Abstract text (incl. figure legends and references)

Valence electron energy loss spectroscopy, in transmission electron microscopy mode (VEELS-TEM) is an important tool to elucidate the complex dielectric function (CDF) and opto-electronic properties (O-EP) of insulators. The motivation of this work is to present a comparative study of CDF and the O-EP of lead-free Ba_0.8Ca_0.2Ti_0.9Zr_0.1O₃ (BCZT) and Ba_0.8Ca_0.2Ti_0.85Zr_0.1V_0.05 (BCZTV) electro-ceramics.

The samples were prepared by the modified Pechini method to obtain nanoparticles. A single tetragonal perovskite structure, considering the P4mm space group was stabilized at 700 ^oC for 1 h (1). This structure was confirmed by Rietveld refinement of the x-ray diffraction pattern for both compositions using Fullprof suite software. The microstructural parameters such as average crystallite size and shape in the nanometer range were simulated using the spherical harmonics method implemented in the GFourier program. The VEELS–TEM experiment scanned the energy interval from 0 to 50 eV was obtained with an electron energy loss spectrometer (EELS GAT–777 STEMPack) attached to a JEM2200FS (200 kV). All measurements were performed at room temperature. The system has an energy resolution of 1.00 eV, determined by the full-width-half-maximum of the zero-loss peak. The electron probe size was below 1 nm. The energy dispersion was set to 0.05 eV/channel, to maintain the energy resolution. The spectrum was collected with a parallel beam (convergence semi-angle α = 0 mrad). The corresponding collection semi-angle was β = 20.3 mrad. For the EELS acquisition, 10 consecutive exposures were accumulated. The deconvolution and subtraction of the zero-loss peak (ZLP) from the VEELS region, the Fourier log method to remove plural scattering and Kramers-Kronig analysis performed in the single scattering distribution were carried out using the routines available in the Digital Micrograph™ Gatan Microscopy Suite (GMS 3) software. Absolute thickness of samples was determined via log ratio method with a value of 51.1 nm.

The spectroscopic analysis started with the chemical identification of the atoms that conforms the BCZT solid-solution. Bulk and surface plasmon were located at 27.2 eV and 12.9 eV for BCZT and 26.9 eV and 12.3 eV for BCZTV, respectively, in the normalized energy loss function. Dielectric constant was calculated from the real part of the complex dielectric function, while the inter-band transitions were identified in the joint density of states function. The refraction index n and the extinction coefficient k, as a function of energy, were obtained from the complex dielectric function. The band gap energy was determined using a polynomial fit in the optical absorption coefficient plot with an Eg = 3.2 eV (BCZT) and 3.18 eV (BCZTV) considering the relativistic correction using the difference method. The Cole-Cole plot corroborates the electronic contributions by the presence of two semicircles due to the inter-band transitions and plasmon region. As an important remark is that Vanadium plays an important role in the microstructural effects, in the covalent character and also in the opto-electronic properties of BCZT compounds.

G. Herrera-Perez would like to thank Mexico Catedra-CONACYT Grant No. 2563, SNI 1-CONACYT for complementary support and CONACyT–SEP Basic Research Project No. 253605.

References:

(1) G. Herrera-Pérez, et al. Micron 149 (2021) 103124.