Processing-composition-structure effects on the optical band gap of KNbO3-based ceramics

This present work is focused on band-gap engineering of solid-solutions based on KNbO3, which was proposed as a promising photoferroelectric (Grinberg et al., 2013). The strategy to narrow the band-gap of the parent KNbO3 (3.22 eV), relies on replacing Nb5+ by lower valence transition metals (Me3+) and K+1 by cations which maintain the compositions stoichiometric. Ceramic processing of KNbO3 by conventional route was optimised in order to minimise K losses, which leads to the formation of a hygroscopic secondary phase, K4Nb6O17. This phase impairs the structural integrity of the samples. In addition, single-phase KNbO3 ceramics have the tendency to absorb moisture from the environment, increasing its conductivity near room temperature. Subsequently, all solid-solutions presented in this work, (1-x) KNbO3-x Ba0.5Bi0.5Nb0.5Zn0.5O3 and (1-x) KNbO3-x BiMeO3 (Me= Mn, Co and Ni) systems in a compositional range of 0≤x≤0.25, 0.90 KNbO3-0.1 BaNb0.5Ni0.5O3 and 0.98 K0.5Na0.5NbO3-0.02 BaNb0.5Ni0.5O3 compounds, were prepared by the same route as KNbO3. X-Ray Diffraction (XRD), Raman spectroscopy and Scanning Electron Microscopy (SEM) revealed compositionally inhomogeneities, suggesting difficulties in cation diffusion for low concentration of solutes by conventional routes. The systems evolve from orthorhombic (x=0) to pseudo-cubic symmetry with an increase of x, suggested by XRD, Raman spectroscopy, ferroelectric and dielectric response. Indeed, these two symmetries seem to coexist for intermediary concentrations. A solubility limit for orthorhombic KNbO3 phase is determined for each system. In addition, a continuous band-gap narrowing was observed in all systems. Nevertheless, (1-x) KNbO3-x BiFeO3 (0≤x≤0.25) system maintained the polar phase up to x=0.25 and its band-gap was narrowed down to 2.22 eV. Indeed, a photocurrent of 0.24 μA/cm2 was measured for 0.75 KNbO3- 0.25 BiFeO3 which is higher than reported for the controversial 0.90 KNbO3-0.1 BaNb0.5Ni0.5O3 compound (Grinberg et al., 2013). The literature does not agree about its band-gap value, which varies from 1.3 eV to 3 eV. Hypothetically, the impossibility of preparing chemically homogenised samples by solid-state reaction may lead to the occurrence of intraband states, which can be misinterpreted. Similar conclusions are reached for 0.98 K0.5Na0.5NbO3-0.02BaNb0.5Ni0.5O3.