Magnetic and Crystallographic Transitions in theα−Mn2O3−Fe2O3System

Abstract

X-ray diffraction, M\"ossbauer spectroscopic, and magnetic susceptibility measurements have led to a determination of the phase diagram including magnetic phases of the system ${({\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})}_{2}{\mathrm{O}}_{3}$, $0\ensuremath{\le}x\ensuremath{\le}0.63$, the limit of ${\mathrm{Fe}}^{3+}$ cation solubility. $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Mn}}_{2}{\mathrm{O}}_{3}$ itself is cubic (space group $\mathrm{Ia}3$) above 308\ifmmode^\circ\else\textdegree\fi{}K and orthorhombic (space group $\mathrm{Pcab}$) below, the crystallographic transition being apparently higher than first order; it becomes antiferromagnetic at 79\ifmmode^\circ\else\textdegree\fi{}K and at 25\ifmmode^\circ\else\textdegree\fi{}K appears to go through a first-order transition to another antiferromagnetic structure. When 0.75 cation% of ${\mathrm{Mn}}^{3+}$ is replaced by ${\mathrm{Fe}}^{3+}$ ion, the lower transition occurs at 19\ifmmode^\circ\else\textdegree\fi{}K. About 0.75% ${\mathrm{Fe}}^{3+}$ ion makes the structure cubic at room temperature; both the crystallographic and upper magnetic transition temperatures (${T}_{t}$ and ${T}_{N1}$, respectively) decrease monotonically and rather rapidly with increasing ${\mathrm{Fe}}^{3+}$ ion content. It is highly probable that when $0.08\ensuremath{\lesssim}x\ensuremath{\lesssim}0.09$, ${T}_{N1}={T}_{t}$; when $0.09<x\ensuremath{\le}0.63$, the structure apparently remains cubic down to 0\ifmmode^\circ\else\textdegree\fi{}K and ${T}_{N1}$ is almost independent of composition. X-ray powder data on $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Mn}}_{2}{\mathrm{O}}_{3}$ at 6.5\ifmmode^\circ\else\textdegree\fi{}K show no additional lines or symmetry change, implying that the transition at 25\ifmmode^\circ\else\textdegree\fi{}K involves a shift in the symmetry center of the structure---a diffusionless but probably first-order transition; a change in space group is not required. Published neutron-diffraction data on $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Mn}}_{2}{\mathrm{O}}_{3}$ and our own observations lead to the hypothesis that the magnetic space group of both orthorhombic phases is $\mathrm{Pcab}$ and of the cubic phase, ${I}_{p}a3$. In the latter, the directions of the ordered spins of cations in the $8b$ sites are absolutely fixed by symmetry, while those in $24d$ sites are constrained to lie in planes perpendicular to the twofold axes. There are no symmetry restrictions on the spins in $\mathrm{Pcab}$, but it is probable that all three magnetic structures are closely related. At all temperatures, the ${\mathrm{Mn}}^{3+}$ ions prefer the less symmetric $24d$ sites, presumably because of the asymmetrical nature (Jahn-Teller distortion) of the electron distribution of the ${\mathrm{Mn}}^{3+}$ ion.