Abstract
Oxygen non-stoichiometry, electrical conductivity and thermal expansion of La 2− x Sr x NiO 4− δ phases with high levels of strontium-substitution (1 ≤ x ≤ 1.4) have been investigated in air and oxygen atmosphere in the temperature range 20–1050 °C. These phases retain the K 2NiF 4-type structure of La 2NiO 4 (tetragonal, space group I4/ mmm). The oxygen vacancy fraction was determined independently from thermogravimetric and neutron diffraction experiments, and is found to increase considerably on heating. The electrical resistivity, thermal expansion and cell parameters with temperature show peculiar variations with temperature, and differ notably from La 2NiO 4± δ in this respect. These variations are tentatively correlated with the evolution of nickel oxidation state, which crosses from a Ni 3+/Ni 4+ to a Ni 2+/Ni 3+ equilibrium on heating.
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