Structure and high-temperature properties of the (Sr,Ca,Y)(Co, Mn)O 3-y perovskites - Perspective cathode materials for IT-SOFC

Oxygen deficient perovskites Sr0.75Y0.25Co1 � xMnxO3� y, x¼ 0.5 and 0.75, were prepared by using the citrate route at 1373-1573 K for 48 h. The cubic Pm-3m perovskite structure for x¼ 0.5 was confirmed by electron diffraction study and refined using neutron powder diffraction (NPD) data. For x ¼0.75, the superstructure corresponding to a¼ ffiffiffi 2 p � aper, b¼ 2 � aper, c ¼ ffiffiffi pa per (a 0 bbtilt system, space group Imma) was revealed by electron diffraction. The solid solution Sr0.75 � xCaxY0.25Co0.25Mn0.75O3 � y, 0.1r xr 0.6 and compound Ca0.75Y0.25Mn0.85Co0.15O2.92 were prepared in air at 1573 K for 48 h. The crystal structure of Ca0.75Y0.25Mn0.85Co0.15O2.92 was refined using NPD data (S.G. Pnma, a¼ 5.36595(4), b¼ 7.5091(6), c ¼ 5.2992(4) ˚ A, Rp¼ 0.057, Rwp¼ 0.056, w 2 ¼ 4.26). High-temperature thermal expansion properties of the prepared compounds were studied in air using both dilatometry and high- temperature X-ray powder diffraction data (HTXRPD). They expanding non-linearly at 298-1073 K due to the loss of oxygen at high temperatures. Calculated average thermal expansion coefficients (TECs) for Sr0.75Y0.25Co1� xMnxO3 � y, x¼ 0.5, 0.75 and Ca0.75Y0.25Mn0.85Co0.15O2.92(1) are 15.5, 15.1, and 13.8 ppm K � 1 , respectively. Anisotropy of the thermal expansion along different unit cell axes was observed for Sr0.15Ca0.6Y0.25Co0.25Mn0.75O3 � y and Ca0.75Y0.25Mn0.85Co0.15O2.92. Conductivity of Sr0.75Y0.25Co1� xMnxO3� y, x¼ 0.5 and 0.75 increases with the temperature reaching 110 S/cm for x¼ 0.5 and 44 S/cm for x¼ 0.75 at 1173 K. Samples of Sr0.75 � xCaxY0.25Co0.25Mn0.75O3� y, 0.1r yr 0.6 were found to be n-type conductors at room temperature with the similar temperature dependence of the conductivity and demonstrated the increase of the s value from � 1t o� 50 S/cm as the temperature increases from 300 to 1173 K. Their conductivity is described in terms of the small polaron charge transport with the activation energy (Ep) increasing from 340 to 430 meV with an increase of the calcium content from x¼ 0t ox¼ 0.6.