Samples of CeO2 doped with oxides such as CaO and Gd203 were prepared. Their conductivities and expansions on reduction were measured at 1000~ and the thermal expansion coefficients in the range 50 to 1000~ were determined. The ionic and electronic conductivity were derived from curves of total conductivity vs. oxygen partial pressure. For both types of conductivity a dependence on dopant valency was observed. The electronic conductivity was independent of dopant radius in contrast to the ionic which was highly dependent. These measured physical properties are compared with the ideal requirements for solid oxide fuel cell anodes. Not all requirements are fulfilled. Measures to compensate for this are discussed.

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The new compounds La4Al2O9, Pr4Al2O9, and Tb4Al2O9 were identified in the system 2Ln2O3.Al2O3. X‐ray powder diffraction showed that they were isomorphous with the previously characterized Ln4Al2O9 (Ln = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Y) and Ln4Ga2O9 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd). La4Al2O9: P21/c, a = 7.97 Å, b = 11.09 Å, c = 11.41 Å, β= 108.8°, Z = 4; Pr4Al2O9: P21/c, a = 7.80 Å, b = 11.03 Å, c = 11.31 Å, β= 109.6, Z = 4; and Tb4Al2O9: P21c, a= 7.40 Å, b= 10.58 Å, c = 11.16 Å, β= 107.2°, Z = 4. The lanthanum and praseodymium compounds were metastable phases in the Ln4Al2O9 systems and transformed into LaAlO3 and La2O3, and PrAlO3 and PrO1.83 upon prolonged heating at 1200°C, respectively. Ln4Al209 (Ln = La, Pr) were synthesized by precipitation of A1(OH)4−with Ln3+(aq) and subsequent heating at 900°C for 10 h. Tb4AI2O9 required calcination at 1200°C for 10 h. The formation of La4Al2O9 and Pr4Al2O9 was followed by TGA‐DSC, which was also used to assess their thermal stability. Several other members of the above family of compounds were similarly synthesized by the new synthesis route. Particularly, for Nd4Al2O9 this represented a significantly improved method.

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