THERMAL-BEHAVIOR IN AIR AND REACTIVITY IN ACID-MEDIUM OF COBALT MANGANESE SPINELS MNXCO3-XO4 1-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-3) SYNTHESIZED AT LOW-TEMPERATURE

The thermal behavior in air of manganese cobalt oxides MnxCo3-xO4 (1 l ess than or equal to x less than or equal to 3) obtained at low temper ature has been studied by X-ray powder diffraction using the Rietveld method of structural refinement. The existence of three well-defined s olid solutions is established, and their field of existence delimited. Cubic Fd3m spinel-type solid solution is formed in the compositional range x less than or equal to 1.4, the cell parameter ranging from 8.0 769(2) for Co3O4 (x = O) to 8.3216(3) Angstrom (x = 1.4). The same tre nd is observed for the cell constants a and c of the tetragonal (space group I4(1)/amd) single phase formed in the interval 2.0 less than or equal to x less than or equal to 3.0, with distortion parameter (c/a' , a' a root 2) ranging from 1.139 to 1.162. For samples 1.4 < x ( 2.0 quenched from 1000 degrees C, the formation of a tetragonal single pha se has been attained for the first time. For this solid solution, the distortion parameter varies from 1.04 to 1.135. Soaking of spinel-like oxides with 0.25 M HCl for 24 h at room temperature yields cubic coba lt deficient spinels, through a topotactic reaction. A redox mechanism that accounts for this transformation is pointed out. For the oxide M nxCo3-xO4 (x = 2.6), the acid treatment leads to the formation of a la mbda-MnO2 type phase (a = 8.033(2) Angstrom) with a very low cobalt co ntent (approximate to 3.2%).