samples of Ni0.1A0.7B0.2O2 (A=Ti,Sn; B=Sb, Nb) were also prepared (thermal history B).All compositions were prepared by the ceramic method. The starting materials were NiO (Merck), TiO2 anatase (Panreac), SnO2 (Panreac), Sb2O3 (Panreac) and Nb2O5 (Merck) of reagent grade chemical quality. The appropriate amounts of the starting materials were mixed and homogenized in acetone in a planetary ball mill for 15 min. Residual acetone was removed by evaporation. Dried samples were put into refractory crucibles and fired between 800 and 1400 C for 12 h of soaking times at each temperature.