Temperature Dependence of the Absorption Edge in Ferroelectric KNbO₃ Crystals

Crystal Structure, Phase Transitions and Photoferroelectric Properties of KNbO3-Based Lead-Free Ferroelectric Ceramics: A Brief Review

“…Table 3 lists the fundamental bandgaps for the different phases of KN calculated by Schmidt et al (2017) using the HSE-30 functional. It should be noted that these calculations correspond to electronic bandgaps, however, the trend with increasing symmetry is consistent with the experimental optical bandgap values reported by Kaifu and Komatsu (Kaifu et al, 1967), as shown in Figure 8.…”

“…In 1967, Kaifu and Komatsu (1967) measured the temperature dependence of the absorption edge (based on an absorption coefficient α = 300 cm −1 ) of KN, which was ascribed to transitions between 2p oxygen states and the d-like conduction states. The results are reproduced in Figure 8, which shows the bandgap to decrease from ∼3.58 eV at −173 • C to ∼2.72 eV at 525 • C. The temperature coefficient of the bandgap is approximately −10 −3 eV/ • C. Moreover, anomalous shifts of the absorption edge are observed at temperatures similar to those of the dielectric anomalies shown in Figure 3A.…”

“…Their calculations for the orthorhombic phase show the valence band maximum (VBM) to be located at T symmetry point, while the conduction band minima (CBM) is positioned at symmetry point, as illustrated in Figure 9. Hence, KN should be an indirect bandgap Kaifu and Komatsu (Kaifu et al, 1967)]. Diffuse reflectance spectroscopy 3.25 (Zhang et al, 2015) semiconductor with a transition of 3.59 eV between T and symmetry points.…”

“…FIGURE 8 | Temperature dependence of the forbidden band width of undoped KN crystal. [Redrawn fromKaifu and Komatsu (Kaifu et al, 1967)]. …”

Light-driven dynamical tuning of the thermal conductivity in ferroelectrics

“…Note also that a critical parameter is the light penetration depth, Δz, which in the case of being too small could result in a limiting factor for practical applications. Fortunately, from the optical absorption data reported by Kaifu and coworkers 43 we could estimate Δz ≈ 30 μm, which safely exceeds the typical thickness of thin films (∼10-100 nm).…”