Magnetic and Crystal Structure of Titanium Sesquioxide

A single-crystal neutron diffraction study has been made of titanium sesquioxide in the temperature range 1.4 to 711°K. The crystal undergoes a transition at a temperature variously reported as about 450 to 600°K, below which the structure becomes antiferromagnetic. In the crystal studied, $T_N\approx 660°$K. The magnetic moments in this corundum-type structure have an arrangement similar to that in $\alpha$ hematite between 253 and 948°K. Puckered sheets of ${\mathrm{Ti}}^{3+}$, parallel to (111), are ferromagnetic with moments lying in the plane normal to the trigonal axis. Adjacent cation sheets, separated by a layer of oxygen, have antiparallel moments. The magnitude of the moment is about 0.2 ${\mathrm{\mu }}_{\mathit{B}}$ per ${\mathrm{Ti}}^{3+}$. There is no evidence for a moment parallel to the trigonal axis. The magnetic form factor increases with scattering angle, due to some unquenched orbital momentum. An explanation for the observed form factor and for the moment perpendicular to [111] has been given by Blume. The density maxima of antiparallel pairs of moments are displaced toward each other along the trigonal axis.