Effects of Oxygen Partial Pressure and Tolerance Factor on Phase Selection of DyMnO₃

Abstract

The equilibrium crystal structure of LnMnO₃ (Ln: lanthanide) has been reported to be orthorhombic when relatively large ions from La³⁺ to Dy³⁺ are used as Ln³⁺ and hexagonal when relatively small ions from Ho³⁺ to Lu³⁺ are used. It has been reported that the hexagonal phase is formed when the tolerance factor, expressed as functions of radii of the constituent ions, is less than 0.840. In the present study, we attempted to induce oxygen deficiency in DyMnO₃ under the solidification at low oxygen partial pressure using aerodynamic levitator to reduce the tolerance factor through a decrease in the valence of manganese ions and the accompanying increase in the ionic radius. The results showed that the oxygen deficiency increases with decreasing oxygen partial pressure. Assuming that valence of manganese ions decreased due to the increase in oxygen deficiency, the corresponding tolerance factor evaluated from the average ionic radii of manganese and oxygen decreased, which promoted the formation of the hexagonal phase as is the case with the reduction of the ionic radius of Ln³⁺.

Fig. 3　Surface morphology of DyMnO₃ samples solidified under various P_O2 observed by confocal laser scanning microscopy.