Preparation and characterization of Ln9.33+x/3Si6−xAlxO26 (Ln = La, Nd and Sm) with apatite-type structure and its application to a potentiometric O2 gas sensor

The compacted sinters of Ln9.33+x/3Si6−xAlxO26 (0 ≤ x ≤ 2.0), (Ln = La, Nd and Sm) are composed of an apatite-like phase with a hexagonal structure. The existence of Ln2SiO5 was confirmed for x ≤ 1.0 as a minor product, but its formation was suppressed by the increasing Al-content. The densification of the sinters was also improved by partially replacing Si4+ with Al3+. For all the Ln systems, the highest ionic conductivity was observed at x = 1.5 in ambient air. The compacted sinters were used as a potentiometric oxygen sensor. The concentration dependence of EMF was well expressed by the Nernst equation, ΔEobs = (RT/nF  ) ln (PO2(I)/PO2(II))(PO2(I)/PO2(II)). Furthermore, the electron number n is comparable to the theoretical value of 4. The sensing characteristics of the sinters are comparable to those of the sensors with 3 and 8 mol% YSZ.