Visible-Light-Absorbing Potassium Niobate-Titanate-Molybdate Ferroelectrics

The interactions of ferroelectric (FE) perovskite oxides $(AB{\mathrm{O}}_3)$ with light are increasingly being studied for different applications, such as photovoltaics and optoelectronics. The combination of different cations at the A and B sites to form solid solutions allows tuning of the material’s properties and, most importantly, the band gap ($E_g$), which sets the wavelength range of light absorption. Classic FE perovskite oxides, such as ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$, ${\mathrm{K}\mathrm{Nb}\mathrm{O}}_3,$ and ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3,$ exhibit $E_g$ > 3 eV, which limits their implementation in visible-light-absorbing devices. Furthermore, the tuning of their $E_g$ via a solid solution strategy to a lower $E_g$ range is limited by the requirement for the presence of a $d^0$ metal at the B site, which is necessary for the FE distortion, but leads to a larger $E_g$. This gives rise to the challenge of decreasing $E_g$, while maintaining FE distortion. Here, we use first-principles calculations to explore the FE and optical properties of the $({\mathrm{K}\mathrm{Nb}\mathrm{O}}_3{)}_x({\mathrm{K}\mathrm{Ti}}_{1/2}{\mathrm{Mo}}_{1/2}{\mathrm{O}}_3{)}_{1-x}$ (KNTM) perovskite oxide solid solution. The introduction of ${\mathrm{Ti}}^{4+}$ and ${\mathrm{Mo}}^{6+}$ into the parent ${\mathrm{K}\mathrm{Nb}\mathrm{O}}_3$ decreases the $E_g$ to about 2.2 eV for x = 0.9, while preserving or enhancing polarization. Experimental fabrication and characterization show that the obtained KNTM material at x = 0.9 has an orthorhombic structure at room temperature and a direct gap of <2.2 eV, confirming first-principles-based predictions. These properties make KNTM a promising candidate for further studies and applications as a visible-light-absorbing FE material.

Figure 1

Relative energy within composition ΔE (a), P_tot (b), and E_g (c) of the studied 87.5-12.5KNTM structures and parent KNO.

Figure 2

Relative energy within composition ΔE (a), P_tot (b), and E_g (c) of the studied 75-25KNTM structures and parent KNO.

Figure 3

Characterization of fabricated 90-10KTNM ceramics: (a) XRD pattern obtained at room temperature compared with that of KNO. (b) Dependence of relative permittivity on temperature measured at 100 Hz–100 kHz; the inset shows the DSC curve compared with those of KNO. (c) Dependence of dielectric loss on temperature measured at 100 Hz–100 kHz. (d) Tauc relation at room temperature, compared with that of KNO; the inset shows the appearance of the fabricated ceramics. (e) FE hysteresis loops measured in the dark, at room temperature, and with different frequencies compared with that of KNO.