X‐ray Powder Diffraction Structural Phase‐Transition Study of La(Cr _1‐x Mn _x )O ₃ (x = 0 to 0.25) Using the Rietveld Method of Analysis

Abstract

The La(Cr 1‐x Mn x )O 3 (x = 0 to 0.25) system exhibited a displacive phase transformation from an orthorhombic to a rhombohedral structure at temperatures ranging from about 256°C at x = 0 to 305°C at x = 0.25. The Rietveld method of X‐ray analysis was used to determine structural characteristics and to assay phase concentrations when multiple phases were present. Indications were that the Mn ions on the B site affected the transition temperature through an ionic size effect; i.e., Mn 3+ , with a larger ionic size than Cr 3+ , increased the transition temperature while the presence of the smaller Mn 4+ ions reduced it. A hysteresis was also observed with respect to the phase transformation as a function of temperature. The phase transformation was associated with a change in the rotation of the oxygen octahedra from an antiphase to an in‐phase tilt sequence.