Magnetic and Crystallographic Transitions in the α−Mn2O3−Fe2O3 System

Abstract

X-ray diffraction, Mössbauer spectroscopic, and magnetic susceptibility measurements have led to a determination of the phase diagram including magnetic phases of the system ${\left({\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\right)}_2{\mathrm{O}}_3$, $0\le x\le 0.63$, the limit of ${\mathrm{Fe}}^{3+}$ cation solubility. $\alpha -{\mathrm{Mn}}_2{\mathrm{O}}_3$ itself is cubic (space group $\mathrm{Ia}3\mathrm{}$) above 308°K and orthorhombic (space group $\mathrm{Pcab}$) below, the crystallographic transition being apparently higher than first order; it becomes antiferromagnetic at 79°K and at 25°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°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 $T_{N1\mathrm{}}$, respectively) decrease monotonically and rather rapidly with increasing ${\mathrm{Fe}}^{3+}$ ion content. It is highly probable that when $0.08\lesssim x\lesssim 0.09$, $T_{N1\mathrm{}}=T_t$; when $0.09<x\le 0.63$, the structure apparently remains cubic down to 0°K and $T_{N1\mathrm{}}$ is almost independent of composition. X-ray powder data on $\alpha -{\mathrm{Mn}}_2{\mathrm{O}}_3$ at 6.5°K show no additional lines or symmetry change, implying that the transition at 25°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 $\alpha -{\mathrm{Mn}}_2{\mathrm{O}}_3$ 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, $I_{p}a3\mathrm{}$. 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 ${\mathrm{Mn}}^{3+}$...