Optical spectroscopy and band gap analysis of hybrid improper ferroelectric Ca3Ti2O7

Abstract

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.

Polarization switching dynamics and switchable diode effect in hybrid improper ferroelectric Ca3Ti2O7 ceramics

“…Such a broad tail and narrowed band gap in CTO ceramic could be again ascribed to the oxygen vacancies. The presence of oxygen vacancies in CTO ceramics will raise the position of the valence band due to the overlap of the nonlocalized oxygen vacancy states and the valence band edge . A similar result has been found in oxygen deficient SrTiO 3 .…”

“…From intersection of the extrapolation the band gap of CTO ceramic is estimated to be 3.27 eV. This value of band gap is much smaller than that of the single crystal (3.9 eV), due to the presence of oxygen vacancies that is in well agree with theoretical prediction . Moreover, there is a so‐called broad tail that persists to lower energies in the curve vs h ν.…”

“…Hence, the oxygen vacancies existing in CTO sample will dramatically influence their electronic energy band. Furthermore, it can be predicted that the photo‐excitation of the electrons from the top of the valence band to bottom of the conduction band will have the effect of bringing electrons from O 2p → Ti 3d state via charge‐transfer excitations . The influence of oxygen vacancies on the electronic structure is studied via the plot of the density of states using 2 × 1 × 2 super‐cell CTO with three oxygen vacant sites, as shown in Figure D.…”

“…Yoon et al provided the first experimental evidence of ferroelectric polarization and ferroelastic domain structure at room temperature in single‐crystal samples of Ca 3− x Sr x Ti 2 O 7 ( x = 0, 0.54, and 0.85) . Moreover, optical spectroscopy and band gap analysis of single crystals CTO samples have been investigated through theoretically calculation and experimental verification . Nowadnick et al used group theory methods and first‐principles calculations to investigate the possible ferroelectric switching pathway in CTO with low‐energy barriers via an orthorhombic twin domain, an antipolar stacking domain, or a rhombohedral‐like phase .…”

Infrared nano-spectroscopy of ferroelastic domain walls in hybrid improper ferroelectric Ca3Ti2O7

“…To verify this observation, we calculated the band gap for the bulk C2mm structure (the hypothetical structure realized at the midpoint of the domain wall). We find that the computed gap is 0.26 eV less than that of the A2 1 am structure -not even close to closing the 3.94 eV gap in this system [42].…”