FORMATION OF COPPER-ZINC CHROMITES

The compounds of a hydrotalcite structure formed from copper-zinc-chro mium hydroxycarbonate at an increasing temperature of calcination are studied. At temperatures ranging from 300 to 350 degrees C, the decomp osition of-hydroxycarbonate occurs to yield anion-modified oxides in w hich chromium cations are partially present in the degree of oxidation +6. The structure of the anion-modified oxide is governed by the natu re of a bivalent cation and differs from that of spinel. The formation of the spinel structure in the copper-chromium system is observed at temperatures above 450 degrees C. The distribution of cations over the ir crystallographic positions in the copper-zinc-chromium spinels depe nds on the Cu :Zn ratio and calcination temperature.