Size-dependent optical properties of $\alpha$-Fe$_{2}$O$_{3}$ nanoparticles

We investigated the variable temperature optical properties of nanoscale hematite ($\alpha$-Fe$_{2}$O$_{3}$) with special attention to the parity-forbidden Fe$^{3+}$ $\textit{d-d}$ excitation that is activated by hybridization and symmetry-breaking phonons. An oscillator strength analysis of the rhombohedra, cubes, and rice reveals that the energy of the coupling phonon scales as (size)$^{-1}$. Moreover, preliminary work in high magnetic fields shows a field-induced color change. These findings are important for more deeply understanding finite length scale effects in this iconic material and other nanoscale transition metal oxides.