Tuning the high-temperature properties of Pr2NiO4+δby simultaneous Pr- and Ni-cation replacement
Novel Pr 2Àx Sr x Ni 1Àx Co x O 4AEd (x ¼ 0.25; 0.5; 0.75) oxides with the tetragonal K 2 NiF 4 -type structure have been prepared. Room-temperature neutron powder diffraction (NPD) study of x ¼ 0.25 and 0.75 phases together with iodometric titration results have shown the formation of hyperstoichiometric oxide for x ¼ 0.25 (d ¼ 0.09(2)) and a stoichiometric one for x ¼ 0.75. High-temperature X-ray powder diffraction (HT XRPD) showed substantial anisotropy of the thermal expansion coefficient
... EC) along the a-and c-axis of the crystal structure, which increases with increasing the Co content from TEC High-temperature NPD (HT NPD) study of the x ¼ 0.75 sample reveals that a very high expansion of the axial (Ni/Co)-O bonds (75.7 ppm K À1 in comparison with 9.1 ppm K À1 for equatorial ones) is responsible for such behaviour, and is caused by a temperature-induced transition between low-and high-spin states of Co 3+ . This scenario has been confirmed by high-temperature magnetization measurements on a series of Pr 2Àx Sr x Ni 1Àx Co x O 4AEd samples. For compositions with high Ni content (x ¼ 0.25 and 0.5) we synthesised K 2 NiF 4 -type oxides Pr 2ÀxÀy Sr x+y (Ni 1Àx Co The studies of the TEC, high-temperature electrical conductivity in air, chemical stability of the prepared compounds in oxygen and toward interaction with Ce 2Àx Gd x O 2Àx/2 (GDC) at high temperatures reveal optimal behaviour of Pr 1.35 Sr 0.65 Ni 0.75 Co 0.25 O 4+d . This compound shows stability in oxygen at 900 C and does not react with GDC at least up to 1200 C. It features low TEC of 13 ppm K À1 and high-temperature electrical conductivity in air of 280 S cm À1 at 900 C, thus representing a promising composition for use as a cathode material in intermediate temperature solid oxide fuel cells (IT-SOFC).