The lattice constants and the molar fractions of Mn
3+ ions in manganese aluminum oxides heat-treated under various oxygen partial pressures (
Po2) were measured and the effects of the atmosphere of heat-treatment on the valence and the coordination state of manganese ions were investigated.
Manganese carbonate and α-alumina were mixed in an equivalent ratio, and the mixture was heat-treated at 1400°C, 1300°C, or 1200°C under various
Po2 from 2.1×10
-1atm to 2.7×10
-14atm. The lattice constants of the manganese aluminum oxides obtained were determined precisely. For two kinds of samples, the change of weight was measured at 1400°C, 1300°C, or 1200°C under various
Po2, and then the molar fractions of Mn
3+ ions in these samples were obtained as a function of
Po2.
The lattice constants and the molar fraction of Mn
3+ ions changed similarly with
Po2,
i. e. they were almost constant at low
Po2, increased abruptly at a certain
Po2, and then increased gradually with increasing
Po2. It is supposed that the substitution of Al
3+ ions by Mn
3+ ions taking place at high
Po2 caused the increase of lattice constants. The abrupt increase in molar fraction of Mn
3+ ions and the lattice constants is interpreted as follows: At low
Po2 the distribution of cations in the spinel structure is normal as Mn
2+ ions have the tetrahedral site preference. The increase of
Po2 favors the formation of Mn
3+ ions, which prefer the octahedral sites. Therefore, the structure becomes intermediate between the normal and the inverse, and the molar fraction of Mn
3+ ions and the lattice constants increased abruptly.
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