mixtures of nickel and iron mixtures of nickel and manganese mixtures of nickel and cobalt mixtures of nickel and titanium mixtures of nickel and cobalt and chromium mixtures of nickel and cobalt and chromium and manganese mixtures of nickel and chromium and manganese chromium nickel copper and or of particles made of an oxide of spinel structure cd 2 o 4 or d c d o 4 which may comprise one indeed even several of the following optional characteristics the element c of the spinel structure is chosen from the group g c 1 formed by nickel ni in a molar fraction of between 0 and 0 2 or in a molar fraction equal to 1 copper cu in a molar fraction of between 0 and 0 2 iron fe in a molar fraction of between 0 2 and 0 6 or in a molar fraction equal to 1 zinc zn in a molar fraction of between 0 and 0 2 or in a molar fraction equal to 1 manganese mn in a molar fraction of between 0 and 0 4 cobalt co in a molar fraction of between 0 and 0 4 or in a molar fraction of between 0 4 and 1 tin sn in a molar fraction of between 0 and 0 2 or in a.