We bring together optical absorption spectroscopy, photoconductivity, and first principles calculations to reveal the electronic structure of the room temperature ferroelectric Ca3Ti2O7. The 3.94 eV direct gap in Ca3Ti2O7 is charge transfer in nature and noticeably higher than that in CaTiO3 (3.4 eV), a finding that we attribute to dimensional confinement in the n = 2 member of the Ruddlesden-Popper series. While Sr substitution introduces disorder and broadens the gap edge slightly, oxygen deficiency reduces the gap to 3.7 eV and gives rise to a broad tail that persists to much lower energies.
Bibliographical noteFunding Information:
Research at Tennessee was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Science Division under Award No. DE-FG02-01ER45885 (J.L.M.). The work performed at Rutgers was supported by the National Science Foundation under Award No. DMREF-1233349 (S.W.C. and D.V.).
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