Apatite-type lanthanum silicates of general formula La9.33+2x/3(SiO4)6O2+x have appeared recently as a new promising class of oxide ion conductors with potential applications as electrolytes for solid oxide fuel cells (SOFCs). They have been shown to demonstrate relatively high oxide ion conductivity at moderate temperatures as well as at low oxygen partial pressures. In this paper, the diffusion pathways and the conduction mechanism of oxide ions in these phases are reinvestigated. This is done by means of atomic scale computer modeling techniques with both semiempirical and bond valence methods. Our results support that oxide ion conduction along the c-axis proceeds by an interstitial mechanism. They also support the presence of interstitial sites located within the conduction channel. However, contrarily to recent research, it is shown that the channel oxide ions are involved in the conduction process by a push−pull type mechanism. This mechanism brings into play a cooperative movement of both two adjacent interstitial oxide ions forming a complex defect and the channel oxide ions. This complex defect is shown to move along the c-axis via a nonlinear pathway different from the conduction path proposed in literature to date. The calculated migration energy of this mechanism is found to be equal to 0.32 eV, which compares well with activation energy measured along the c-axis for Nd9.33(SiO4)6O2 single crystals.

We have successfully synthesized the highly c-axis-oriented polycrystals of apatite-type lanthanum silicate by the reactive diffusion technique. When the La 2 SiO 5 /La 2 Si 2 O 7 diffusion couples were isothermally heated at 1773−1873 K for 5−100 h, the apatite polycrystals were readily produced in the form of a layer at the interfacial boundaries. The annealed couples were characterized using optical microscopy, micro-Raman spectroscopy, X-ray diffractometry, and electron probe microanalysis. The product layers were composed of the highly c-axis-oriented prismatic crystallites, with their elongation directions being almost parallel to the diffusion direction. The formation of the apatite layer was controlled by volume diffusion, the overall reaction of which is described by (10 + 6x)La 2 SiO 5 + (4−3x)La 2 Si 2 O 7 → 3La 9.33+2x (SiO 4 ) 6 O 2+3x (0.01 ≤ x ≤ 0.13). The apatite layer formed at 1873 K was characterized by the steady decrease of the x-value along the diffusion direction from 0.13 at the La 2 SiO 5 /apatite interface to 0.01 at the apatite/La 2 Si 2 O 7 interface. We have also prepared sandwich-type La 2 Si 2 O 7 /La 2 SiO 5 /La 2 Si 2 O 7 diffusion couples and heated them at 1873 K for 100 h. The annealed couple was mechanically processed, and the thin-plate electrolyte consisting of the highly c-axis-oriented polycrystal was obtained. The oxide-ion conductivity was determined from the impedance spectroscopy data at 573−973 K, which steadily increased from 2.4 × 10 −3 S/cm to 2.39 × 10 −2 S/cm with increasing temperature. The empirical activation energy of conduction was 0.35 eV, which compares well with the calculated migration energy of 0.32 eV in a previous study.

We have prepared the highly c-axis-oriented polycrystalline material of apatite-type La 9.50 Si 6 O 26.25 by isothermal heating of the sandwich-type La 2 SiO 5 /La 2 Si 2 O 7 / La 2 SiO 5 diffusion couple at 1873 K for 50 h. The resulting polycrystal was characterized using optical microscopy, X-ray diffractometry, and impedance spectroscopy. The annealed couple was mechanically processed, and the thin-plate electrolyte consisting of the textured polycrystal was obtained. The oxide-ion conductivity along the c-axis steadily increased from 2.0 × 10 −2 S/cm to 7.9 × 10 −2 S/cm with increasing temperature from 723 to 1073 K. The conductivity of this material was, at 723−973 K, about 2.5 times higher than that of the c-axis-oriented apatite polycrystal of La 9.33 Si 6 O 26 . These two materials have the identical activation energy of conduction (0.35 eV), and hence the conduction mechanism must be the same. Both crystal structures of La 9.50 Si 6 O 26.58 and La 9.33 Si 6 O 26 at ambient temperature (space group P6 3 /m) showed the appreciable positional disordering of O atoms (12i site) that are bonded to Si atoms, together with the anharmonic displacements of La atoms (4f and 6h sites). The former structure is further characterized by the positional disordering of channel oxide ions (2a and 4e sites) as well as the presence of interstitial oxide ions (6h site), which would contribute to the higher conductivity along the c-axis.

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International audienceThe oxide-ion conductivity was successfully improved for the apatite-type lanthanum silicate polycrystal by combining the beneficial use of germanium doping and texturing methods. We synthesized the highly c-axis-oriented polycrystalline material La9.33(Si0.87Ge0.13)6O26 by isothermal heating of the sandwich-type La2Si2O7/La2(Si0.833Ge0.167)O5/La2Si2O7 diffusion couples at 1873 K for 50 h. The resulting polycrystal was subsequently characterized using optical microscopy, micro-Raman spectroscopy, X-ray diffractometry, and impedance spectroscopy. The annealed couple was mechanically processed, and the thin-plate electrolyte consisting of the textured polycrystal was obtained. The oxide-ion conductivity along the c-axis steadily increased from 2.0 × 10-3 S/cm to 6.92 × 10-2 S/cm with increasing temperature from 673 to 973 K, with the empirical activation energy of conduction being 0.75 eV. The crystal structure at ambient temperature (space group P63/m) showed the appreciable positional disordering of O atoms (12i site) that are bonded to Si atoms, together with the anharmonic displacements of La atoms (4f and 6h sites). The disordered structure might be closely related to the fast oxide-ion conduction at high temperatures

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We have prepared the highly c-axis-oriented polycrystalline material of Si-deficient apatite-type lanthanum silicate by isothermal heating of the sandwich-type La 2 SiO 5 / La 2 Si 2 O 7 /La 2 SiO 5 diffusion couple at 1873 K for 100 h. The resulting polycrystal of La 9.50 (Si 5.87 □ 0.13 )O 26 , where □ denotes a vacancy in Si site, was characterized using optical microscopy, X-ray diffractometry, and impedance spectroscopy. The annealed couple was mechanically processed, and the textured thin-plate electrolyte was obtained. The ionic conductivity (σ) along the c-axis steadily increased from 1.6 × 10 −2 S/cm to 1.26 × 10 −1 S/cm with increasing temperature from 623 to 1073 K. The Arrhenius plot of σ showed the marked slope change at ca. 800 K; the activation energies of conduction were, above and below 800 K, 0.53 and 0.17 eV, respectively. The crystal structure of La 9.50 (Si 5.87 □ 0.13 )O 26 at ambient temperature (space group P6 3 /m) showed the appreciable positional disordering of O atoms (12i site) that are bonded to Si atoms, together with the anharmonic displacements of La atoms (4f and 6h sites). The Si-deficient apatite was formed by the extraction of the SiO 2 component from the La 2 O 3 -excess apatite according to La 9.33+2x Si 6 O 26+3x − 1.5xSiO 2 → La 9.33+2x (Si 6−1.5x □ 1.5x )O 26 (x ∼ 0.087).

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