ИСТИНА

Complex perovskite-related cobaltates with Co3+ cations exhibit excellent electrocatalytic properties in the oxygen reduction reaction and are considered as prospective cathode materials in intermediate temperature solid oxide fuel cell (IT-SOFC) [1]. However, they suffer from high thermal expansion coefficient (TEC) due to thermally activated transition between low (LS) and high-spin (HS) state of Co3+. In the present work, we discuss influence of this transition on the high-temperature thermal expansion properties of perovskite-related Co-containing oxides. Pr2-xSrxNi1-xCoxO4-y (x= 0.25; 0.5; 0.75) oxides with the tetragonal K2NiF4-type structure have been prepared. Room-temperature neutron powder diffraction (NPD) study of x = 0.25 and 0.75 phases together with iodometric titration results have shown the formation of hyperstoichiometric oxide for x=0.25 (y=0.09(2)) and a stoichiometric one for x=0.75. High-temperature X-ray powder diffraction (HT XRPD) showed substantial anisotropy of the thermal expansion coefficient (TEC) along the a- and c-axis of the crystal structure, which increases with increasing the Co content from TEC(c)/TEC(a) = 2.4 (x= 0.25) to 4.3 (x=0.75). High-temperature NPD (HT NPD) study of the x = 0.75 sample reveals that a very high expansion of the axial (Ni/Co)–O bonds (75.7 ppm K-1 in comparison with 9.1 ppm K-1 for equatorial ones) is responsible for such behaviour, and is caused by a temperature-induced transition between low- and high-spin states of Co3+ [2]. One of the ways to decrease TEC of cobaltates is to stabilize HS Co3+ in the ground state. Such compounds can be found among cobaltates with the brownmillerite-type structure. NPD at 2K together with soft X-ray absorption spectroscopy at the Co-L2,3 edges studies showed the presence of HS Co3+ in the ground state in CoO6 octahedra of Sr2Co1.2Ga0.8O5 with the brownmillerite structure [3]. This cobaltate is found to possess as low TEC as 13.1 ppm K-1 (298-1073K).