Synthesis and structural characterisation of the apatite-type phases La10−xSi6O26+z doped with Ga

Solid State Ionics 167 (2004) 17 – 22 www.elsevier.com/locate/ssi

Synthesis and structural characterisation of the apatite-type phases La10 xSi6O26+z doped with Ga J.E.H. Sansom, J.R. Tolchard, P.R. Slater *, M.S. Islam Department of Chemistry, University of Surrey, Stag Hill, Guildford, Surrey, GU2 7XH UK Received 6 March 2003; received in revised form 20 October 2003; accepted 12 December 2003

Abstract Apatite-type oxides of general formula (La/Sr)10 xSi6O26 + y have been attracting considerable interest recently, because of their observed high oxide ion conductivity. In this paper, we report the effects on the conductivity of Ga doping at the Si site. For samples that are nominally stoichiometric in oxygen, La9.33 + x/3Si6 xGaxO26 (0 V x V 2), we find that the oxide ion conductivity increases with Ga content up to a maximum value (r500 jC = 1.3  10 3 S cm 1) for x = 1.5, before decreasing with further Ga incorporation. The conductivity data is discussed in relation to results from other doping studies and neutron powder diffraction structural data collected for the samples La9.67Si5GaO26 and La10Si4Ga2O26. In terms of the latter, the data shows a difference in space group (from P63 to P63/m) between the samples. In addition, there is a larger anisotropy of the thermal displacement parameters for the channel oxygens in La9.67Si5GaO26, which is consistent with the higher conductivity of this sample. D 2004 Elsevier B.V. All rights reserved. PACS: 66.30.Dn Keywords: Oxide ion conduction; Apatite; Synthesis; Neutron diffraction; Solid oxide fuel cells

1. Introduction Apatite-type phases with the general formula (La/ M)10 x(Si/Ge)6O26 + y (M = Mg, Ca, Sr, Ba) are attracting considerable interest as a new class of oxide ion conductor [1– 15]. These phases have the potential to be used in a variety of technological applications, including electrolyte materials in solid oxide fuel cells. Conductivities higher than YSZ at intermediate temperatures have been reported (e.g., La10Si6O27: r = 4.3  10 3 S cm 1, versus 1  10 3 S cm 1 for YSZ at 500 jC [2]). Most of the present research has focused on the Si-based systems, despite the fact that higher conductivities have been reported for the Ge-based systems, (La/Sr)10 xGe6O26 + y. This is due to the fact that these latter systems suffer from significant problems attributed to Ge loss [9,12,15]. One particular property of these apatite-type systems, which gives them huge potential for modifying the oxide

* Corresponding author. Tel.: +44-1483-686847; fax: +44-1483686851. E-mail address: [email protected] (P.R. Slater). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.12.014

ion conductivity, is the wide range of potential substitutional possibilities. Apatites have general formula A10 x M6O26 F y, where A and M can be a range of cations (e.g., A = rare earths, alkaline earths, M = Si, Ge, P, V, etc.). The structure of the apatite La9.33Si6O26 is shown in Fig. 1; it consists of isolated SiO4 tetrahedra with the La cations located in seven-coordinate and nine-coordinate cavity sites. The extra oxide ions occupy channels running through the structure, and it is these oxide ion channels that are responsible for the high oxide ion conduction. We have been recently investigating these systems, with a view to rationalising them in terms of structure and properties. It was found that fully stoichiometric systems, e.g., La8Sr2Si6O26, had much lower conductivities and higher activation energies than systems that contained either cation vacancies, e.g., La9.33Si6O26, or oxygen excess, e.g., La9SrSi6O26.5. Neutron diffraction experiments showed that these materials have significant complexities in their structures particularly within the channel oxygen sites [5]. Such studies showed that the channel oxygen atoms were essentially ordered close to the ideal (0,0,0.25) sites in the stoichiometric systems, while in the

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Fig. 1. The structure of La9.33Si6O26 (tetrahedra = SiO4).

non-stoichiometric systems, significant disorder within the channels, in the form of interstitial oxygens, was indicated. This importance of interstitial oxygens was supported by our recent computer modelling studies on La9.33Si6O26 and La8Sr2Si6O26, which led to good agreement with experimentally observed activation energies, and suggested that oxide ion conduction proceeded via an interstitial mechanism in La9.33Si6O26, while for La8Sr2Si6O26, the mechanism involved vacancy migration [13]. These modelling studies suggested a complicated interstitial migration mechanism, which was strongly dependant on the ability of the silicate substructure to relax towards the La sites that contained the cation vacancies. The importance of cation non-stoichiometry has also been supported by subsequent studies on the La9.33 + x/ 3Si6 xAlxO26 system by Abram et al. [6], who showed that the conductivity reached a maximum value at x = 1.5 and then decreased as the fully cation stoichiometric system was Table 1 Summary of conductivity data for Ga-doped La10 Sample

r (S cm 1) at 500 jC

La9.33Si6O26 La9.5Si5.5Ga0.5O26 La9.58Si5.25Ga0.75O26 La9.67Si5GaO26 La9.75Si4.75Ga1.25O26 La9.83Si4.5Ga1.5O26 La9.92Si4.25Ga1.75O26 La10Si4Ga2O26 La10Si5GaO26.5

1.13  10 4.56  10 7.64  10 1.04  10 1.06  10 1.32  10 5.15  10 4.1  10 2.39  10

4 4 4 3 3 3 5 6 3

approached. Thus, the initial studies on these Si-based apatite-type systems have indicated that for high oxide ion conduction, cation vacancies and/or oxygen excess are required, and recent preliminary work on Ge-based systems suggests that these factors are also important in these particular cases [14]. In order to examine whether these factors apply to all dopants, we have performed similar studies with Ga and In doping. The results showed that while Ga doping was successful, In doping led to significant impurities suggesting negligible solubility in the apatite structure. In this paper we report our work on a range of Ga-doped samples. Conductivity data were collected for the samples, La9.33 + x/3Si6 x GaxO26 (0 V x V 2), in which the lower charge on Ga doping is balanced by increased La content, thus reducing the number of cation vacancies. We also report data for the cation stoichiometric, oxygen excess sample La 10Si5-

xSi6O26 + z

Ea (eV) 0.78 0.67 0.62 0.70 0.64 0.73 0.76 0.72 0.70

Fig. 2. Conductivity plots for the samples (a) La9.33Si6O26, (b) La10Si4Ga2O26 and (c) La9.83Si4.5Ga1.5O26.

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Table 3 Selected bond distances for La9.67Si5GaO26

Fig. 3. A plot of log r at 500 jC versus Ga content (x) for the series La9.33 + x/3Si6 xGaxO26.

GaO26.5 to examine the effect of the incorporation of excess oxygen into the doped system. In addition to the synthesis and conductivity data, neutron powder diffraction structural data for the samples La9.67Si5GaO26 and La10Si4Ga2O26 are reported. The conductivity data is discussed with reference to the neutron diffraction data, our recent computer modelling studies of La9.33Si6O26, and previously reported results for other doped phases [5,6,10,13].

Bond

˚) Bond distance (A

La1UO1 La1UO2 La1UO3 La2UO1 La2UO2 La2UO4 La3UO1 La3UO2 La3UO3 La3UO4 La3UO5 Si/GaUO1 Si/GaUO2 Si/GaUO3 Si/GaUO4

2.439(6) [  3] 2.559(6) [  3] 2.933(5) [  3] 2.548(7) [  3] 2.512(7) [  3] 2.814(4) [  3] 2.752(2) 2.504(2) 2.597(6), 2.478(8) 2.614(6), 2.456(7) 2.320(1) 1.635(2) 1.664(2) 1.69(1) 1.62(1)

the samples remained single phase after sintering. Both sides of the pellet were then coated with Pt paste to act as electrical contact for the measurements. Neutron powder diffraction data were collected on diffractometer HRPD, ISIS, Rutherford Appleton Laboratory for the samples La9.67Si5GaO26 and La10Si4Ga2O26. Structure refinement was performed by the Rietveld method, using the GSAS suite of programs [16].

2. Experimental 3. Results High-purity La2O3, Ga2O3 and SiO2 were used to prepare a range of samples La9.33 + x/3Si6 xGaxO26 (0 V x V 2) and La10Si5GaO26.5. The dried starting materials were ground in the appropriate ratios and heated to 1300 jC for 16 h. The samples were then reground and reheated to 1350 jC for a further 16 h. Phase purity was examined using powder Xray diffraction (Seifert 3003TT X-ray diffractometer), which indicated single phase samples for the complete range of samples prepared. Conductivities were determined using a.c. impedance measurements (Hewlett Packard 4192A Impedance analyser). Pellets (1.6 cm diameter) of each sample were pressed at 6000 kg cm 2 and were then placed on Pt foil and sintered at 1700 jC for 2 h. X-ray diffraction confirmed that

3.1. Synthesis and conductivities A range of single phase samples (La9.33 + x/3Si6 xGaxO26 (0 V x V 2) and La10Si5GaO26.5) with Ga doping were successfully prepared by synthesis at 1350 jC. In contrast, in order to prepare similar samples with Al doping, temperatures in excess of 1500 jC were required [6]. The solubility of Ga in this system therefore appears higher than that of Al at temperatures below 1500 jC. Sintering the pellets at 1700 jC for 2 h gave pellets with high densities (>90% theoretical), allowing good resolution of bulk and grain boundary semicircles. However, it was only possible to observe the bulk semicircle at

Table 2 Structural parameters for La9.67Si5GaO26 Atom

Site

x

y

La1 La2 La3 Si/Ga O1 O2 O3 O4 O5

2b 2b 6c 6c 6c 6c 6c 6c 2a

1/3 2/3 0.2320(1) 0.4002(2) 0.3222(2) 0.5976(2) 0.3504(8) 0.3380(9) 0.0000

2/3 1/3 0.0116(2) 0.3713(2) 0.4862(2) 0.4717(2) 0.2542(8) 0.2524(8) 0.0000

z

Uiso (  100)

SOF

U11

U22

U33

0.022(1) 1.3(2) 0.955(6) 0.022(2) 1.7(2) 0.879(6) 0.234(2) 1.19(3) 1.000 0.241(2) 1.15(5) 0.833/0.167 0.234(2) 2.44* 1.0000 3.2(1) 2.5(1) 2.5(1) 0.234(2) 1.81* 1.0000 1.4(1) 1.1(1) 2.7(1) 0.0508(4) 2.28* 1.0000 2.2(3) 2.4(3) 2.6(3) 0.08468(5) 2.38* 1.0000 6.3(5) 1.1(3) 1.0(3) 0.228(3) 2.52* 1.0000 1.21(9) 1.21(9) 5.1(2) ˚ . Cell volume = 594.632(2) A ˚ 3. Rwp = 5.96%, Chi2 = 3.24. Space group = P63 (no. 173), a = 9.74413(2), c = 7.23155(2) A

U12

2.0(1) 0.39(8) 1.5(3) 1.5(3) 0.61(5)

U13

1.5(4) 0.9(4) 0.4(2) 1.9(3)

U23

0.1(5) 0.1(5) 0.3(3) 0.1(3)

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Table 4 Structural parameters for La10Si4Ga2O26 Atom

Site

x

y

La1 La2 Si/Ga O1 O2 O3 O4

4f 6h 6h 6h 6h 12i 2a

1/3 0.0099(2) 0.3724(2) 0.4902(2) 0.4718(2) 0.2527(1) 0.0000

2/3 0.2329(1) 0.4006(2) 0.3228(2) 0.6017(2) 0.3461(1) 0.0000

z

Uiso (  100)

SOF

U11

0.0001(2) 0.25 0.25 0.25 0.25 0.0642(1) 0.25

U22

1.28(3) 1.0000 1.04(2) 1.0000 1.08(3) 0.667/0.333 2.42* 1.0000 1.9(1) 3.3(1) 1.67* 1.0000 1.13(8) 1.57(8) 2.37* 1.0000 2.06(6) 3.25(8) 1.92* 1.0000 1.18(8) 1.18(8) 3 ˚ ˚ Space group = P63/m (no. 176), a = 9.76992(2), c = 7.26788(3) A. Cell volume = 600.788(3) A . Rwp = 5.64%,

low to intermediate temperatures ( < 600 jC) for some samples (e.g., La9.83Si4.5Ga1.5O26). At higher temperatures, only a partial grain boundary semicircle or electrode response was observed for these samples, making measurement of the bulk contribution inaccurate. Therefore, a summary of bulk conductivity data at 500 jC is given for each sample in Table 1. Conductivity plots are shown in Fig. 2 for the samples La9.33Si6O26, La10Si4Ga2O26 and La9.83Si4.5Ga1.5O26, while a plot of log r500 jC versus Ga content for the series La9.33 + x/3Si6 xGaxO26 is given in Fig. 3. As can be seen from the data, the conductivity increases significantly for the La9.33 + x/3Si6 xGaxO26 (0 V x V 2) samples with increasing Ga content up to x = 1.5 and then decreases above that level, consistent with the similar work on Al doping of Abram et al. [6]. The highest conductivity at 500 jC is, however, observed for the sample with oxygen excess, La10Si5GaO26.5, which is consistent with previous doping studies on the La site showing the highest conductivities for samples with oxygen excess [2,4,5,10]. 3.2. Structural refinement: La9.67Si5GaO26 and La10Si4 Ga2O26 For both samples, three possible space groups were investigated P-3, P63 and P63/m in accordance with previous studies of apatite-type systems. It was found for the sample La9.67Si5GaO26 that the space groups P-3 and P63 gave better fits than space group P63/m. Since the fits for the

Table 5 Selected bond distances for La10Si4Ga2O26 Bond

˚) Bond distance (A

La1UO1 La1UO2 La1UO3 La2UO1 La2UO2 La2UO3

2.471(1) 2.529(1) 2.861(1) 2.792(2) 2.500(2) 2.607(1) 2.464(1) 2.325(1) 1.667(2) 1.702(2) 1.689(1)

La2UO4 Si/GaUO1 Si/GaUO2 Si/GaUO3

[  3] [  3] [  3]

[  2], [  2]

[  2]

U33

U12

U13

1.69(9) 2.14(9) 1.51(5) 3.38(17)

1.50(9) 0.68(8) 1.45(6) 0.59(4)

– – 0.08(5) –

U23

– – 0.71(5) –

Chi2 = 2.70.

former two space groups were very similar, we have chosen the higher symmetry space group P63. The refined structural parameters are given in Table 2 with selected bond distances in Table 3. In the case of La10Si4Ga2O26, it was found that all three space groups gave similar fits, and so again we have chosen the highest symmetry space group, in this case P63/m. The refined structural parameters for this sample are given in Table 4 with selected bond distances in Table 5. As expected, there is an increase in the average Si/GaUO bond distances in moving from La9.67Si5GaO26 to La10Si4Ga2O26 due to the larger size of Ga3 + compared to Si4 +, along with an increase in unit cell size. The main differences between space groups P63 and P63/ m are the splitting of some of the sites, e.g., the La1 site becomes two sites in space group P63. It is these sites that cation vacancies are located, and so the lowering of the space group symmetry allows for variations in the occupancies of these two sites, as observed in the refinement (Table 2). It can therefore be proposed that it is this feature that lowers the symmetry for the cation deficient sample La9.67Si5GaO26 relative to the cation stoichiometric sample La10Si4Ga2O26. The observed, calculated and difference profiles for both samples are given in Figs. 4 and 5.

4. Discussion The results add further support to previous conclusions that the presence of cation vacancies and/or oxygen excess enhances the oxide ion conductivities of these apatite-type systems. The conductivities for La9.83Si4.5Ga1.5O26 and La10 Si 5 GaO 26.5 recorded at 500 jC were very high, 1.3  10 3 and 2.4  10 3 S cm 1, respectively. In contrast, the conductivity of the fully stoichiometric La10Si4Ga2O26 was very low at this temperature (4.1  10 6 S cm 1). In addition, the results suggest that the level of cation vacancies is an important factor. The conductivity appears to initially increase with a decrease in the number of cation vacancies, i.e., from La9.33Si6O26 towards La9.83Si4.5 Ga1.5O26, and then decreases significantly on moving to full stoichiometry, i.e., La10Si4Ga2O26, similar to the results of Abram et al. [6] for Al-doped systems.

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Fig. 4. Observed, calculated and difference neutron diffraction profiles for La9.67Si5GaO26.

The origin of the enhancement of oxide ion conductivity on introduction of cation vacancies needs further discussion in relation to the structural data on these systems. The structures of both samples were similar, with the main difference being the reduction in space group symmetry from P63/m to P63 in moving from La10Si4Ga2O26 to La9.67Si5GaO26. Our previous structural studies of the systems La9.33Si6O26 and La8Sr2Si6O26 suggested the displacement of a significant number ( c 14%) of the channel oxygen atoms away from the ideal (0,0, c 0.25) site to a new interstitial site (O6) at (0,0, c 0.38) for the former, creating Frenkel-like defects. It was suggested that the presence of these defects accounted for the higher conductivity of La9.33Si6O26. In both the Ga-doped samples (La9.67Si5GaO26 and La10Si4Ga2O26) examined, there was,

however, no evidence for significant numbers of oxygen atoms in this interstitial O6 site, although it is possible that there could be the displacement of a small fraction of oxide ions into such a site. In support of this, the thermal displacement parameters for the channel oxygen atoms were high and showed significant anisotropy, with the values being particularly high for U33 which corresponds to the displacement down the channels. A comparison of the values for La9.67Si5GaO26 and La10Si4Ga2O26 shows that the anisotropy is significantly higher for the former consistent with the higher conductivity of this phase. Other interesting observations are the high thermal displacement parameters for the silicate oxygens and distortion of the Si/GaO4 tetrahedra for La9.67Si5GaO26, as seen from the bond distances (Table 3). Although high values would

Fig. 5. Observed, calculated and difference neutron diffraction profiles for La10Si4Ga2O26.

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be expected for the thermal displacement parameters due to the presence of both Si and the larger Ga on this site (with corresponding longer GaUO bond distances), the distortion of the tetrahedra, which is higher for La9.67Si5GaO26 compared to La10Si4Ga2O26, might not be expected. It is also interesting to note that high thermal displacement parameters were also observed in our previous studies of La9.33Si6O26, which has no Ga, and the magnitudes of these were shown to increase substantially with temperature [5,17]. Of further relevance to this discussion is the fact that our modelling studies have suggested that the oxide ion migration in these systems involves significant cooperative relaxation of the silicate units. This has the effect of opening up the channels to conduction [13]. In addition, these studies suggested the presence of an alternative interstitial site away from the channel centre towards the periphery. Such a position is practically impossible to model using diffraction data, since the presence of oxygens in these positions will cause significant localised distortions in the silicate sublattice as shown by the modelling work, while diffraction techniques can only give the average structure. The high thermal displacement parameters for the silicate oxygens and distortion of the tetrahedra for La9.67Si5GaO26 may, however, give some support to the presence of such sites in this sample. Also, of relevance to this discussion is a comparison of the present results with other doping studies in La9.33Si6O26, which shows an interesting discrepancy. Although the importance of cation vacancies appears uniformly applicable to all samples, the optimum cation vacancy concentration observed on Al and Ga doping on the Si site does not seem to apply for doping on the La site. Our previous studies of Ba, Sr, and Ca doping showed that the conductivities of La8.67MSi6O26 (M = Ca, Sr, Ba) were comparable to that of the undoped phase, La9.33Si6O26 (e.g., r500 jC = 1.4  10 4 S cm 1 for La8.67BaSi6O26) [10] and so are an order of magnitude lower than the Ga-doped sample with comparable cation vacancies, La9.67Si5GaO26. This appears to add further support to the conclusions from the modelling work that the tetrahedral sites have a key effect on the conductivity. Finally, one unusual feature of the conductivity data was that the activation energies for all the samples were similar in value, c 0.70 eV, including the nominally cation and oxygen stoichiometric compound La10Si4Ga2O26. This is in contrast to our previous studies of cation and oxygen stoichiometric samples, such as La8M2Si6O26 (M = Ca, Sr, Ba) [5,10], for which the conductivity was low and the activation energy was high. The low conductivity combined with low activation energy for La10Si4Ga2O26 suggests that the oxide ion defects are readily mobile, but are too few in number and therefore not readily available to facilitate conductivity.

5. Conclusions We have shown that it is possible to prepare single phase hexagonal apatite phases in the range La9.33 + x/3Si6 xGaxO26 (0 V x V 2). The results reinforce the importance of the presence of cation vacancies for fast oxide ion conduction in these apatite-type phases. For the oxygen stoichiometric systems, the highest conductivity was observed for x = 1.5 with the conductivity of the nominally stoichiometric phase, x = 2 La 10Si 4Ga2O 26 being comparatively low. Higher conductivities were observed for these Ga-doped samples than for samples with comparable levels of cation vacancies involving doping on the La site. These results along with structural and modelling studies of La9.33Si6O26 suggest the importance of the tetrahedral cation sublattice in the conduction process. Incorporating excess oxygen also leads to high conductivity, even in cation stoichiometric phases, with La10Si5GaO26.5 exhibiting the highest conductivity of the samples studied. The value reported (2.4  10 3 S cm 1 at 500 jC) is higher than that of YSZ at this temperature.

Acknowledgements We would like to thank EPSRC and Merck for funding.

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