Cobalt aluminate CoAl 2 O 4 powder, constituted of nano-sized crystallites, is prepared, involving the reactivity of AlCl 3 and CoCl 2 with molten alkali-metal nitrates. The reaction at 450 °C for 2 h leads to a mixture of spinel oxide Co 3 O 4 and amorphous γ-Al 2 O 3 . It is transformed into the spinel oxide CoAl 2 O 4 by heating at 1000 °C. The powders are mainly characterized by XRD, FTIR, ICP, electron microscopy and diffraction, X-EDS and diffuse reflection. Their properties are compared to those of powders obtained by solid state reactions of a mechanical mixture of chlorides or oxides submitted to the same thermal treatment.

Submicronic CoAl 2 O 4 powders were prepared by double decomposition reaction between solid LiAlO 2 and molten KCoCl 3 at 500 • C for 24 h. The reaction mechanism involves the dissolution of LiAlO 2 shifted by the precipitation of CoAl 2 O 4 until complete transformation and the reaction leads to powders with a very homogeneous chemical composition. The powders obtained were mainly characterized by XRD, FTIR, ICP, X.EDS, electron microscopy and diffraction and diffuse reflexion. The blue pigments obtained exhibit a high thermic stability allowing their use for the colouring of ceramic tiles.