pretations disagree on its nature (6-8). The thermal depen-Materials with La 2 NiO 4؉␦ composition, ␦ ranging between 0 dence of the conductivity of La 2 NiO 4ϩͳ (0 Յ ͳ Յ 0.05) has and 0.25, have been prepared either by solid state reaction or been described by Arrhenius laws with activation energies electrochemical oxidation; their electrical conductivity, thermoin the range 50-100 meV or in terms of Mott variable electric power, and magnetic susceptibility behaviors are rerange hopping (6, 9-13).
ported. The compounds remain semiconducting in the whole
The magnetic behavior of extra-oxygen doped range of composition and, at low temperature, the resistivity La 2 NiO 4ϩͳ materials exhibits many similarities with that of increases with ␦. The sign of the Seebeck coefficient changes the high T C superconductors La 2 CuO 4ϩͳ . Both compounds for ␦ Ȃ 0.2 and the thermal variation shows complex behavior. remain antiferromagnetically ordered above room temper-La 2 NiO 4.03 exhibits a thermally activated hopping and an antiature when nearly stoichiometric; this magnetic ordering ferromagnetic ordering. When ␦ increases, variable range hopdisappears for La 2 CuO 4ϩͳ for ͳ Ն 0.02 (1, 14) whereas for ping or nearest neighbor hopping, involving at least two types of La 2 NiO 4ϩͳ it has been observed for ͳ values up to Ȃ0.15 carriers, can account for the transport properties. The magnetic behavior of materials with ␦ Յ 0.18 reveals either 3D or 2D (5). The magnetic susceptibility as well as the Ne ´el tempermagnetic correlations whereas the susceptibility of La 2 NiO 4.25 ature of La 2 NiO 4ϩͳ strongly depend on the ͳ value (5, (La 8 Ni 4 O 17 ) follows a Curie-Weiss law below 170 K; a change 15-19) and highly stoichiometric La 2 NiO 4 shows nearly in the electronic configuration of the nickel ions arises beyond temperature independent behavior above 80 K (15).