CHARACTERIZATION OF COPRECIPITATED ALUMINUM-CHROMIUM MIXED HYDROXIDESAND OF THE PRODUCTS OF THEIR CALCINATION

Crystalline phases have been coprecipitated from Al/Cr mixed-nitrate a queous solutions with ammonium carbonate. These phases have been chara cterized as probably layered hydroxycarbonates with the likely composi tion (NH4)(2)(Al,Cr)(6)(OH)(14)(CO3)(3) . H2O, using XRD, FTIR and TG- DTA analyses. The calcination of these materials gives rise to amorpho us mixed oxides where Cr is partly in the hexavalent state, while furt her calcination at 1173 K gives rise to polyphasic materials. Samples with Al:Cr atom ratio between 5.1 and 1.1 give rise to four phase mate rials with three corundum-type phases and a spinel-type phase. All the se phases contain both Al and Cr.