Crystal Structure and Oxide-Ion Conductivity along c-Axis of Apatite-Type Lanthanum Silicate with Excess Oxide Ions

Abstract

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.

Study of the formation of the apatite-type phases La9.33+x(SiO4)6O2+3x/2 synthesized from a lanthanum oxycarbonate La2O2CO3

“…This value is measured on polycrystalline material, and is higher than what can be obtained on oriented or monocrystalline material. However these results are in good agreement with what is reported in literature [11,13,17,32] and assume that the oxide-ionic conductivity in this apatite-type structure is due to interstitial oxide ions [10,15,16,23]. This new elaboration route has no damaging effect on the ionic conductivity.…”

“…No difference in activation energy values between these compositions is observed. It suggests that both materials present the same oxygen interstitial mechanism along the channel [6,13,16].…”

“…This explains that they can exhibit high anisotropic conductivity in the c direction increasing with the incorporation of interstitial oxygen inside large channel (formula with x > 0) [6e8, 10,13e16]. For example, highly oriented La 9.5 (SiO 4 ) 6 O 2.5 apatite crystals of a few micrometers have been prepared from La 2 Si 2 O 7 and La 2 SiO 5 with a long time heating at 1650 C and exhibit a high conductivity value along c-axis of 7.9 Â 10 À2 S cm À1 at 800 C with a low activation energy value of 0.35 eV [13]. This conductivity is higher than the 2.39 Â 10 À2 S cm À1 measured at 800 C on a c-oriented apatite polycrystal of La 9.33 (SiO 4 ) 6 O 2 [6].…”

Local structure and oxide-ion conduction mechanism in apatite-type lanthanum silicates

“…They are also very sensitive to the amount of extra oxide ions present in the structure [4–6]. The conductivities of the La 9.50 (SiO 4 ) 6 O 2.25 compound measured at 723−973 K are, for example, larger by a factor of 2.5 with respect to those of the oxygen stoichiometric La 9.33 (SiO 4 ) 6 O 2 compound [6]. Their structure is made of isolated SiO 4 tetrahedral units forming two distinct channels parallel to the c-axis (see Figure 1) [7,8].…”

Chemical solution deposition of the highly c-axis oriented apatite type lanthanum silicate thin films

“…Actually, high temperature operation of SOFCs can cause several problems, such as limitations on the component materials (collector, housing, etc. 1600°C) required for sintering, [14][15][16][17][18] and the other is difficulty in the fabrication of thinner films with thicknesses of few micrometers for SOFC electrolyte applications. Therefore, new electrolyte materials have been explored to enable efficient operation at lower temperatures (<500°C).…”