Effect of calcination temperature on Co(II) oxalate dihydrate–iron(II) oxalate dihydrate mixture: DTA–TG, XRD, Mössbauer, FT-IR and SEM studies (Part II)

Effect of calcination temperature on the physico-morphological properties of CoC2O4·2H2O–FeC2O4·2H2O (1:2 mole ratio) mixture has been investigated using DTA–TG, XRD, Mössbauer effect (ME) spectroscopy, FT-IR and SEM measurements. DTA–TG results showed that the decomposition proceeds via four well-defined steps. The first is attributed to dehydration and formation of anhydrous mixture which decomposed in the second step to give CoC2O4–Fe2O3 mixture. In the third step, CoC2O4 decomposed and Co3O4–Fe2O3 mixture is formed. This mixture is thermally stable up to 920 °C, then Co3O4 reduced to CoO, which reacts directly with α-Fe2O3 to form CoFe2O4, in the fourth step. ME spectra demonstrated that part of α-Fe2O3 is formed in a superparamagnetic doublet state in the early stages of decomposition. As the temperature increased to 800 °C, the crystallites are grown and the superparamagnetism disappeared. At 1000 °C, a ferrimagnetic ordering ME spectrum fitted into two Zeeman sextets due to Fe3+ at the two distinct sites of CoFe2O4 inverse spinel has been obtained. XRD patterns of samples calcined at 250 and 280 °C revealed none of XRD lines characteristic of α-Fe2O3, while the pattern of that fired at 1000 °C confirmed the presence of single-phase cubic spinel structure with no evidence of impurities. FT-IR and SEM studies are consistent with the previous results. Kinetic analysis of the dehydration and the oxalate decomposition reactions was performed under non-isothermal conditions using different integral methods of analysis. The dynamic TG curves showed a sigmoid shape and obeyed the Avrami–Erofeev equation characteristic of the solid state nucleation–growth mechanism. The activation parameters were calculated and discussed.