Effects of deviations from stoichiometry, oxygen excess and substitution of Cr for Fe on the chemical transport of hematite with TeCl 4 as a transporting agent

Abstract

Single crystals of pure hematite (α-Fe 2O 3) and hematite in which the iron is partially replaced by chromium (Fe 2- y Cr y O 3) can be obtained by chemical transport using TeCl 4 as a transporting agent. A thermodynamic analysis has shown the transport to be substantially affected by some additional conditions such as slight deviations from stoichiometry and small amounts of excess oxygen and dopants. The transport of pure, practically stoichiometric hematite occurs at temperatures above 900°C in the direction from the hot to the cold zone. At pressures below 1 MPa and temperatures ranging from 750 to 900°C (which correspond to about 12 mg/ml TeCl 4), the transport proceeds in the opposite direction. However, in a similar temperature range, the transport reaction proceeds in the direction hot → cold again, on introducing small excess oxygen amounts into the ampoule and in cases when the hematite shows some oxygen deficiency. From a mixture of Fe 2O 3/Fe 3O 4, the hematite is transferred to the cold zone. Chromium-containing hematite is also transported to the cold zone. The composition of the mixed Fe 2- y Cr y O 3 crystals obtained is different from that of the initial substance. All results of the thermodynamic analysis were confirmed by experiments.