Size-dependent vibronic coupling in α-Fe₂O₃

We report the discovery of finite length scale effects on vibronic coupling in nanoscale α-Fe₂O₃ as measured by the behavior of vibronically activated d-d on-site excitations of Fe³⁺ as a function of size and shape. An oscillator strength analysis reveals that the frequency of the coupled symmetry-breaking phonon changes with size, a crossover that we analyze in terms of increasing three-dimensional character to the displacement pattern. These findings demonstrate the flexibility of mixing processes in confined systems and suggest a strategy for both enhancing and controlling charge-lattice interactions in other materials.