Low-temperature preparatioin and characterization of the perovskite lanthanide chromites and related oxides

Abstract

Conclusions

The thermal decomposition of lanthanum biscitrato chromate(III) dihydrate was investigated in both static air and flowing argon atmospheres. The complex decomposes in four distinct steps, forming oxycarbonate and orthochromate(V) as intermediate products. Simple citrates such as chromium citrate and lanthanum citrate were synthesized, characterized, and their thermal decomposition behavior studied in static air. Probable decomposition schemes have been proposed for these compounds. The decomposition behavior of a mechanical mixture of lanthanum citrate (LC) and chromium citrate (CC) was compared with that of the citrate complex LCC and the individual citrates. These studies confirm the single-phase nature of LCC. A logical decomposition scheme has been proposed for the thermal breakdown of lanthanum biscitrato chromate(III) dihydrate in air. LaCrO? is obtained at a temperature as low as 600?°C by isothermal decomposition of LCC, which is significantly lower than the conventional ceramic method (>900?°C). Lanthanide biscitrato chromate(III) hydrates of the general formula Ln[Cr(C?H?O?)?]·nH?O (where Ln = Pr, Nd, Sm, Gd, Dy, Ho, Yb, Lu, Y) were prepared and characterized using various physicochemical techniques. Thermal decomposition of these complexes (Ln = Pr, Nd, Dy, Ho) was studied in static air and flowing argon. Stable oxycarbonates and orthochromates(V) were isolated as intermediates, and a general decomposition scheme was proposed. The formation temperature of heavier lanthanide chromites (e.g., Dy and Ho at ~750?°C) is considerably higher than that of lighter lanthanides (e.g., Pr and Nd at ~600?°C). Additional Findings Controlled combustion of lanthanide chromium biscitrato complexes enables the synthesis of phase-pure lanthanide chromates(V) at relatively low temperatures. Formation of the chromate(V) phase is strongly dependent on oxygen partial pressure; in non-oxidizing atmospheres such as flowing argon, this phase does not form. The thermal stability of LnCrO?(V) can be significantly influenced by varying oxygen partial pressure and by partial substitution of chromium with Ti?? or Cu²?. Such substitutions also modify the morphology, physicochemical properties, and spectral characteristics of these chromates(V).