Abstract
The change of the lattice constant of manganese, iron, or cobalt alminate with the spinel structure by the valence and/or the coordination state of the transition metal was investigated. The carbonate or the oxide of manganese, iron, or cobalt and a-alumina was mixed in the equivalent ratio, and the mixture was heat-treated at 1300°C or 1400°C. The oxygen partial pressure (PO2) of the atmosphere was varied from 2.1 × 10-1 atm to 4.33 × 10-12 atm for the heattreatment at 1300°C, and from 2.1 × 10-1 atm to 3.98 × 10-11 atm at 1400°C. The crystalline phases of the product were identified by the X-ray powder diffractometry. The results are listed in Table 1. For the spinel phase, the lattice 'constant was determined by the precise measurement. The lattice constant of the spinel phase is shown as a function of Po2 in Fig.3.
( 1 ) Co-Al-O system: The lattice constant was not changed for all the temperature an d Po2 The spinel phase was CoALO, and it is supposed that the valence of the cobalt ions w ere kept bivalent. ( 2 ) Fe-Al-O system: The lattice constant was held almost constant in the lower Po2 region, and increased with the increase of Po2. The spinel phase was the solid solution of FeAl2O4 and Fe3O4, i. e. the Fes+ ions were formed in the higher Po2 region, and substituted for the AP+ ions. ( 3 ) Mn-Al-O system: MnAAl2O4 was formed. The lattice constant was held almost constant in the lower Po2 region, and increased with the increase of Po2. It is supposed that the Mils ions were formed in the higher Po2 region, and substituted for the AP+ions, similarly to the Fe-Al-O system. There observed a bending of the curve in Fig.3. It would be interpreted as follows; In the lower Po2 region, the distribution of the spine' phase was normal as the Mn2+ ions have the tetrahedral site preference. The increase of Po2 favored the formation of the Mns3+ ions, which prefer the octahedral site. Therefore the spinel was intermediate phase between normal and inverse, and the lattice constant increased abruptly.
