Impedance spectroscopy and optical properties of lanthanum-modified Bi2FeMnO6 for NTC thermistor applications

Abstract

The double perovskite Bi_1.75La_0.25FeMnO₆ (BLFMO) ceramic was prepared by a solid-state reaction method and characterized by different techniques such as X-ray diffraction, scanning electron microscope, energy dispersive X-ray, transmission electron microscope, and optical, dielectric, and electrical property analysis. The synthesized material has a monoclinic crystal structure with an average crystallite size of 63.7 nm and lattice strain of 0.0013, as revealed by X-ray diffraction (XRD) data. The surface morphology of the prepared sample was studied by the scanning electron microscope (SEM) technique, which shows spherical-shaped well-developed grains having clear grain boundaries with an average grain size of 55.6 μm. The EDX spectrum and elemental color mapping checked the purity and homogeneity of the sample. The transmission electron microscope (TEM) technique displayed that the particles are well connected in the synthesized material, which may be a possible reason for the better physical properties. The Brunauer–Emmet–Teller (BET) surface area was 0.371 m² g⁻¹, with a pore volume of 7.202 cc g⁻¹ and an average pore diameter of 19.635 Å. The optical properties were studied from Fourier transform infrared (FTIR) and ultraviolet diffuse reflectance spectroscopy (UV-DRS). The FTIR spectrum revealed the vibrational modes of all the constituent elements in the sample. The direct bandgap energy of 2.71 eV was calculated from the UV-DRS spectrum, which is suitable for optoelectronic device applications. The sample exhibited high dielectric constant, low loss (from dielectric study), negative temperature coefficient of resistance behavior (from impedance study), non-Debye relaxation (from modulus study), and a thermally activated conduction mechanism (from ac conductivity study). The occurrence of Maxwell–Wagner dispersion was known from the dielectric study. The semi-circular arcs in the Nyquist and Cole–Cole's plots explained their semi-conducting nature. The resistance versus temperature curve indicated the semiconducting nature of the sample and its potential application as a negative temperature coefficient (NTC) thermistor. Thus, the prepared sample has unique characteristics for different applications related to optoelectronics and sensors.