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

Abstract

The equilibrium crystal structure of LnMnO₃ (Ln: lanthanide) is known to be orthorhombic when larger ions from La³⁺ to Dy³⁺ are used as Ln³⁺, and hexagonal when smaller ions from Ho³⁺ to Lu³⁺ are used. Research indicates that the hexagonal phase forms when the tolerance factor, expressed as functions of radii of the constituent ions, is less than 0.840. In this study, we attempted to induce oxygen deficiency in DyMnO₃ through solidification at low oxygen partial pressure using an aerodynamic levitator. The objective was to decrease the tolerance factor by reducing the valence of manganese ions and thereby increasing their ionic radius. The results showed an increase in oxygen deficiency as the oxygen partial pressure decreased. Based on the assumption that the manganese ions’ valence decreased due to an increase in oxygen deficiency, the corresponding tolerance factor evaluated from the average ionic radii of manganese and oxygen also decreased. This decrease promoted the formation of the hexagonal phase, similar to the effect observed when the ionic radius of Ln³⁺ is reduced.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 87 (2023) 226–230. The abstract and figure captions are modified.