A study was made of the shift of the absorption edge in BaTiO3 caused by an applied electric field near the phase transition from the tetragonal phase to the cubic one. The absorption edge shifted to shorter wavelengths is independent of the direction of the field. This shift was measured as a function of temperature and field strength and shows a sharp maximum at the Curie temperature θ, a linear field dependence below θ, and a quadratic field dependence above θ.
V. M. Fridkin and K. A. Verkhovskaya, "Anomalous Shift of the Absorption Edge in Ferroelectric BaTiO3, under an Electric Field," Appl. Opt. 6, 1825-1827 (1967)