酸化アルミニウムマンガン中のMn³⁺生成に及ぼす焼成雰囲気の影響

抄録

The lattice constants and the molar fractions of Mn³⁺ 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 Mn³⁺ ions in these samples were obtained as a function of Po₂.
The lattice constants and the molar fraction of Mn³⁺ 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 Al³⁺ ions by Mn³⁺ ions taking place at high Po₂ caused the increase of lattice constants. The abrupt increase in molar fraction of Mn³⁺ ions and the lattice constants is interpreted as follows: At low Po₂ the distribution of cations in the spinel structure is normal as Mn²⁺ ions have the tetrahedral site preference. The increase of Po₂ favors the formation of Mn³⁺ ions, which prefer the octahedral sites. Therefore, the structure becomes intermediate between the normal and the inverse, and the molar fraction of Mn³⁺ ions and the lattice constants increased abruptly.