Effect of Isovalent Substitution of Bi Cations by La Cations on the Crystal Structure and Thermodynamic Properties of Nanocomposites Based on Bismuth Ferrite

The work is devoted to the study of nanocomposites of bismuth ferrites (BFO) and bismuth lanthanum ferrites (BLFO). The effect of substitution of Bi cations by La cations on the crystal structure and thermodynamic properties of bismuth ferrites was studied. These composites were prepared by the modern method of sol-gel synthesis. Scanning electron microscope (SEM) images of the prepared samples were obtained. Using SEM method, it was established that the morphology of BFO and BLFO is similar. All samples have a granular microstructure with granule sizes from 20 to 100 nm. This means that, at a scale above 200 nm, all samples are morphologically indistinguishable, while structural features appear at smaller scale levels. X-ray diffraction analysis was preformed, and the thermodynamic properties of these bismuth ferrites were studied. X-ray diffraction study revealed the crystallinity of bismuth ferrites at a scale level of less than one nanometer. It was found that the formation of nanocomposites based on bismuth ferrites occurs according to the nucleation mechanism of non-isothermal crystallization. When cooled, the BFO sample is a three-component material. As a result of the substitution of La cations for Bi cations, a more homogeneous two-component BLFO structure is formed, which consists of BiLaFeO3 crystallites and Bi2Fe4O9 crystallites with approximately equal dimensions (about 0.400 nm3). It was also found a decrease in the heat capacity, enthalpy, entropy and Gibbs energy modulus in BLFO compared to BFO, which indicates an improvement in the thermodynamic compatibility of components due to a decrease in heat losses at structural defects.