(3), 619.84(5), and 618.90(5) Å3 for La9.50Al0.5, La9.60Al0.5, La9.67Al0.5, and La9.75Al0.5, respectively. The unit cell volume decreases as lanthanum content increases, as it was found in the La10-x0x(GeO4)6O3-1.5x series.14 We have also prepared and analyzed two samples with higher Al content, La9.500.5(Ge5Al1O24)O1.75 (La9.5Al1.0) and La9.500.5(Ge4.5Al1.5O24)O1.50 (La9.5Al1.5). The synthesis procedure was identical to that described above. The Rietveld analysis of the LXRPD data showed that La9.5Al1.0 contained 0.86(3)% (w/w) of LaAlO3. Likewise, La9.5Al1.5 contained 1.94(5)% (w/w) of LaAlO3 and 9.8(2)% (w/w) of La2GeO5. Thus, these samples were not further analyzed. Crystal Structures. We only report the hexagonal structures (space group P63/m) for La9.50Al0.5 at RT, 500 °C, and 900 °C, as they were obtained from NPD data. Given that the oxyapatite structure is well-known, the original part of this study is the investigation about the existence of the interstitial oxygen in this oxygen stoichiometric compound, La9.500.5(Ge5.5Al0.5O24)O2.0. This type of oxygen, O(5), was predicted by the atomistic simulations in oxygen stoichiometric La9.3300.67(SiO4)6O2.016 and it has been experimentally confirmed by NPD in two Si and Ge oxyapatites, La9.5500.45(SiO4)6O2.32 and La9.6000.40(GeO4)6O2.4, which have oxygen content higher than 2.0.15 The room-temperature NPD pattern of La9.500.5(Ge5.5Al0.5O24)O2.0 was analyzed by the Rietveld method with the GSAS suite of programs.20 The starting model was based on the structure of La9.600.4(Ge6O24)O2.415 but the stoichiometry was adjusted. The nominal aluminum content was placed at the tetrahedral site because La9.50Al0.5 is single (18) Steil, M. C.; Thevenot, F.; Kleitz, M. J. Electrochem. Soc. 1997, 144, 390. (19) Navarro, L.; Marques, F.; Frade, J. J. Electrochem. Soc. 1997, 144, 267. (20) Larson, A. C.; von Dreele, R. B. GSAS Program. Los Alamos National Lab Report LA-UR-86748; Los Alamos National Laboratory: Los Alamos, NM, 1994.

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Leo´ n-Reina et al. Table 1. Structural Parameters (space group P63/m) for La9.500.5(Ge5.5Al0.5O24)O2 at Different Temperatures Refined from NPD Data RT

500 °C

900 °C

a/Å c/Å V/Å3 Rwp/% Rp/% RF/%

9.9206(4) 7.2893(3) 621.29(6) 3.03 2.35 1.30

9.9593(3) 7.3173(3) 628.56(5) 2.26 1.69 2.10

10.0039(3) 7.3360(2) 635.81(5) 1.99 1.54 2.14

La(1), 6h, (x, y, 1/4) x y Uiso × 100/Å2

0.2326(3) -0.0101(4) 0.93

0.2297(4) -0.0109(4) 1.70

0.2292(3) -0.0105(4) 2.40

La(2), 4f, (1/3, 2/3, z)a z Uiso × 100/Å2

-0.0000(5) 2.06

0.0028(7) 2.47

0.0022(7) 3.19

Figure 1. Observed (crosses), calculated (full line), and difference (bottom) room-temperature neutron powder diffraction Rietveld plot (λ ) 1.88 Å) for La9.500.5(Ge5.5Al0.5O24)O2.

Ge, Al, 6h, (x, y, 1/4)b x y Uiso × 100/Å2

0.4003(3) 0.3725(3) 0.73

0.3995(4) 0.3725(3) 1.55

0.3993(4) 0.3725(3) 2.36

phase. Moreover, the occupation factor of O(4), at the center of the channels, was constrained to that of O(5), at the interstitial position, to maintain the electroneutrality in the refined structural formula. The refined occupation factor for O(5) was 0.024(2), and, as a consequence, there are vacancies at the O(4) site [constrained occupation factor 0.85(1)]. The difference in the numbers is due to the multiplicities ratio, 6, between the two sites. Thus, 2.4% occupancy at O(5) implies 15% vacancies at O(4) site. The refinement without interstitial oxygen and full occupation of O(4) led to worse R-factors (RWP ) 3.14 and RF ) 1.45%) when compared to those obtained in the refinement with interstitial oxygen (RWP ) 3.03 and RF ) 1.30%). This worsening of the R-factors supports the presence of interstitial oxygen ion in oxygen stoichiometric La9.50Al0.5. Figure 1 shows the final Rietveld fit of the NPD data. Refined positional atomic parameters and occupation factors are given in Table 1 and bond distances are given in Table 2. Anisotropic thermal factors, Table S1, are deposited as Supporting Information. The lanthanum vacancies are located at the La(2) site, which is nine-coordinated, in full agreement with previous results.12-15 The average 〈Ge,Al-O〉 bond distance in La9.500.5(Ge5.5Al0.5O24)O2.0 is 1.74 Å (Table 2). This value is very similar to that obtained for La9.600.4(GeO4)6O2.4, 1.73 Å,15 as the ionic radii of Ge4+ and Al3+ in 4-fold coordination are almost equal, 0.39 Å.21 The interstitial oxygen O(5) is very close to the average position of O(3) (1.2 Å, see Table 2) but the atomistic calculations showed that the lattice can accommodate the interstitial oxygen through a local relaxation.16 The distance between these two oxygens must be much larger but powder diffraction cannot measure it as this technique only determines average distances. However, an indirect signature of this local lattice relaxation is the very high value of U11 for O(3), see Table S1 in the Supporting Information, which has been previously associated with the local lattice relaxation due to the presence of interstitial oxygens.15 High-temperature NPD data for La9.50Al0.5 were collected at 500 and 900 °C. The results of the Rietveld refinements

O(1), 6h, (x, y, 1/4) x y Uiso × 100/Å2

0.3148(5) 0.4886(5) 3.30

0.3167(6) 0.4892(6) 3.99

0.3191(6) 0.4907(6) 5.55

O(2), 6h, (x, y, 1/4) x y Uiso × 100/Å2

0.6019(4) 0.4739(4) 1.46

0.6034(5) 0.4743(6) 3.31

0.6019(5) 0.4745(5) 4.44

O(3), 12i, (x, y, z) x y z Uiso × 100/Å2

0.3424(4) 0.2487(3) 0.0612(3) 4.67

0.3398(4) 0.2485(4) 0.0628(4) 5.34

0.3392(4) 0.2499(4) 0.0641(4) 6.69

O(4), 2a, (0, 0, 1/4) Uiso × 100/Å2 Occ. f.

3.40 0.85(1)

4.19 0.89(2)

5.81 0.95(2)

O(5), 12i, (x, y, z)c x y z Occ. f.

0.01(1) 0.21(2) 0.62(1) 0.024(2)

0.02(2) 0.20(2) 0.62(2) 0.019(3)

0.02(6) 0.17(4) 0.58(4) 0.008(3)

(21) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.

a The occupation factor for La(2) is 0.875. b The occupation factors for Ge and Al are 0.917 and 0.083, respectively. c The Uiso× 100 for O(5) was fixed to 3/Å2 at the three temperatures.

Table 2. Selected Bond Distances (Å) for La9.500.5(Ge5.5Al0.5O24)O2 at Different Temperatures RT

500 °C

900 °C

La(1)-O(1) La(1)-O(2) La(1)-O(3) × 2 La(1)-O(3) × 2 La(1)-O(4)

2.777(6) 2.523(4) 2.448(2) 2.622(4) 2.360(2) 2.54

2.804(7) 2.555(5) 2.631(5) 2.462(3) 2.344(2) 2.56

2.843(7) 2.569(5) 2.643(5) 2.478(3) 2.347(2) 2.58

La(2)-O(1) × 3 La(2)-O(2) × 3 La(2)-O(3) × 3

2.480(4) 2.564(4) 2.924(3) 2.66

2.476(4) 2.583(5) 2.965(4) 2.67

2.485(5) 2.599(5) 2.987(4) 2.69

Ge, Al-O(1) Ge, Al-O(2) Ge, Al-O(3) × 2 〈Ge, Al-O〉

1.739(4) 1.733(4) 1.739(3) 1.74

1.729(5) 1.759(5) 1.738(3) 1.74

1.730(5) 1.755(5) 1.728(3) 1.74

O(5)-O(3)

1.2(1)

1.3(2)

1.5(3)

are given in Tables 1, 2, and S1. There is no symmetry change on heating, and the thermal expansion coefficient, determined from the unit cell volumes at the three temperatures, was Rv ) 8.9(1)‚10-6 K-1. It must be noted that the high-temperature patterns were collected in a steel can, and

Al-Doped Germanium Oxyapatite

Figure 2. Imaginary part of the electrical modulus versus frequency at several temperatures for La9.600.4(Ge5.5Al0.5O24)O2.15. The inset shows the spectroscopic plots of Z′′ and M′′ versus logf at 250 °C for the same composition.

hence, the patterns contained the holder diffraction peaks. These regions were removed from the analyses, and therefore, the results from these refinements have higher errors than those obtained at RT. The refined occupation factors of the interstitial oxygen, O(5), were 0.024(2), 0.019(3), and 0.008(3) at RT, 500 °C, and 900 °C, respectively. Hence, it seems that there is a slight variation in the amount of interstitial oxygens (and vacancies at the center of the channels) with temperature. However, a more detailed NPD study is needed to ensure this point. Impedance Study. Figure 2 shows the imaginary part of the complex electrical modulus (M′′) data at different temperatures for La9.60Al0.5 on a double-logarithmic scale. Well-defined maxima can be observed which show power law behavior at both sides of the peaks. The experimental data were replotted as the imaginary parts of the impedance, Z′′, and electric modulus, M′′, against log(frequency), see inset in Figure 2. The maxima of both curves, with capacities of ca. 4 pF, are very close which indicates that the impedance peak is associated with the same RC element responsible for the modulus peak. Therefore, the impedance data can be considered as bulk responses. A significant relaxation at the grain boundaries has not been detected. Complex impedance spectra (not shown) displayed the common features of ionic conductors. The low-frequency spike due to the electrode effect is developed at intermediate temperatures (ca. 500 °C). This spike collapses to a semicircular arc above 600 °C indicating that oxygen molecules are able to diffuse through the entire thickness of the electrode. Figure 3 shows the measured (bulk) ionic conductivities of this series versus temperature in the Arrhenius format. Similar data for GDC (10%Gd) and YSZ-8, extracted from the bibliography,2,18 are also shown for the sake of comparison. We have also measured YSZ-8 in our conductivity jig and both data sets (measured and from bibliography) are quite close indicating the correctness of our experimental setup. The key feature in Figure 3 is that La9.50Al0.5 has an oxide conductivity 1 order of magnitude higher than that of

Chem. Mater., Vol. 17, No. 3, 2005 599

Figure 3. Ionic conductivity data versus reciprocal temperature (Arrhenius plot) for several electrolytes: La9.50Al0.5 (0, /) from two syntheses, La9.60Al0.5 (O), La9.67Al0.5 (4), La9.75Al0.5 (3), and YSZ-8 (9); reference YSZ-8 and GDC-10 as solid lines. The inset shows the electronic conductivity for La9.50Al0.5 at 950 °C (0) and 1000 °C (O) from the ionblocking technique.

YSZ-818 and even slightly higher than GDC2 above 600 °C. Two regimes with different Ea are observed for the La10-xAl0.5 series as for the Al-free series.14 Below ca. 730 °C (regime II), Eas range between 1.07(1) and 1.25(1) eV. Above 730 °C (regime I), Eas are much smaller and close to 0.5 eV. Bulk conductivities at 800 °C were 0.16(1), 0.014(1), 0.016(1), and 0.017(1) S‚cm-1 for La9.50Al0.5, La9.60Al0.5, La9.67Al0.5, and La9.75Al0.5, respectively. The analogous bulk conductivity for La9.5500.45(GeO4)6O2.32 was 0.0082 S‚cm-1.14 Thus, a very significant enhancement in conductivity caused by the aluminum doping has been observed for La9.50Al0.5. La9.500.5(Ge5.5Al0.5O24)O2.0 has the highest amount of vacancies in the lanthanum sublattice and also an appreciable population of interstitial oxygen ions due to the existence of vacancies at O(4) site. The vacancies at the lanthanum site are important as they allow the local lattice relaxation around the migrating interstitial oxygen ion. There are many oxyapatite stoichiometries with full occupancy at the lanthanum site and all samples showed much lower conductivities. The aluminum-doped samples show higher conductivity likely because Al3+ interacts with the migrating oxygen ion weaker than Ge4+. Aluminum doping in the La10-x0x(SiO4)6O3-1.5x series8 also led to an increase in conductivity but the conductivity data were taken only up to 450 °C. Furthermore, the reported conductivities were much lower than those given here. It has been recently reported for Ga3+ doping in silicon oxyapatites that the conductivities at 800 °C improve from 0.002 S‚cm-1 for La9.3300.67(SiO4)6O2 to 0.03 S‚cm-1 for La9.8300.17(Si4.5Ga1.5O24)O2.11 The electronic conductivity for La9.50Al0.5, together with its possible variation with the oxygen partial pressure, has been measured using the ion-blocking Hebb-Wagner technique. The values of the electronic conductivity (σe) were

600 Chem. Mater., Vol. 17, No. 3, 2005

extracted from the curves of electronic current vs voltage as follows: σe ) L/S(dI/dV). The inset of Figure 3 gives the values of the electronic conductivity as a function of the oxygen partial pressure at two temperatures. These values increase at low oxygen partial pressures (below 10-9 atm) but they are always at least 3 orders of magnitude lower than the ionic conductivities. The ionic transport number was obtained by combination of impedance and ion-blocking data, and it is higher than 0.99 in the studied oxygen partial pressure range of 0.21 to 10-20 atm. Conclusions Pellets of La9.500.5(Ge5.5Al0.5O24)O2 show higher oxide conductivities than Ce0.9Gd0.1O1.95 above 600 °C with negligible n-type electronic conductivity. This very high oxide conductivity is very likely due to an interstitial oxygen ion mechanism. The apatite structure is highly anisotropic and the interstitial oxygen migration takes place along the

Leo´ n-Reina et al.

c-axis. Electrical measurements in apatite single crystals showed higher conductivities along the c-axis.9 Therefore, a challenge in this field is to grow apatite electrolytes as dense c-axis oriented polycrystalline films to join the cathode and anode in the SOFC units. These films would show higher conductivities than those reported here using pellets, which may lead to lower operating temperatures. Acknowledgment. We are thankful for financial support from MAT2003-7483-C2-1 research grant. This work was partially performed at the spallation neutron source SINQ, Paul Scherrer Institut, Villigen, Switzerland. Supporting Information Available: Table S1, anisotropic thermal parameters for La9.500.5(Ge5.5Al0.5O24)O2 at RT, 500 °C, and 900 °C (pdf). This material is available free of charge via the Internet at http://pubs.acs.org. CM048361R