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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

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768107024317/lm5010sup1.cif
Contains datablock La10(GeO4)5(GeO5)O2

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108768107024317/lm5010sup2.hkl
Contains datablock I

txt

Text file https://doi.org/10.1107/S0108768107024317/lm5010sup3.txt
Supplementary material

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
La10Ge6O27β = 88.079 (5)°
Mr = 2256.70γ = 120.890 (4)°
Triclinic, P1V = 616.8 (1) Å3
Hall symbol: -P_1Z = 1
a = 9.9346 (7) ÅF(000) = 978
b = 9.9132 (7) ÅDx = 6.076 Mg m3
c = 7.3021 (5) ÅNeutron radiation, λ = 1.8834 Å
α = 90.960 (5)°T = 300 K
Data collection top
Local
diffractometer
Data collection mode: reflection
Ge 115 monochromatorScan method: Stationary detector
Specimen mounting: vanadium can with He exchange gas
Refinement top
Least-squares matrix: fullExcluded region(s): 0.027 to 8.97 degrees unreliable due to background
Rp = 0.039129 parameters
Rwp = 0.0500 restraints
Rexp = 0.07023 constraints
RBragg = 0.014Weighting scheme based on measured s.u.'s
3001 data points
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzBiso*/BeqOcc. (<1)
La10.341 (1)0.641 (1)0.006 (2)0.2 (2)
La20.302 (1)0.667 (1)0.508 (2)0.2 (2)
La30.235 (1)0.993 (1)0.241 (2)0.4 (1)
La40.010 (1)0.251 (1)0.264 (2)0.4 (1)
La50.771 (1)0.797 (1)0.228 (2)0.4 (1)
Ge10.398 (1)0.370 (1)0.282 (2)0.2 (1)
Ge20.614 (1)0.015 (1)0.254 (2)0.2 (1)
Ge30.992 (3)0.597 (3)0.231 (3)0.2 (1)0.500
Ge3a0.936 (3)0.574 (3)0.257 (3)0.2 (1)0.500
O10.293 (2)0.465 (2)0.281 (2)0.62 (9)
O20.502 (2)0.818 (2)0.258 (2)0.62 (9)
O30.212 (4)0.701 (5)0.207 (5)0.62 (9)0.500
O3a0.149 (4)0.665 (4)0.202 (5)0.62 (9)0.500
O40.615 (2)0.484 (2)0.295 (2)0.62 (9)
O50.529 (2)0.135 (2)0.231 (2)0.62 (9)
O60.882 (4)0.389 (4)0.201 (5)0.62 (9)0.500
O6a0.824 (4)0.388 (4)0.240 (5)0.62 (9)0.500
O70.362 (2)0.278 (2)0.060 (2)0.62 (9)
O80.769 (2)0.052 (2)0.094 (2)0.62 (9)
O90.885 (4)0.637 (4)0.069 (5)0.62 (9)0.500
O9a0.914 (4)0.661 (5)0.020 (5)0.62 (9)0.500
O100.676 (2)0.778 (2)0.549 (2)0.62 (9)
O110.252 (2)0.901 (2)0.558 (2)0.62 (9)
O120.068 (4)0.300 (4)0.577 (5)0.62 (9)0.500
O12a0.017 (3)0.204 (4)0.615 (4)0.62 (9)0.500
O130.004 (2)0.020 (3)0.237 (3)2.9 (4)
O140.028 (4)0.477 (4)0.511 (5)0.62 (9)0.500
 

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