research papers
Oxygen conduction at low temperatures in apatites make these materials potentially useful as electrolytes in solid-oxide fuel cells, but our understanding of the defect structures enabling ion migration is incomplete. While conduction along [001] channels is dominant, considerable inter-tunnel mobility has been recognized. Using neutron powder diffraction of stoichiometric `La10(GeO4)6O3', it has been shown that this compound is more correctly described as an La10(GeO4)5(GeO5)O2 apatite, in which high concentrations of interstitial oxygen reside within the channel walls. It is suggested that these framework interstitial O atoms provide a reservoir of ions that can migrate into the conducting channels of apatite, via a mechanism of inter-tunnel oxygen diffusion that transiently converts GeO4 tetrahedra to GeO5 distorted trigonal bipyramids. This structural modification is consistent with known crystal chemistry and may occur generally in oxide apatites.
Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768107024317/lm5010sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108768107024317/lm5010sup2.hkl | |
Text file https://doi.org/10.1107/S0108768107024317/lm5010sup3.txt |
Computing details top
Data collection: Shimadzu XRD 6000; cell refinement: TOPAS (Bruker, 2005); data reduction: TOPAS (Bruker, 2005); program(s) used to solve structure: TOPAS (Bruker, 2005); program(s) used to refine structure: TOPAS (Bruker, 2005); molecular graphics: ATOMS (Shape, 2004), GFOURIER (Gonzales-Platas, J. & Rodriguez-Carvajal, J. (2006).
La10(GeO4)5(GeO5)O2 top
Crystal data top
La10Ge6O27 | β = 88.079 (5)° |
Mr = 2256.70 | γ = 120.890 (4)° |
Triclinic, P1 | V = 616.8 (1) Å3 |
Hall symbol: -P_1 | Z = 1 |
a = 9.9346 (7) Å | F(000) = 978 |
b = 9.9132 (7) Å | Dx = 6.076 Mg m−3 |
c = 7.3021 (5) Å | Neutron radiation, λ = 1.8834 Å |
α = 90.960 (5)° | T = 300 K |
Data collection top
Local diffractometer | Data collection mode: reflection |
Ge 115 monochromator | Scan method: Stationary detector |
Specimen mounting: vanadium can with He exchange gas |
Refinement top
Least-squares matrix: full | Excluded region(s): 0.027 to 8.97 degrees unreliable due to background |
Rp = 0.039 | 129 parameters |
Rwp = 0.050 | 0 restraints |
Rexp = 0.070 | 23 constraints |
RBragg = 0.014 | Weighting scheme based on measured s.u.'s |
3001 data points |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
x | y | z | Biso*/Beq | Occ. (<1) | |
La1 | 0.341 (1) | 0.641 (1) | 0.006 (2) | 0.2 (2) | |
La2 | 0.302 (1) | 0.667 (1) | 0.508 (2) | 0.2 (2) | |
La3 | 0.235 (1) | 0.993 (1) | 0.241 (2) | 0.4 (1) | |
La4 | 0.010 (1) | 0.251 (1) | 0.264 (2) | 0.4 (1) | |
La5 | 0.771 (1) | 0.797 (1) | 0.228 (2) | 0.4 (1) | |
Ge1 | 0.398 (1) | 0.370 (1) | 0.282 (2) | 0.2 (1) | |
Ge2 | 0.614 (1) | 0.015 (1) | 0.254 (2) | 0.2 (1) | |
Ge3 | 0.992 (3) | 0.597 (3) | 0.231 (3) | 0.2 (1) | 0.500 |
Ge3a | 0.936 (3) | 0.574 (3) | 0.257 (3) | 0.2 (1) | 0.500 |
O1 | 0.293 (2) | 0.465 (2) | 0.281 (2) | 0.62 (9) | |
O2 | 0.502 (2) | 0.818 (2) | 0.258 (2) | 0.62 (9) | |
O3 | 0.212 (4) | 0.701 (5) | 0.207 (5) | 0.62 (9) | 0.500 |
O3a | 0.149 (4) | 0.665 (4) | 0.202 (5) | 0.62 (9) | 0.500 |
O4 | 0.615 (2) | 0.484 (2) | 0.295 (2) | 0.62 (9) | |
O5 | 0.529 (2) | 0.135 (2) | 0.231 (2) | 0.62 (9) | |
O6 | 0.882 (4) | 0.389 (4) | 0.201 (5) | 0.62 (9) | 0.500 |
O6a | 0.824 (4) | 0.388 (4) | 0.240 (5) | 0.62 (9) | 0.500 |
O7 | 0.362 (2) | 0.278 (2) | 0.060 (2) | 0.62 (9) | |
O8 | 0.769 (2) | 0.052 (2) | 0.094 (2) | 0.62 (9) | |
O9 | 0.885 (4) | 0.637 (4) | 0.069 (5) | 0.62 (9) | 0.500 |
O9a | 0.914 (4) | 0.661 (5) | 0.020 (5) | 0.62 (9) | 0.500 |
O10 | 0.676 (2) | 0.778 (2) | 0.549 (2) | 0.62 (9) | |
O11 | 0.252 (2) | 0.901 (2) | 0.558 (2) | 0.62 (9) | |
O12 | 0.068 (4) | 0.300 (4) | 0.577 (5) | 0.62 (9) | 0.500 |
O12a | 0.017 (3) | 0.204 (4) | 0.615 (4) | 0.62 (9) | 0.500 |
O13 | 0.004 (2) | 0.020 (3) | 0.237 (3) | 2.9 (4) | |
O14 | 0.028 (4) | 0.477 (4) | 0.511 (5) | 0.62 (9) | 0.500 |