Ben J.
Corrie
a,
J. Felix
Shin
a,
Steve
Hull
b,
Kevin S.
Knight
b,
Maria C.
Vlachou
c,
John V.
Hanna
*c and
Peter R.
Slater
*a
aSchool of Chemistry, University of Birmingham, Birmingham, B15 2TT, UK. E-mail: p.r.slater@bham.ac.uk
bISIS Facility, Rutherford Appleton Laboratory, Chilton, Oxon, UK
cDepartment of Physics, University of Warwick, Coventry CV4 7AL, UK. E-mail: j.v.hanna@warwick.ac.uk
First published on 17th November 2015
Apatite silicates are attracting significant interest as potential SOFC electrolyte materials. They are non-conventional oxide ion conductors in the sense that oxide ion interstitials, rather than vacancies, are the key defects. In this work we compare the structures of La9.6Si6O26.4 and La8Sr2Si6O26, both before and after hydration in order to gather information about the location of the interstitial oxide ion site. Neutron diffraction structural studies suggest that in the as-prepared La8Sr2Si6O26 and hydrated La8Sr2Si6O26, the interstitial oxide ion sites are close to the apatite channel centre. For La9.6Si6O26.4, a similar site close to the channel centre is observed, but on hydration of this particular sample, the interstitial site is shown to be significantly displaced away from the channel centre towards the SiO4 units. This can be explained by the need for additional displacement from the channel centre to accommodate the large amount of interstitial anions in this hydrated phase. The solid state 29Si MAS NMR spectra are shown to be very sensitive to the different speciation exhibited by the La8Sr2Si6O26 and La9.6Si6O26.4 systems, with the former being dominated by regular SiO4 framework species and the latter being dominated by interruptions to this network caused by cation vacancies and interstitials. The corresponding 17O MAS NMR study identifies a strong signal from the O atoms of the SiO4 groups, thus demonstrating that all of the O species in these systems are exchangeable O under heterogeneous gas phase conditions. In addition, interstitial O species attributed to pendant OH linkages on the Si positions are clearly identified and resolved, and these are removed on dehydration. This observation and assignment is corroborated by corresponding 1H MAS NMR measurements. Overall the neutron diffraction work indicates that the interstitial site location in these apatite silicates depends on the anion content with progressive displacement towards the SiO4 tetrahedra on increasing anion content, while the observation of exchangeable O on the SiO4 groups is consistent with prior modelling predictions as to the importance on the silicate units in the conduction process.
For the water incorporation, half of each sample was heated in water in a hydrothermal vessel (model 4749 Parr digestion vessel with 23 ml capacity) at 200 °C for 48 hours, as described previously.33 The water contents were assessed through thermogravimetric analysis (Netzsch STA 449 F1 Jupiter Thermal Analyser). The TGA experiments were carried out in N2 with a heating rate of 10 °C min−1 up to 700 °C.
In order to gain additional information on the thermal stability of the water in the apatite structure, the hydrated La9.6Si6O26.4 sample was investigated further through high temperature X-ray diffraction, utilising a Bruker D8 diffractometer. Measurements were made between 50 and 550 °C in air.
The structures of both as prepared and hydrated La9.6Si6O26.4 and La8Sr2Si6O26 samples were determined by Rietveld refinement using neutron diffraction data. Room temperature data for as prepared La9.6Si6O26.4 and La8Sr2Si6O26, and hydrated La9.6Si6O26.4 samples were collected on diffractometer HRPT at the SINQ, Paul Scherrer Institut while room temperature data for hydrated La8Sr2Si6O26 were collected on the HRPD diffractometer, ISIS, Rutherford Appleton Laboratory. All structural refinements employed the GSAS suite of Rietveld refinement software.61
All 29Si MAS and CPMAS NMR measurements were performed at an external B0 field of 9.4 T using a Bruker DSX-400 spectrometer operating at a 29Si Larmor frequency of 59.61 MHz. Each MAS and CPMAS NMR experiment was undertaken using a Bruker 4 mm dual channel (HX) MAS probe in which MAS frequencies (νr) of 12 kHz were achieved. The 29Si pulse time calibration was performed on a sample of solid kaolinite where a π/2 pulse width of 4 μs was measured, and the reported 29Si MAS NMR data were acquired using single pulse (Bloch decay) experiments with high power 1H decoupling (B1 = 50 kHz) during acquisition. A π/4 excitation pulse of 2 μs and a recycle delay of 30 s were common to all measurements and provided a quantitative description of the Si speciation, although checks with longer recycle delays of up to 120 s were also undertaken. For the 29Si CPMAS measurements an initial 1H π/2 time of 4 μs and a Hartmann-Hahn contact period of 5 ms were also calibrated on the kaolinite sample, with a recycle delay of 5 s being used. All 29Si chemical shifts are reported against the primary TMS solution reference (δ 0.0 ppm) via the secondary kaolinite solid reference (δ −92.0 ppm).
The corresponding 17O and 1H MAS NMR measurements were performed on 17O enriched samples at an external B0 field of 14.1 T using a Bruker Avance II-600 spectrometer operating at 17O and 1H Larmor frequencies of 81.30 and 600.13 MHz, respectively. Since, the natural abundance of 17O is very low, it is important to enrich the samples prior to measurement of the 17O NMR. To achieve 17O enrichment, the samples (1 g) were initially hydrated (0.5 cm3 90% 17O enriched water) under hydrothermal conditions (200 °C, 48 hours) as described earlier. These 17O MAS NMR experiments were undertaken using a Bruker 2.5 mm triple channel (HXY) MAS probe in which MAS frequencies (νr) of 31.25 kHz were achieved. The 17O pulse time calibration was performed on a water sample where a ‘non-selective’ (solution) π/2 pulse width of 3 μs was measured, and the reported 17O MAS NMR data were acquired using a rotor synchronized spin echo (θ − τ − 2θ − τ − acq.) experiment. The ‘selective’ (solids) pulses used θ and 2θ pulses of 1 and 2 μs duration (representing flip angles of π/2 and π), respectively, and a recycle delay of 10 s was employed. All 17O chemical shifts are reported against the primary solution reference of water (δ 0.0 ppm). The 1H pulse time calibration was performed on a water sample where a π/2 pulse width of 3 μs was measured, and the reported 1H MAS NMR data were acquired using a rotor synchronized spin echo (θ − τ − 2θ −τ − acq.) experiment. All reported 1H chemical shifts are referenced to the TMS primary reference (δ 0.0 ppm).
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Fig. 2 Observed, calculated and difference neutron diffraction profiles for as-prepared La8Sr2Si6O26. |
Space group | a/b (Å) | c (Å) | R wp | R p | χ 2 |
---|---|---|---|---|---|
P63/m | 9.70680 (8) | 7.23791 (7) | 2.54 | 1.92 | 5.884 |
Atom | Site | x | y | z | U iso × 100 (Å) | SOF |
---|---|---|---|---|---|---|
La(1) | 2b | 1/3 | 2/3 | −0.0006(2) | 0.597(20) | 0.5 |
Sr(1) | 2b | 1/3 | 2/3 | −0.0006(2) | 0.597(20) | 0.5 |
La(2) | 2b | 0.01292(11) | 0.24500(9) | 1/4 | 0.436(15) | 1 |
Si | 6c | 0.40092(18) | 0.37049(17) | 1/4 | 0.160(30) | 1 |
O(1) | 6c | 0.32170(13) | 0.48327(14) | 1/4 | 1 | |
O(2) | 6c | 0.59401(13) | 0.47124(15) | 1/4 | 1 | |
O(3) | 6c | 0.34263(11) | 0.25364(10) | 0.07024(11) | 1 | |
O(4) | 2a | 0 | 0 | 1/4 | 1 | |
100× | U 11 | U 22 | U 33 | U 12 | U 13 | U 23 |
O(1) | 1.40(6) | 1.04(6) | 0.867(67) | 1.049(56) | 0 | 0 |
O(2) | 0.43(6) | 0.42(5) | 1.229(58) | 0.116(50) | 0 | 0 |
O(3) | 2.02(5) | 0.77(4) | 0.552(34) | 0.739(38) | −0.573(33) | −0.287(30) |
O(4) | 0.67(5) | 0.67(5) | 2.427(107) | 0.335(26) | 0 | 0 |
Bond | Bond distance (Å) |
---|---|
La(1)/Sr(1)–O(1) | 2.5048(12), 2.5045(12), 2.5041(12) |
La(1)/Sr(1)–O(2) | 2.5474(12), 2.5480(12), 2.5475(12) |
La(1)/Sr(1)–O(3) | 2.8835(10), 2.8825(10), 2.8832(10) |
La(2)–O(1) | 2.7204(15) |
La(2)–O(2) | 2.4970(15) |
La(2)–O(3) (×2) | 2.4731(8) |
La(2)–O(3) (×2) | 2.6005(12) |
La(2)–O(4) | 2.3180(7) |
Si(1)–O(1) | 1.6222(18) |
Si(1)–O(2) | 1.6236(18) |
Si(1)–O(3) (×2) | 1.6302(11) |
On hydration, a small expansion in the cell volume was observed, and TGA studies indicated a water content of 0.18 molecules per formula unit. The presence of water means the occupancy of interstitial sites by the extra oxide ions from the water. The neutron diffraction structural studies were in agreement with this, indicating the presence of interstitial oxide ions at a position of (−0.0247, 0.1416, 0.6617). It was not possible to locate the proton site, most likely due to the low occupancy, thermal motion of these protons, and the presence of a range of different H sites with significant local displacement in these positions. The final refined structural parameters and bond distances are given in Tables 3 and 4, while the observed calculated and difference profiles are given in Fig. 3.
Space group | a/b (Å) | c (Å) | R wp | R p | χ 2 |
---|---|---|---|---|---|
P63/m | 9.72828(11) | 7.25273(9) | 1.58 | 2.23 | 7.773 |
Atom | Site | x | y | z | U iso × 100 (Å) | SOF |
---|---|---|---|---|---|---|
La(1) | 2b | 1/3 | 2/3 | −0.0009(1) | 0.531(16) | 0.5 |
Sr(1) | 2b | 1/3 | 2/3 | −0.0009(1) | 0.531(16) | 0.5 |
La(2) | 2b | 0.01320(9) | 0.24494(8) | 1/4 | 0.454(12) | 1 |
Si | 6c | 0.40086(16) | 0.3709(2) | 1/4 | 0.301(22) | 1 |
O(1) | 6c | 0.32136(12) | 0.4831(2) | 1/4 | 1 | |
O(2) | 6c | 0.59393(11) | 0.4711(3) | 1/4 | 1 | |
O(3) | 6c | 0.34321(9) | 0.2541(1) | 0.07047(8) | 1 | |
O(4) | 2a | 0 | 0 | 1/4 | 0.981(6) | |
O(i) | 6c | −0.0246(33) | 0.1416(32) | 0.6617(35) | 0.031(2) | |
100× | U 11 | U 22 | U 33 | U 12 | U 13 | U 23 |
O(1) | 1.49(5) | 1.11(6) | 0.496(47) | 1.084(49) | 0 | 0 |
O(2) | 0.24(5) | 0.37(4) | 1.260(57) | −0.001(41) | 0 | 0 |
O(3) | 2.39(5) | 0.68(4) | 0.404(26) | 0.774(34) | −0.798(31) | −0.234(27) |
O(4) | 0.84(6) | 0.84(6) | 2.60(12) | 0.418(29) | 0 | 0 |
O(i) | 2.5 | 2.5 | 2.5 | 0 | 0 | 0 |
Bond | Bond distance (Å) |
---|---|
La(1)/Sr(1)–O(1) | 2.5111(13), 2.5108(13), 2.5105(13) |
La(1)/Sr(1)–O(2) | 2.5519(13), 2.5525(13), 2.5520(13) |
La(1)/Sr(1)–O(3) | 2.8876(11), 2.8866(11), 2.8873(11) |
La(2)–O(1) | 2.7233(17) |
La(2)–O(2) | 2.5078(16) |
La(2)–O(3) (×2) | 2.4810(9) |
La(2)–O(3) (×2) | 2.6081(13) |
La(2)–O(4) | 2.3202(8) |
La(2)–O(i) | 2.40(4), 1.843(34), 2.40(4) |
Si(1)–O(1) | 1.6207(20) |
Si(1)–O(2) | 1.6258(21) |
Si(1)–O(3) (×2) | 1.6331(13) |
Si(1)–O(i) (×2) | 2.24(4) |
Accompanying the water incorporation, there was a small decrease in the AO6 metaprism twist angle from 23.23 to 23.20°, leading to a small expansion of the channels to accommodate this water.
The structural parameters and bond distances for the as prepared La9.6Si6O26.4 sample using neutron diffraction data are given Table 5 and 6, with the observed, calculated and difference profiles in Fig. 4. The data indicated a refined composition of La9.50Si6O26.26, close to that expected from the starting stoichiometry. The position of the interstititial oxide ions is similar to the position observed by Bechade et al. and others.38,44,45 Furthermore, as in the prior studies, the presence of oxide ion interstitials is accompanied by some vacancies in the ideal channel oxide ion site. In their work, Bechade et al. proposed a defect complex (O′′i– –O′′i (Kröger–Vink notation, O′′i = interstitial oxide ion with a double effective negative charge,
= oxide ion vacancy with a double effective positive charge)), and in the present study, a similar complex can be proposed. The observed length scale in this study for this complex is 2.27 Å which is smaller than the predicted value of 2.93 Å from the modelling work.10 However, it should be noted that the atomic displacement parameter for O(6) is very high perpendicular to the channel (100 × U22 = 34 Å2). This suggests significant local displacement from the refined position, which may hence allow an increase in the O′′i–
length. Overall it would suggest the presence of various interstitial oxide ion sites with differing displacements from the channel centre. In particular, the anisotropic thermal ellipsoids for the O(6) sites are directed towards channel lanthanum, La(3) (Fig. 5), which is not unexpected as O(6) is highly underbonded (bond valence sum calculations for this oxygen site give a value of only −1.08), and so some local displacement might be expected to aid the stability of this oxide ion site.
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Fig. 4 Observed, calculated and difference neutron diffraction profiles for as-prepared La9.6Si6O26.4. |
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Fig. 5 The anisotropic thermal ellipsoids around the apatite channel position of as prepared La9.6Si6O26.4, viewed down the c-axis. |
Space group | a/b (Å) | c (Å) | R wp | R p | χ 2 |
---|---|---|---|---|---|
P63 | 9.72441(4) | 7.18726(5) | 2.00 | 1.58 | 3.299 |
Atom | Site | x | y | z | U iso × 100 (Å) | SOF |
---|---|---|---|---|---|---|
La(1) | 2b | 1/3 | 2/3 | −0.0161(5) | 1.28(2) | 0.874(2) |
La(2) | 2b | 2/3 | 1/3 | −0.0139(5) | 1.28(2) | 0.874(2) |
La(3) | 6c | 0.2286(1) | −0.0119(1) | 0.2427(5) | 1.09(1) | 1 |
Si | 6c | 0.4028(2) | 0.3725(1) | 0.2488(5) | 0.69(2) | 1 |
O(1) | 6c | 0.3235(2) | 0.4847(2) | 0.2429(9) | 1 | |
O(2) | 6c | 0.5948(1) | 0.4729(2) | 0.2438(10) | 1 | |
O(3) | 6c | 0.3531(4) | 0.2580(5) | 0.0592(8) | 1 | |
O(4) | 6c | 0.6643(6) | 0.7466(5) | 0.9204(7) | 1 | |
O(5) | 2a | 0 | 0 | 1/4 | 0.853(5) | |
O(i) | 6c | 0.0033(23) | 0.0193(41) | 0.4049(21) | 0.092(3) | |
100× | U 11 | U 22 | U 33 | U 12 | U 13 | U 23 |
O(1) | 2.98(7) | 2.51(7) | 2.43(9) | 2.38(7) | −1.64(20) | −0.52(21) |
O(2) | 1.01(6) | 0.98(5) | 2.47(7) | 0.37(5) | 0.37(20) | 0.30(21) |
O(3) | 2.22(11) | 1.84(14) | 1.75(14) | 1.29(11) | −0.80(9) | −0.51(14) |
O(4) | 8.27(20) | 1.18(14) | 1.59(15) | 2.08(16) | −2.73(12) | −0.84(15) |
O(5) | 0.28(5) | 0.27(5) | 13.4(3) | 0.14(3) | 0 | 0 |
O(i) | 8.9(18) | 33.8(36) | 8.3(15) | 13.7(24) | 15.0(16) | 19.3(24) |
Bond | Bond distance (Å) |
---|---|
Si–O(1) | 1.620(1) |
Si–O(2) | 1.618(1) |
Si–O(3) | 1.674(1) |
Si–O(4) | 1.587(1) |
La(1)–O(1) (×3) | 2.535(1) |
La(1)–O(2) (×3) | 2.503(1) |
La(1)–O(4) (×3) | 2.944(1) |
La(2)–O(1) (×3) | 2.453(1) |
La(2)–O(2) (×3) | 2.591(1) |
La(2)–O(3) (×3) | 2.806(1) |
La(3)–O(1) | 2.762(1) |
La(3)–O(2) | 2.519(1) |
La(3)–O(3) | 2.630(1), 2.476(1) |
La(3)–O(4) | 2.464(1), 2.586(1) |
La(3)–O(5) | 2.284(1) |
O(i)–La(3) | 2.632(1), 2.677(1), 2.405(1) |
Space group | a/b (Å) | c (Å) | R wp | R p | χ 2 |
---|---|---|---|---|---|
P63 | 9.79242(10) | 7.17565(9) | 2.47 | 1.93 | 5.309 |
Atom | Site | x | y | z | U iso × 100 (Å) | SOF |
---|---|---|---|---|---|---|
La(1) | 2b | 1/3 | 2/3 | 0.0058(9) | 1.93(4) | 0.882(3) |
La(2) | 2b | 2/3 | 1/3 | 0.0028(9) | 1.93(4) | 0.882(3) |
La(3) | 6c | 0.2360(1) | −0.0077(2) | 0.2438(6) | 1.28(2) | 1 |
Si | 6c | 0.4032(3) | 0.3757(3) | 0.2475(12) | 1.51(5) | 1 |
O(1) | 6c | 0.3269(3) | 0.4877(3) | 0.24941(8) | 1 | |
O(2) | 6c | 0.5939(2) | 0.4724(3) | 0.2515(9) | 1 | |
O(3) | 6c | 0.3624(4) | 0.2649(6) | 0.0628(6) | 1 | |
O(4) | 6c | 0.6664(6) | 0.7494(6) | 0.9169(6) | 1 | |
O(5) | 2a | 0 | 0 | 1/4 | 0.890(11) | |
O(i) | 6c | 0.1334(20) | 0.1425(12) | 0.4192(17) | 0.200(7) | |
100× | U 11 | U 22 | U 33 | U 12 | U 13 | U 23 |
O(1) | 6.09(17) | 4.29(17) | 0.71(10) | 4.70(15) | −1.62(25) | −2.36(18) |
O(2) | 2.06(12) | 1.74(11) | 2.25(12) | 0.04(10) | −0.99(30) | −0.97(31) |
O(3) | 5.29(24) | 4.75(27) | 0.64(16) | 4.03(23) | 0.79(14) | 0.17(16) |
O(4) | 8.46(29) | 1.82(20) | 2.18(18) | 2.34(23) | −3.68(15) | −1.09(19) |
O(5) | 1.64(15) | 1.64(15) | 22.5(8) | 0.82(7) | 0 | 0 |
O(i) | 8.5(13) | 0.20(58) | 3.09(71) | 3.07(66) | 0.52(70) | −2.21(51) |
Bond | Bond distance (Å) |
---|---|
Si–O(1) | 1.609(1) |
Si–O(2) | 1.618(1) |
Si–O(3) | 1.628(1) |
Si–O(4) | 1.614(1) |
La(1)–O(1) (×3) | 2.455(1) |
La(1)–O(2) (×3) | 2.580(1) |
La(1)–O(4) (×3) | 3.010(1) |
La(2)–O(1) (×3) | 2.505(1) |
La(2)–O(2) (×3) | 2.552(1) |
La(2)–O(3) (×3) | 2.740(1) |
La(3)–O(1) | 2.830(1) |
La(3)–O(2) | 2.479(1) |
La(3)–O(3) | 2.651(1), 2.520(1) |
La(3)–O(4) | 2.483(1), 2.538(1) |
La(3)–O(5) | 2.350(1) |
O(i)–La(3) | 2.489(1), 2.302(1), 2.538(1) |
The water content of the hydrated sample was determined by TGA measurements (see Fig. 9). Unlike the single stage dehydration suggested by the high temperature X-ray diffraction data, there was evidence for a two stage water loss from the TGA data: firstly there was an abrupt loss in mass at around 280 °C with a second mass loss at around 470 °C. Since high temperature X-ray diffraction data did not show any change at higher temperature, the second mass loss was attributed to the decomposition of an impurity phase (most likely amorphous). As the structure refinement indicated a lower La content than the starting ratio, it was presumed that this may be amorphous La(OH)3. La(OH)3 is known to decompose in two steps, via an LaOOH intermediate as outlined below, and so it was proposed that the second mass loss observed in the TGA plots was due to the LaOOH dehydrating. This would suggest that part of the first mass loss was due to La(OH)3 dehydrating to LaOOH.
In order to estimate the contribution from this proposed amorphous La(OH)3 impurity phase, firstly, the mass of LaOOH was estimated from the mass loss at 470 °C and subsequently, the mass of La(OH)3 was calculated (1.7(2) wt%). Eliminating the contribution of the above process, the calculated level of water incorporated was 0.75 H2O per formula unit, which is similar to the composition La9.53Si6O26.295·0.685H2O and the calculated interstitial content (≈1 O per formula unit) from the diffraction studies. The dehydration temperature difference between X-ray diffraction study and the TGA result is due to the fact that the TGA measurement was performed with a 10 °C min−1 ramp rate, and so the experiment is performed under non-equilibrium conditions.
The 29Si CPMAS NMR data shown in Fig. 10(b) contrast markedly with the corresponding MAS NMR data discussed above. It is immediately evident that the signal-to-noise of these CPMAS data is greatly inferior thus suggesting that, despite there being an abundance of H species (i.e. OH and H2O) to facilitate 1H–29Si cross-polarisation, the motion of the intercalated OH/H2O modulates the 1H–29Si dipolar interaction and greatly diminishes the efficiency of the Hartmann-Hahn condition. For both of the hydrated La8Sr2Si6O26 and La9.6Si6O26.4 systems only the monomeric SiO4 positions exhibit some observable intensity above the noise level, and this is probably facilitated by proximity to a component of immobile OH interstitial species. However, the dehydration of these systems shows that both the mobile and immobile OH and H2O species are largely removed, as evidenced by the absence of a 29Si signal above the baseline noise.
From Fig. 10(c) the 17O MAS NMR data shows that similar O speciation from both systems is observed. Each spectrum is dominated by 17O resonances at an apparent shift of δ ∼180–200 ppm that are attributed to O species associated with the framework SiO4 element. In addition, there are two additional signals attributed to interstitial species observed from these data; a clearly resolved 17O resonance at δ ∼40–50 ppm, which may represent a Si-based (Si–O–Si or Si–OH) interstitial O species bridging between SiO4 framework moieties, and a partially resolved shoulder at δ ∼260–270 ppm which is assigned to the less prevalent Si–O–La interstitial species. The latter assignment is consistent with the observation of interstitial O species in previously reported silicate- and germanate-based rare earth apatite SOFC materials.47,49 It can be observed that upon dehydration of these systems both of these interstitial species are essentially removed, with the more complete removal of the Si–O–Si/Si–OH interstitial species being observed from the La8Sr2Si6O26 system, suggesting that this component is mostly related to Si–OH (see Fig. 10(b)). Previous studies on the La8Y2Ge6O27 rare earth apatite phase has clearly shown that a resonance observed at δ ∼ 580 ppm is associated with O channel species;49 this channel species is not clearly evident in the silicate-based La8Sr2Si6O26 and La9.6Si6O26.4 systems under study here. In addition, the resonance at δ ∼380–390 ppm (marked with an asterisk) in Fig. 10(c) is identified as a 17O background signal arising from the ZrO2 MAS rotor material.
Although nominal O interstitial species within the hydrated and dehydrated La8Sr2Si6O26 and La9.6Si6O26.4 phases are indicated by 17O resonances at δ ∼40–50 ppm, a more precise identification of these species is afforded by the accompanying 1H MAS NMR data shown in Fig. 10(d). A substantial proportion of these interstitials are hydroxylated or –Si–OH species as suggested by the narrow 1H resonance at δ 1.0 ppm. The near-complete elimination of these species upon dehydration of the La8Sr2Si6O26 system, coupled with their partial removal upon dehydration of the La9.6Si6O26.4 system is demonstrated by the concomitant reduction of the δ ∼40–50 ppm resonance(s) in the 17O MAS NMR data (see Fig. 9(c)) and the δ 1.0 ppm resonance in the 1H MAS NMR data (see Fig. 10(d)). The resonance at δ ∼8 ppm represents H-bonded OH species residing in more occluded and sterically crowded environments; these are characterized by the larger downfield shift caused by the deshielding of the H-bonding arrangement and a broader residual linewidth induced by a much larger homonuclear 1H–1H dipolar interaction. In addition, the smaller resonances at δ 3.9 and 4.9 ppm represent different H2O environments within each unit cell. These resonances are also affected by the dehydration process.
These results demonstrate that the 17O enrichment via heterogeneous gas phase exchange under autoclave conditions facilitates the exchange of the O positions comprising the SiO4 groups in both the as-synthesized La8Sr2Si6O26 and La9.6Si6O26.4 systems. These O exchange characteristics are similar to those reported for the analogous germanate systems.49 This work therefore suggests that while a direct interstitial oxide ion conduction mechanism may occur down the channels, as proposed by Bechade et al.,38 there is likely to be significant contribution from exchange processes throughout the SiO4 tetrahedra network. In particular, such processes would explain the observation of significant conductivity perpendicular to the channel direction from single crystal studies.51
In comparison to the germanate (La10−xGe6O27−3x/2) apatite systems, there appears, however, to be less association of the interstitial oxide ion defects with the MO4 tetrahedra (for the germanate systems, the 17O NMR data indicated that the presence of interstitial oxide ions led to the formation of GeO5 units49). This closer association may explain the higher activation energies for oxide ion conduction for the germanate compared to the silicate apatites, with a degree of trapping of the oxide ion interstitials due to this association. This trapping contribution is supported by recent high temperature Raman studies on germanate apatites.43
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