Hexaamminecobalt(III) Chromate and Dichromate as Precursors of Nanoscale Spinels Co(Co1−xCrx)2O4 (x = 0.75; 0.90; 1)

The work develops methods for synthesizing complex salts: [Co(NH3)6]2(CrO4)3, [Co(NH3)6](CrO4)(NO3), [Co(NH3)6](CrO4)Cl·3H2O, [Co(NH3)6]2(Cr2O7)3·5H2O, [Co(NH3)6](Cr2O7)(NO3)·H2O and [Co(NH3)6](Cr2O7)Cl·H2O. The compounds were characterized by PXRD, IR and elemental analysis. It was demonstrated that mixed-anion complex salts ([Co(NH3)6](CrO4)(NO3) or [Co(NH3)6](CrO4)Cl·3H2O) form in the [Co(NH3)6]3+/(CrO4)2− system depending on the counterion of hexaamminecobalt(III), [Co(NH3)6](NO3)3 or [Co(NH3)6]Cl3, respectively. Conversely, it was more difficult to obtain mixed-anion complexes in the [Co(NH3)6]3+/(Cr2O7)2− system. Thus, to obtain [Co(NH3)6](Cr2O7)Cl·H2O an excess of chloride ions in the solution is required. At the same time, when the initial dichromate solution is acidified, the complex [Co(NH3)6]2(Cr2O7)3·5H2O is formed, exhibiting structural polymorphism: the crystal structure changes without altering the chemical composition when stored in a closed container. The process of thermal decomposition of all synthesized complex compounds in an inert atmosphere has been studied. The final product of the thermolysis of most compounds at a temperature of 600 °C is a nanoscale single-phase solid solution Co(Co1−xCrx)2O4 with a spinel structure (with crystallite sizes of no more than 15 nm), where the metal ratio corresponds to that of the initial complex salt. The smallest crystallite size (3–4 nm) was obtained through the thermolysis of [Co(NH3)6](CrO4)Cl.