Oxide ion conduction anisotropy deconvoluted in polycrystalline apatite-type lanthanum silicates

Effect of Crystallographic Anisotrophy and Lanthanum Stoichiometry on Microstructural Evolution of Lanthanum Silicate Electrolytes

“…There can be also a qualitative change in defect chemistry at the stoichiometric point. The oxide ion conductivity of the lanthanum silicates below 600°C was shown to fall into two groups with different activation energy and conductivity magnitude . There is also difference attributed to the phase stability.…”

“…The observation can be attributed to the sluggish phase transformation kinetics as discussed above. In our previous work, a solid solution for −0.5≤ x ≤ 0 was suggested based on the composition variation with x in the high‐temperature sintered samples, which, however, should be considered as the average value of the two phases, La 9.33+ x (SiO 4 ) 6 O 2+1.5 x and La 2 Si 2 O 7 , as clearly indicated by the BSE image compared with the SE image in Fig. .…”

“…Similar behavior in lanthanum silicates has been also reported, where the glassy phase appears to form even at moderate sintering temperatures of 1550°C. Compositional analysis on the inner region of the large grains also suggests the saturation of lanthanum content x = +0.4 in the bulk apatite material at 1700°C but more accurate examination seems necessary to determine the exact solid solution extent. As indicated in Figs.…”

“…Another batch series (#2) with x = −0.5, 0, +0.5, +0.75, +1, +1.25, and +1.5 was prepared without the sieving processes and sintered at lower temperature of 1700°C. The third series batches were prepared by milling using a planetary mill (Pulverisette 6, Fritsch, Germany) before and after calcination in addition to the procedure for batch series #1 as reported elsewhere . The X‐ray diffraction patterns of the calcined powders (batch series #1) and sintered powders (batch series #3) were obtained over a 2θ range of 10–80° (XRD‐7000, Shimadzu, Japan).…”

“…The third series batches were prepared by milling using a planetary mill (Pulverisette 6, Fritsch, Germany) before and after calcination in addition to the procedure for batch series #1 as reported elsewhere. 15 The X-ray diffraction patterns of the calcined powders (batch series #1) and sintered powders (batch series #3) were obtained over a 2h range of 10-80°(XRD-7000, Shimadzu, Japan). Macroscale microstructural examination of as-sintered surfaces was performed by optical microscopy (FS-70, Mitutoyo, Japan).…”