Ferroelectric materials have a permanent electric dipole, an internal separation of the centres of positive and negative ionic lattices, that can be flipped by applying an electric field. They also undergo a structural change at a material dependent temperature. known as the Curie temperature, above which this dipole behaviour disappears. Despite having permanent dipoles, ferroelectrics are insulating materials. These properties make them valuable in technologies such as sensors, actuators, and memory devices. 

In this work, the researchers study the band gaps of ferroelectric materials to better enable their use in energy conversion, catalysis, and optoelectronic devices, where understanding light absorption and electron behaviour is essential. Traditionally, the band gap in ferroelectrics has been treated as a single number. However, ferroelectrics are not conventional semiconductors. They contain localized charges, polarons, internal dipoles, and structural disorder. These features give rise to three distinct band gaps, not one. 

There is the intrinsic fundamental band gap, defined as the ground state difference between the fully occupied valence band and the completely empty conduction band. The smaller optical gap is associated with light induced transitions involving bound electron-hole pairs (excitons), and the even smaller transport gap associated with electrical conduction via localised electronic carriers. 

In this study, the authors determine the fundamental, optical, and transport gaps using X‑ray photoelectron spectroscopy, optical spectroscopy, and electrical conductivity measurements, respectively, for NBT‑6BT and NaNbO₃. The fundamental gap values are further supported by DFT calculations. Because these three gaps differ by about 1 eV or more, different experiments have actually been probing different gaps all along, meaning past optical and electrical results were often compared incorrectly, leading to widespread misinterpretation. The conclusion establishes that ferroelectrics possess three fundamentally different energy gaps, explains why they differ, provides a framework for measuring them, confirms their values theoretically, and highlights why this distinction is crucial for designing future energy and electronic technologies. 

Do you want to learn more about this topic?

Prospects and applications near ferroelectric quantum phase transitions: a key issues review by P Chandra, G G Lonzarich, S E Rowley and J F Scott (2017)