Optical gap of strontium titanate (Deviation from urbach tail behavior)

The absorption of SrTiO3 below the fundamental edge shows, near liquid-nitrogen temperature, an oscillatory behavior, for which an explanation is proposed in terms of indirect transitions Γ15→X3, centered around 3.27 eV and involving a 51-meV LO phonon. There is no evidence of excitonic effects. The first direct transition is probably X5′→X3 and is located at ∼3.46 eV. Broadening effects mask the indirect structure at higher temperatures where the absorption curves gradually evolve into normal Urbach tailing. © 1970 The American Physical Society