Band gap, complex dielectric function, and optical properties of Sb-doped lead-free KNN piezo-ceramics analyzed by VEELS-TEM

Sb-doped KNN (Na0.9K0.1Nb0.97Sb0.03O3, KNNS) is an environment-friendly lead-free ferroelectric and it has high piezoelectric response. A band gap energy Eg = 3.58 eV for KNNS was evaluated by valence electron energy loss spectroscopy (VEELS), which has not been reported in the literature. The zero loss peak (ZLP) was extracted by the reflected tail model, results were gotten by the Fourier-log method. Single scattering dispersion (SSD) was used to execute the Kramers-Krӧnig analysis, thence: complex dielectric function (ε1 and ε2) based on KNNS, and optical properties were obtained; The static refractive index n(0)(nKNNS) resulted to be 2.21, which mathematically is about a square root of dielectric constant, coinciding with that obtained experimentally. Also, a peak at n1=4.1 eV and n2=9.42 eV located in the near ultraviolet was observed. Understanding the different applications of KNNS in optoelectronic devices is vital, also it is worth mentioning that many articles overlook the relativistic effects in the band gap, dielectric function, and the optical properties measurements.